Cellular Telecommunications

Cellular Telecommunications

- articles and information on cellular telecommunication / cell phone technology

There are many mobile or cell phone systems that are in use all over the world and there is a considerable amount of equipment ranging from the cell phones themselves to cellular base stations, antennas and the network infrastructure. In addition to the basic equipment there is new cell phone technology to provide the many new services that are now available, enabling cell phone users to enjoy many new applications from games, and ringtone downloads to picture and video downloads. With technologies ranging from GSM, GPRS, EDGE to UMTS or W-CDMA and cdmaOne (IS-95) to CDMA2000 1X, EV-DO and EV-DV and mobile TV technologies such as MediaFLO, DMB, and DVB-H, there are plenty of technologies in use.

There is plenty of terminology associated with cellular telecommunications and cell phone technology. Our terminology glossary explains the most commonly used terms associated with cellular telecommunications / cell phone technology:

The technology behind cellular systems and cell phones has developed from the first generation (1G) systems to the second geenration (2G) systems and then to the third generation (3G) systems. At each stage the performance improved and further facilities were available, from SMS messaging to video downloads.
- Major Mobile Phone Systems (a tabular overview)
- The Route from Analogue to 3G

Cellular telecommunications basics.
There are a number of basic concepts behind cellular telecommunications systems. These include the idea of cells themselves as well as how the networks are set up, what is in a mobile phone and how some of the transmission technologies such as CDMA, TDMA and the like operate.
- The basic concept of a cellular telecommunications system
- The multiple access schemes used by mobile phones
- The duplex schemes used by mobile phone networks - FDD and TDD
- The operation and electronics within a mobile phone
- The basics of a cellular network
- The way in which a mobile phone registers onto a network
- Handover or handoff, the way in which cellular calls are transferred from one cell to another
- Code Division Multiple Access (CDMA) What it is and how it works
- Orthogonal Frequency Division Multiplex (OFDM) What it is and how it works
- MIMO - Multiple Input Multiple Output What it is and how it works

Cellular testing
With vast numbers of phones and cellular systems deployed and new ones beong launched all the time, it is essential that new equipment is thoroughly tested ebfore it is launched ontoth e market. Additionally ones systems are deployed it is essential to esnure that they are operating to their best.
- Cellular conformance testing overview
- Conformance testing for GSM and UMTS
- cdma interoperability test

Global System for Mobile Communications (GSM).
GSM is the system that was developed in Europe and is now the most popular system in use around the globe.
- The development of the Global System for Mobile communications - GSM
- GSM technical overview (5 pages)
- General Packet Radio Service, GPRS, tutorial (5 pages)
- Enhanced Data for GSM Evolution (EDGE)

- UMA - unlicensed mobile access and GAN generic access network
- UMA / GAN conformance testing

CDMA System including cdmaOne (IS-95) and CDMA2000 (IS-2000)
The CDMA family of technologies including cdmaOne (IS-95) to CDMA 2000 were the first technologies to use CDMA technology.
- What is CDMA - cdmaOne / CDMA2000? A Guide to the CDMA system and its evolution.
- IS-95 (cdmaOne)
- CDMA2000 1xEV-DO
- CDMA Band Classes and Frequency Allocations

Universal Mobile Telecommunications System (UMTS) / Wideband CDMA (W-CDMA)
- UMTS / W-CDMA Tutorial (5 pages)
- High Speed Downlink Packet Access (HSDPA)
- High Speed Uplink Packet Access (HSUPA) The companion to HSDPA.
- UMTS TDD The scheme being used for mobile broadband.
- 3G LTE - Long Term Evolution an overview of the developments being undertaken for the next generation cellualr systems The scheme being used for mobile broadband.

Miscellaneous sytems and features
- Pacific or Personal Digital Cellular (PDC)
- i-mode for e-mails and internet surfing
- TD-SCDMA the 3G system being developed in China
Location services:
- Assisted GPS
Mobile video:
- Mobile video broadcasting video to phones using T-DMB and DVB-H
- DVB-H Digital Video Broadcasting - Handheld
- DMB Digital Multimedia Broadcasting

Private Mobile Radio PMR
Private or Professional Mobile Radio (PMR), or Public Access Mobile Radio (PAMR) is used in many countries around the world, and new technology is being developed to ensure it keeps up with today's needs. It is widely used by the emergency services, and this is particularly true of the new Tetra system that is being widely deployed.
An overview of private mobile radio (PMR) - the various standards that are available to use.
An overview of the basic "local" PMR system
Trunking using the MPT1327 system
An Overview of TETRA private mobile radio (PMR)

Overview of Mobile Phone Systems

The large number of different mobile phone or cellphone systems that are talked about today can be very confusing. Whilst not all are in use today, some of the older systems have been superseded and some of the newer systems have not all been rolled out yet, nevertheless many different names and technologies are talked about. The table below gives a summary of the main systems that have been used, are being used or are due for introduction.
Cellphone System	Generation	Channel Spacing	Access Method	Comments
AMPS	1G	30 kHz	FDMA	Advanced Mobile Phone System, this analogue system first developed and used in the USA
NAMPS	1G	10 kHz	FDMA	Narrow band version of AMPS chiefly used in the USA and Israel based on a 10 kHz channel spacing.
TACS	1G	25 kHz	FDMA	Analogue system originally in the UK. Based around 900 MHz, this system spread world wide. After the system was first introduced, further channels were allocated to reduce congestion, in a standard known as Extended TACS or ETACS
NMT	1G	12.5 kHz	FDMA	Nordic Mobile Telephone. This analogue system was the first system to be widely used commercially being launched in 1979. It was used initially on 450 MHz and later at 900 MHz. It was used chiefly in Scandinavia but it was adopted by up to 30 other countries including Oman.
NTT	1G	25 kHz	FDMA	Nippon Telegraph and Telephone. The system used in Japan, using a 900 MHz frequency band, and 55 MHz transmit receive spacing. (A high capacity version is known as HICAP).
C450	1G	20 kHz	FDMA	The system adopted in West Germany (East Germany was separate at this time). It used a band in the region of 450 MHz along with a 10 MHz receive / transmit spacing.
GSM	2G	200 kHz	TDMA	Originally called Groupe Speciale Mobile, the initials later stood for Global System for Mobile communications. It was developed in Europe and first introduced in 1991. The service is normally based around 900 MHz although some 850 MHz allocations exist in the USA.
DCS 1800	2G	200 kHz	TDMA	1800 MHz derivation of GSM and is also known as GSM 1800.
PCS 1900	2G	200 kHz	TDMA	1900 MHz derivation of GSM, and is also known as GSM 1900.
TDMA	2G	30 kHz	TDMA	Although it was originally known as US Digital Cellular (USDC) and was introduced in 1991. It is sometimes called North America Digital Cellular and also known by its standard number IS-54 that was later updated to standard IS136. It is a 2G digital system that was designed to operate alongside the AMPS system.
PDC	2G	25 kHz	TDMA	Pacific or Personal Digital Cellular. The system found only in Japan where it has gained very widespread use. It has many similarities with IS-54 although it uses a different speech coder and a 25 kHz bandwidth.
GPRS	2.5G	200 kHz	TDMA	General Packet Radio Service. A data service that can be layered onto GSM. It uses packet switching instead of circuit switching to provide the required performance. Data rates of up to 115 kbps attainable.
EDGE	2.5 / 3G	200 kHz	TDMA	Enhanced Data rates for GSM Evolution. The system uses a different form of modulation (8PSK) and packet switching which is overlayed on top of GSM to provide the enhanced performance. Systems using the EDGE system may also be known as EGPRS systems.
CdmaOne	2G	1.25 MHz	CDMA	This is the brand name for the system with the standard reference IS95. It was the first CDMA system to gain widespread use. The initial specification for the system was IS95A, but its performance was later upgraded under IS95B which the cdmaOne specification actually uses. Apart from voice it also carries data at rates up to 14.4 kbps for IS95A and under IS95B data rates of up to 115 kbps are supported.
CDMA2000 1X	2.5G	1.25 MHz	CDMA	This system supports both voice and data capabilities within a standard 1.25 MHz CDMA channel. CDMA2000 builds on cdmaOne to provide an evolution path to 3G. The system doubles the voice capacity of cdmaOne systems and also supports high-speed data services. Peak data rates of 153 kbps are currently achievable with figures of 307 kbps quoted for the future, and 614 kbps when two channels are used.
CDMA2000 1xEV-DO	3G	1.25 MHz	CDMA	The EV-DO stands for Evolution Data Only. This is an evolution of CDMA 2000 that is designed for data only use and its specification is IS 856. It provides peak data rate capability of over 2.45 Mbps on the forward or downlink , i.e. from the base station to the user. The aim of the system is to deliver a low cost per megabyte capability along with an always on connection costed on the data downloaded rather than connection time.
CDMA2000 1xEV-DV	3G	1.25 MHz	CDMA	This stands for Evolution Data and Voice. It is an evolution of CDMA2000 that can simultaneously transmit voice and data. The peak data rate is 3.1 Mbps on the forward link. The reverse link is very similar to CDMA2000 and is limited to 384 kbps.
UMTS	3G	5 MHz	CDMA / TDMA	Universal Mobile Telecommunications System. Uses Wideband CDMA (W-CDMA) with one 5 MHz wide channel for both voice and data, providing data speeds up to 2 Mbps.
TD-SCDMA	3G	1.6 MHz	CDMA	Time Division Synchronous CDMA. A system developed in China to establish their position on the cellular telecommunications arena. It uses the same bands for transmit and receive, allowing different time slots for base stations and mobiles to communicate. Unlike other 3G systems it uses only a time division duplex (TDD) system.

Cell Phone Systems

- overview and history of cell phone from the first analogue systems through to the latest 3G systems

The development and history of the mobile phone has seen a tremendous number of changes since the first cell phones were introduced. It was only at the beginning of the 1980s when mobile phone technology started to be deployed commercially. Since then there have been many new cell phone or mobile phone systems introduced, and many improvements have been made in the technology. The mobile phones themselves as well as the associated equipment including base stations and the other network equipment has become much cheaper and far smaller.

One of the major changes is the level of market penetration that has been achieved. Around one in six of the world's population now has a mobile phone. When the first systems were introduced the operating and ownership costs were such that they were only used by businesses with a real need for their employees to be able to keep in touch all the time. Now they are an almost essential personal belonging for most people. In many countries market penetration has exceeded 70%, and it is not uncommon for many people to have one phone for business and another for personal use. Accordingly in some segments of the population the market penetration is very much higher than 70%.

Development overview
The phones themselves have undergone many changes during their history. The technologies that have been used have improved dramatically. The first systems to be launched were based on analogue technology. The early phones were very large and could certainly not be placed in a pocket like the phones of today.

The first generation (1G) phone systems as they are now known were overtaken in the early 1990s by the first digital systems.

The high levels of use and limited frequency allocation meant that greater spectrum use efficiency was needed. Accordingly the next or second-generation (2G) phone systems were introduced to meet this need.

As the usage of phones increased and people became more mobile, new possibilities emerged for using the phones for datatransfer. They could be used to download information from the Internet, or to send video. The first stage in this migration was to provide a medium speed data transfer capability. These systems were accordingly known as 2.5G.

However the ultimate aim was to provide a relatively high-speed data transfer capability. These full third generation (3G) systems took longer to develop and roll-out than had been originally anticipated as a result of higher development costs and a downturn in the global economy. However they are able to provide a significant improvement in capability over the 2.5 G systems

Analogue Systems
There was an enormous variety of first generation systems that were introduced. Much of the early development of cellular systems had been undertaken in the USA, but the first fully commercial system to be launched was the Nordic Mobile Telephone (NMT) system. Shortly after this a system known as the Advanced Mobile Phone System (AMPS) was launched commercially. This was developed primarily by Bell and was introduced in the USA although many other countries used this system later. A further system known as Total Access Communication System (TACS) developed by Motorola was introduced in the UK and many other countries.

These were the main systems that were developed, although around the globe many variants were developed to suit the needs of the individual countries.

Although there were differences in the specifications of the systems, they were all very similar in concept. The voice information was carried on a frequency-modulated carrier. A control channel was also used to enable the mobile to be routed to a suitable vacant channel. The channel spacing for each system was different. NMT used a 12.5 kHz channel spacing, AMPS, a 30 kHz spacing and TACS a 25 kHz spacing. A later development of AMPS called NAMPS or narrowband AMPS used a 10 kHz channel spacing to conserve spectrum.

Digital systems
The analogue systems were very successful, but their very success started to show some of their shortcomings. The main one was the inefficient way in which they sued the spectrum. With the growth rates that were being seen, there was insufficient spectrum to support the quality of service that was required. By converting to a digital system, considerable savings could be made. A number of systems arose from this initiative. These second-generation systems as they were termed, started to be deployed in the early 1990s and their history is just as remarkable.

The system that was developed in Europe was the result of 26 telecommunications companies working together. Work actually started in 1982, and the roll-out commenced in 1991. The system known by the letters GSM was originally called Groupe Speciale Mobile but this was later changed to Global System for Mobile communications in view of the wide involvement in its development. It used time division multiple access (TDMA) to allow up to eight users to use each of the channels that are spaced 200 kHz apart. The basic system used frequencies in the 900 MHz band, but other bands in the 1800 and 1900 MHz (USA) bands were added. New bands in the 850 MHz region were also added.

In the USA a system specially designed to operate alongside their AMPS system was devised. The system was known under a variety of names including Digital AMPS or DAMPS, and US Digital Cellular (USDC), although it is normally known just as TDMA today as it relies on TDMA technology. The system was originally defined under standard number IS-54, although this was later updated to IS-136 and it uses a 30 kHz channel spacing to make it compatible with the existing AMPS systems in operation.

Another development in the USA from Qualcomm took a major leap in technology. It introduced a totally new concept for multiple access. Based on direct sequence spread spectrum (DSSS) that had previously been used for military transmissions, it used a multiple access system known as code division multiple access (CDMA). The new system offered far greater levels of spectrum efficiency although it required more complicated circuitry in the handsets. The system was defined under standard IS-95 and each carrier had a bandwidth of 1.25 MHz, although many users could use the same channel. The specification was updated from IS-95A to IS-95B. It was this later standard that went under the trade name cdmaOne.

2.5G
Once the second-generation systems became established it soon became apparent that the limited data capabilities of some of the 2G systems were a significant disadvantage. Many applications for data transfer with the increased use of the Internet and laptop computers were seen. Even though the third generation systems were on the horizon, developments were needed to provide a service before they entered the market. One of the first was the General Packet Radio Service (GPRS) development for the GSM system. Its approach centred on the use of packet data. Up until this time all circuits had been dedicated to a given user in an approach known as circuit switched, i.e. where a complete circuit is switched for a given user. This was inefficient when a channel was only carrying data for a small percentage of the time. The new packet switched approach routed individual packets of data from the transmitter to the receiver allowing the same circuit to be used by different users. This enabled circuits to be used more efficiently and charges to be metered according to the data transferred.

Further data rate improvements were made using a system known as EDGE (Enhanced data Rates for GSM Evolution). This basically took the GPRS system and added a new modulation scheme, 8PSK, to enable a much higher data rate to be achieved. Whilst the symbol rate remained the same at 270.833 samples per second, each symbol carried three bits instead of one.

Whilst GPRS and EDGE were applied to GSM networks, enhancements were also applied to the CDMA system that originated in the USA. Here an evolutionary path from 2G through 2.5G to 3G was created. The intermediate stage was development of cdmaOne was CDMA2000 1X. This scheme retained the 1.25 MHz bandwidth of IS95 / cdmaOne, but by adding further channels enabled data transfer rates of 307 kbps to be achieved, thereby doubling the capacity of IS95B.

Third Generation
Although technologies such as GPRS, EDGE and CDMA2000 1X were able to deliver significantly higher data rates than their predecessors, the final migration was to the full 3G service. There were three main technologies.

From Europe there was the UMTS (Universal Mobile Telecommunications System) using wideband CDMA (W-CDMA). This system used a 5 MHz channel spacing and provided data rates of up to 2 Mbps.

Then there were the CDMA2000 evolutions. The first to be launched was CDMA2000 1xEV-DO. Here the letters EV-DO stood for Evolution Data Only. The idea for this system was that many of the applications would only need a data connection, as in the case of a data card for use in a PC to provide a wireless Internet capability over a mobile phone system. For any applications needing both data and voice a standard 1X channel would be required in addition. Although using CDMA technology, the EV-DO system also used TDMA technology as well to provide the throughput whilst still maintaining backward compatibility with IS95 (cdmaOne) and CDMA2000 1X.

The next evolution of the CDMA2000 family was CDMA2000 1xEV-DV. This was an evolution of the 1X system, and totally distinct from 1xEV-DO and it provided a full data and voice capability. Again this system was able to provide backward compatibility with IS95 (cdmaOne) and CDMA2000 1X whilst still being able to provide a data capability of 3.1 Mbps in the forward direction.

These major two players in the 3G scene both used what is called frequency division duplex (FDD) where the forward and reverse links used different frequencies. Within UMTS there is a specification covering a time division duplex (TDD) system where the forward and reverse links used the same frequency but use different timeslots. However the TDD version is not being deployed for some time.

A third 3G system that originated in China uses TDD. Known as time division synchronous CDMA (TD-SCDMA) this system used a 1.6 MHz channel spacing and was thought to be likely to take a significant portion of the Chinese market along with those in neighbouring countries

Summary
It took just over 20 years to migrate from the first analogue systems to the 3G systems capable of high data rate transfers. Now people are working on the 4G standards and it remains to be seen what new services and capabilities this new technology will offer

Basic Cellular Concepts

- an overview or tutorial explaining the basic concept of a cellular telecommunications system and how cells enable efficient frequency re-use

Cellular telecommunications systems are widely used today and need to offer very efficient use of the available frequency spectrum. With billions of mobile phones in use around the globe today, it is necessary to re-use the available frequencies many times over without mutual interference of one cell phone to another.

Early schemes for radio telephones schemes used a single central transmitter to cover a wide area. These radio telephone systems suffered from the limited number of channels that were available. Often the waiting lists for connection were many times greater than the number of people that were actually connected.

The need for a spectrum efficient system
To illustrate the need for efficient spectrum usage for a radio telecommunications system, take the example where each user is allocated a channel. While more effective systems are now in use, the example will take the case of an analogue system. Each channel needs to have a bandwidth of around 25 kHz to enable sufficient audio quality to be carried as well as enabling there to be a guard band between adjacent signals to ensure there are no undue levels of interference. Using this concept it is only possible to accommodate 40 users in a frequency band 1 MHz wide. Even of 100 MHz were allocated to the system this would only enable 4000 users to have access to the system. Today cellular systems have millions of subscribers and therefore a far more efficient method of using the available spectrum is needed.

Cell system for frequency re-use
The method that is employed is to enable the frequencies to be re-used. Any transmitter will only have a certain coverage area. Beyond this the signal level will fall to a limited below which it cannot be used and will not cause significant interference to users associated with a different transmitter. This means that it is possible to re-use a channel once outside the range of the transmitter. The same is also true in the reverse direction for the receiver, where it will only be able to receive signals over a given range. In this way it is possible to arrange split up an area into several smaller regions, each covered by a different transmitter / receiver station.

These regions are conveniently known as cells, and give rise to the name of a cellular telecommunications system. Diagrammatically these cells are often shown as hexagonal shapes that conveniently fit together. In reality this is not the case. They have an irregular boundary because of the terrain over which they travel. Hills, buildings and other objects all cause the signal to be attenuated and diminish differently in each direction.

It is also very difficult to define the exact edge of a cell. The signal strength gradually reduces and towards the edge of the cell performance will fall. As the mobiles themselves will have different levels of sensitivity, this adds a further greying of the edge of the cell. Therefore it is never possible to have a sharp cut-off between cells. In some areas they may overlap, whereas in others there will be a "hole" in coverage.

Cell clusters
To overcome this problem, in a basic cellular system, adjacent cells are allocated different frequency bands so that they can overlap without causing interference. In this way cells can be grouped together in what is termed a cluster.

Often these clusters contain seven cells, but other configurations are also possible. Seven is a convenient number, but there are a number of conflicting requirements that need to be balanced when choosing the number of cells in a cluster:

• Limiting interference levels
• Number of channels that can be allocated to each cell site

It is necessary to limit the interference between cells having the same frequency. The topology of the cell configuration has a large impact on this. The larger the number of cells in the cluster, the greater the distance between cells sharing the same frequencies.

In the ideal world it might be good to choose a large number of cells to be in each cluster. Unfortunately there is only a limited number of channels available. This means that the larger the number of cells in a cluster, the smaller the number available to each cell, and this reduces the capacity.

This means that there is a balance that needs to be made between the number of cells in a cluster, and the interference levels, and the capacity that is required.

Cell size
Even though the number of cells in a cluster can help govern the number of users that can be accommodated, by making all the cells smaller it is possible to increase the overall capacity of the network. However a greater number of transmitter receiver or base stations are required if cells are made smaller and this increases the cost to the operator. Accordingly in areas where there are more users, small low power base stations are installed.

The different types of cells are given different names according to their size and function:

• Macro cells
• Micro cells
• Pico cells
• Selective cells
• Umbrella cells

Macro cells are large cells that are usually used for remote or sparsely populated areas. These may be 10 km or possibly more in diameter. Micro cells are those that are normally found in densely populated areas which may have a diameter of around 1 km. Picocells may also be used for covering very small areas such as particular areas of buildings, or possibly tunnels where coverage from a larger cell is not possible. Obviously for the small cells, the power levels used by the base stations are much lower and the antennas are not position to cover wide areas. In this way the coverage is minimised and the interference to adjacent cells is reduced.

Other types of cell may be used for some specialist applications. Sometimes cells termed selective cells may be used where full 360 degree coverage is not required. They may be used to fill in a hole in the coverage, or to address a problem such as the entrance to a tunnel etc. Another type of cells known as an umbrella cell is sometimes used in instances such as those where a heavily used road crosses an area where there are microcells. Under normal circumstances this would result in a large number of handovers as people driving along the road would quickly cross the microcells. An umbrella cell would take in the coverage of the microcells (but use different channels to those allocated to the microcells). However it would enable those people moving along the road to be handled by the umbrella cell and experience fewer handovers than if they had to pass from one microcell to the next.

Cellular Multiple Access Schemes

- an overview or tutorial explaining the multiple access schemes used by cell phone systems including FDMA, TDMA and CDMA

n any cellular telecommunications or mobile phone system, it is necessary to have a scheme that enables several multiple users to gain access to it and use it simultaneously. These methods are known as multiple access schemes.

There are three main multiple access schemes that are in use at the moment:

• Frequency Division Multiple Access - FDMA
• Time Division Multiple Access - TDMA
• Code Division Multiple Access - CDMA

FDMA
FDMA is the most straightforward of the multiple access schemes that have been used. As a subscriber comes onto the system, or swaps from one cell to the next, the network allocates a channel or frequency to each one. In this way the different subscribers are allocated a different slot and access to the network. As different frequencies are used, the system is naturally termed Frequency Division Multiple Access. This scheme was used by all analogue systems.

TDMA
The second system came about with the transition to digital schemes. Here digital data could be split up in time and sent as bursts when required. As speech was digitised it could be sent in short data bursts, any small delay caused by sending the data in bursts would be short and not noticed. In this way it became possible to organise the system so that a given number of slots were available on a give transmission. Each subscriber would then be allocated a different time slot in which they could transmit or receive data. As different time slots are used for each subscriber to gain access to the system, it is known as time division multiple access. Obviously this only allows a certain number of users access to the system. Beyond this another channel may be used, so systems that use TDMA may also have elements of FDMA operation as well.

CDMA
The final scheme uses one of the aspects associated with the use of direct sequence spread spectrum. It can be seen from the article in the cellular telecoms area of this site that when extracting the required data from a DSSS signal it was necessary to have the correct spreading or chip code, and all other data from sources using different orthogonal chip codes would be rejected. It is therefore possible to allocate different users different codes, and use this as the means by which different users are given access to the system.

The scheme has been likened to being in a room filled with people all speaking different languages. Even though the noise level is very high, it is still possible to understand someone speaking in your own language. With CDMA different spreading or chip codes are used. When generating a direct sequence spread spectrum, the data to be transmitted is multiplied with spreading or chip code. This widens the spectrum of the signal, but it can only be decided in the receiver if it is again multiplied with the same spreading code. All signals that use different spreading codes are not seen, and are discarded in the process. Thus in the presence of a variety of signals it is possible to receive only the required one.

In this way the base station allocates different codes to different users and when it receives the signal it will use one code to receive the signal from one mobile, and another spreading code to receive the signal from a second mobile. In this way the same frequency channel can be used to serve a number of different mobiles.

Situation today
Although the latest cellular telecommunications systems use CDMA as their basis, elements of TDMA and FDMA are also used. Both the major schemes, UMTS and CDMA2000 have a limit on the number of users who are able to use a single channel. In some instances two or more channels may be allocated to a particular cell. This means that the system still uses an element of FDMA.

Additionally UMTS incorporates some timeslots, and this means that the scheme uses elements of TDMA.

While CDMA is currently the dominant technology, both the other forms of access scheme are still in evidence, not just in legacy technologies, but utilised as part of the main access scheme in the latest 3G systems.

Duplex transmission schemes for cellular telecommunications systems

- an overview or tutorial explaining the forms of duplex transmission used by cellular telecommunications systems including FDD and TDD

Basic radio communications systems use a single channel and what is known as a press to talk system, where the user presses a button or "pressel" on the microphone to talk, and then releases the pressel to listen on the same frequency. This system is known as simplex as it uses a single channel. For a phone system a full duplex system is required where it is possible to speak in both directions at the same time. There are two main ways in which this can be achieved. The first is to transmit in one direction on one frequency, and simultaneously transmit in the other direction on another. To achieve this there must be sufficient frequency separation and filtering to ensure that the transmitter does not interfere with the receiver. A scheme that uses one frequency for transmitting traffic in one direction and another frequency for traffic in the other is known as Frequency Division Duplex (FDD).

The other system uses only a single frequency and can be employed where digital or data systems are used. This requires the analogue audio signal to be digitised. A single frequency is used for the radio frequency signal and short packets of data are sent first in one direction, and then the other. As these data bursts are relatively short the user does not notice the short delay introduced by the fact that the digitised speech signal is not sent immediately. This type of system is known as Time Division Duplex (TDD).

It is often necessary to distinguish between the link from the mobile to the base station, and the link from the base station to the mobile. The first, i.e. the link from the mobile to the base station is often called the uplink or the reverse link as the signal is being transmitted up to the base station. The second, i.e. the link from the base station to the mobile is known as the downlink or the forward link.

Mobile phone (Cell phone) electronics

- an overview or tutorial explaining the design of a cell phone and operation of the different circuit functions in the phone

The mobile phone or cell phone as it is often called is equally important to the network in the operation of the completecellular telecommunications network. Despite the huge numbers that are made, they still cost a significant amount to manufacture, discounts being offered to users as incentives to use a particular network. Their cost is a reflection of the complexity of the mobile phone electronics. They comprise several different areas of electronics, from radio frequency (RF) to signal processing, and general processing.

The design of a cell phone is particularly challenging. They need to offer high levels of performance, while being able to fit into a very small space, and in addition tot his the electronics circuitry needs to consume very little power so that the life between charges can be maintained.

Mobile phone contents
Mobile phones contain a large amount of circuitry, each of which is carefully designed to optimise its performance. The cell phone comprises analogue electronics as well as digital circuits ranging from processors to display and keypad electronics. A mobile phone typically consists of a single board, but within this there are a number of distinct functional areas, but designed to integrate to become a complete mobile phone:

• Radio frequency - receiver and transmitter
• Digital signal processing
• Analogue / digital conversion
• Control processor
• SIM or USIM card
• Power control and battery

Radio frequency elements
The radio frequency section of the mobile phone is one of the crucial areas of the cell phone design. This area of the mobile phone contains all the transmitter and receiver circuits. Normally direct conversion techniques are generally used in the design for the mobile phone receiver.

The signal output from the receiver is applied to what is termed an IQ demodulator. Here the data in the form of "In-phase" and "Quadrature" components is applied to the IQ demodulator and the raw data extracted for further processing by the phone.

On the transmit side one of the key elements of the circuit design is to keep the battery consumption to a minimum. For GSM this is not too much of a problem. The modulation used is Gaussian Minimum Shift Keying. This form of signal does not incorporate amplitude variations and accordingly it does not need linear amplifiers. This is a distinct advantage because non linear RF amplifiers are more efficient than linear RF amplifiers.

Unfortunately EDGE uses eight point phase shift keying (8PSK) and this requires a linear RF amplifier. As linear amplifiers consume considerably more current this is a distinct disadvantage. To overcome this problem the design for the mobile phone is organised so that phase information is added to the signal at an early stage of the transmitter chain, and the amplitude information is added at the final amplifier.

Analogue to Digital Conversion
Another crucial area of any mobile phone design is the circuitry that converts the signals between analogue and digital formats that are used in different areas. The radio frequency sections of the design use analogue techniques, whereas the processing is all digital.

The digital / analogue conversion circuitry enables the voice to be converted either from analogue or to digital a digital format for the send path, but also between digital and analogue for the receive path. It also provides functions such as providing analogue voltages to steer the VCO in the synthesizer as well as monitoring of the battery voltage, especially during charging. It also provides the conversion for the audio signals to and from the microphone and earpiece so that they can interface with the digital signal processing functions.

Another function that may sometimes be included in this area of the mobile phone design or within the DSP is that of the voice codecs. As the voice data needs to be compressed to enable it to be contained within the maximum allowable data rate, the signal needs to be digitally compressed. This is undertaken using what is termed a codec.

There are a number of codec schemes that can be used, all of which are generally supported by the base stations. The first one to be used in GSM was known as LPC-RPE (Linear Prediction Coding - Regular Pulse Excitation). However another scheme known as AMR (Adaptive Multi-Rate) is now widely used as it enables the data rate to be further reduced when conditions permit without impairing the speech quality too much. By reducing the speech data rate, further capacity is freed up on the network.

Digital Signal Processing
The DSP components of the mobile phone design undertake all the signal processing. Processes such as the radio frequency filtering and signal conditioning at the lower frequencies are undertaken by this circuitry. In addition to this, equalisation and correction for multipath effects is undertaken in this area of the design.

Although these processors are traditionally current hungry, the current processors enable the signal processing to be undertaken in a far more power effective manner than if analogue circuits are used.

Control processor
The control processor is at the heart of the design of the phone. It controls all the processes occurring in the phone from the MMI (Man machine interface) which monitors the keypad presses and arranging for the information to be displayed on the screen. It also looks after all the other elements of the MMI including all the menus that can be found on the phone.

Another function of the control processor is to manage the interface with the mobile network base station. The software required for this is known as the protocol stack and it enables the phone to register, make and receive calls, terminate them and also handle the handovers that are needed when the phone moves from one cell to the next. Additionally the softwareformats the data to be transmitted into the correct format with error correction codes included. Accordingly the load on this processor can be quite high, especially when there are interactions with the network.

The protocols used to interact with the network are becoming increasingly complicated with the progression from 2G to 3G. Along with the increasing number of handset applications the load on the processor is increasing. To combat this, the design for this area of the phone circuitry often uses ARM processors. This enables high levels of processing to be achieved for relatively low levels of current drain.

A further application handled by this area of the design of the mobile phone is the monitoring the state pf the battery and control of the charging. In view of the sophisticated monitoring and control required to ensure that the battery is properly charged and the user can be informed about the level of charge left, this is an important area of the design.

Battery
Battery design and technology has moved on considerably in the last few years. This has enabled mobile phones to operate for much longer. Initially nickel cadmium cells were used, but these migrated to nickel-metal-hydride cells and then to lithium ion cells. With phones becoming smaller and requiring to operate for longer from a single charge, the capacity of the battery is very important, and all the time the performance of these cells is being improved.

Cellular network overview

- a summary or tutorial about the basics of a cellular telecommunications network, detailing the main elements within it - BTS, BSC, HLR, VLR, etc.

The network forms the heart of any cellular telephone system. The cellular network fulfils many requirements. Not only does the network enable calls to be routed to and from the mobile phones as well as enabling calls to be maintained as the cellphone moves from one cell to another, but it also enables other essential operations such as access to the network, billing, security and much more. To fulfil all these requirements the cellular network comprises many elements, each having its own function to complete.

The most obvious part of the cellular network is the base station. The antennas and the associated equipment often located in a container below are seen dotted around the country, and especially at the side of highways and motorways. However there is more to the network behind this, as the system needs to have elements of central control and it also needs to link in with the PSTN landline system to enable calls to be made to and from the wire based phones, or between networks.

Different cellular standards often take slightly different approaches for the cellular network required. Despite the differences between the different cellular systems, the basic concepts are very similar. Additionally cellular systems such as GSM have a well defined structure, and this means that manufacturers products can be standardised.

Basic cellular network structure
An overall cellular network contains a number of different elements from the base transceiver station (BTS) itself with its antenna back through a base station controller (BSC), and a mobile switching centre (MSC) to the location registers (HLR and VLR) and the link to the public switched telephone network (PSTN).

Of the units within the cellular network, the BTS provides the direct communication with the mobile phones. There may be a small number of base stations then linked to a base station controller. This unit acts as a small centre to route calls to the required base station, and it also makes some decisions about which of the base station is best suited to a particular call. The links between the BTS and the BSC may use either land lines of even microwave links. Often the BTS antenna towers also support a small microwave dish antenna used for the link to the BSC. The BSC is often co-located with a BTS.

The BSC interfaces with the mobile switching centre. This makes more widespread choices about the routing of calls and interfaces to the land line based PSTN as well as the HLR and VLR.

Base transceiver station, BTS
The base transceiver station or system, BTS consists of a number of different elements. The first is the electronics section normally located in a container at the base of the antenna tower. This contains the electronics for communicating with themobile handsets and includes radio frequency amplifiers, radio transceivers, radio frequency combiners, control, communication links to the BSC, and power supplies with back up.

The second part of the BTS is the antenna and the feeder to connect the antenna to the base transceiver station itself. These antennas are visible on top of masts and tall buildings enabling them to cover the required area. Finally there is the interface between the base station and its controller further up the network. This consists of control logic and software as well as the cable link to the controller.

BTSs are set up in a variety of places. In towns and cities the characteristic antennas are often seen on the top of buildings, whereas in the country separate masts are used. It is important that the location, height, and orientation are all correct to ensure the required coverage is achieved. If the antenna is too low or in a poor location, there will be insufficient coverage and there will be a coverage "hole". Conversely if the antenna is too high and directed incorrectly, then the signal will be heard well beyond the boundaries of the cell. This may result in interference with another cell using the same frequencies.

The antennas systems used with base stations often have two sets of receive antennas. These provide what is often termed diversity reception, enabling the best signal to be chosen to minimise the effects of multipath propagation. The receiver antennas are connected to low loss cable that routes the signals down to a multicoupler in the base station container. Here a multicoupler splits the signals out to feed the various receivers required for all the RF channels. Similarly the transmitted signal from the combiner is routed up to the transmitting antenna using low loss cable to ensure the optimum transmitted signal.

Mobile switching centre (MSC)
The MSC is the control centre for the cellular system, coordinating the actions of the BSCs, providing overall control, and acting as the switch and connection into the public telephone network. As such it has a variety of communication links into it which will include fibre optic links as well as some microwave links and some copper wire cables. These enable it to communicate with the BSCs, routing calls to them and controlling them as required. It also contains the Home and Visitor Location Registers, the databases detailing the last known locations of the mobiles. It also contains the facilities for the Authentication Centre, allowing mobiles onto the network. In addition to this it will also contain the facilities to generate the billing information for the individual accounts.

In view of the importance of the MSC, it contains many backup and duplicate circuits to ensure that it does not fail. Obviously backup power systems are an essential element of this to guard against the possibility of a major power failure, because if the MSC became inoperative then the whole network would collapse.

Mobile phone network registration

- a summary or tutorial about the way in which a mobile phone achieves registration onto a cellular telecommunications system or network.

On any cellular telecommunications system the way in which registration and call set-up occur needs to be carefully managed. Not only does the cellular telecommunications network need to provide quick and efficient service for its rightful customers, but it also needs to be able to offer high levels of security for the user and the network.

There are many different cellular telecommunications systems in use around the globe. Older ones are being phased out, and newer cellular systems are being introduced. Accordingly there is no single way in which registration and call set up are managed. However there are some general principles that are used, and these are illustrated here.

Basic requirements
When the mobile phone is turned on it needs to be able to communicate with the cellular telecommunications network. However the phone does not have an allocated channel, time slot or chip code (dependent upon the type of access method used). It is therefore necessary for there to be some methods or allocated means within the cellular telecommunications network, whereby a newly switched on mobile can communicate with the network and set up the standard communication.

Even if a call is not to be made instantly, the network needs to be able to communicate with the mobile to know where it is. In this way the network can route any calls through the relevant base station as the network would be soon overloaded if the notification of an incoming call had to be sent via several base stations.

Registration
There are a variety of tasks that need to be undertaken when a phone is turned on. This can eb seen by the fact that it takes a few seconds from switching the phone on before it is ready for use. Part of this process is the software start-up for the phone, but the majority comes from the registration process with the cellular network. There are several aspects to the regristration. The first is to make contact with the base station, and next the mobile has to register to allow it to have access to and use the network.

In order to make contact with the base station the mobile uses a paging or control channel. The name of this channel, and the exact way in which it works will vary from one cellular standard to the next, but it is a channel that is used that the mobile can access to indicate its presence. The message sent is often called the "attach" message. Once this has been achieved it is necessary for the mobile to register with the cellular network, and to be accepted onto it.

Network elements
It is necessary to have a register or database of users allowed to register with a given network. With mobiles often being able to access the all the channels available in a country, methods of ensuring the mobile registers with the correct network, and to ensure the account is valid are required. Additionally it is required for billing purposes. To achieve this, an entity on the network often known as the Authentication Centre (AuC) is used. The network and the mobile communicate and numbers giving the identity of the subscriber. Here the user information is checked to provide authentication and encryption parameters that verify the user's identity and ensure the confidentiality of each call protecting users and network operators from fraud.

Once accepted onto the network two further registers are normally required. These are the Home Location Register (HLR) and the Visitors Location Register (VLR). These two registers are required to keep track of the mobile so that the network knows where it is at any time so that calls can be routed to the correct base station or general area of the network. These registers are used to store the last known location of the mobile. Thus at registration the register is updated and then periodically the mobile updates its position. Even when the mobile is in what is termed its idle mode it will periodically communicate with the network to update its position and status.

When the mobile is switched off it sends a detach message. This informs the network that it is switching off, and enables the network to update the last known position for the mobile.

Home and abroad
The two registers are required, one for mobiles for which the network is the home network, i.e. the one with whom the contract exists, and the other for visitors. If there was only one register then every time the mobile sent any message to the foreign network, this would need to be relayed back to the home network and this would require international signalling. The approach which is adopted is to send a message back to the HLR when the mobile first enters the new country saying that the mobile is in a different network and that any calls for that mobile should be forwarded to the foreign visited network.

Summary
By undergoing a registration procedure when the mobile is turned on, the cellular network is able to communicate correctly with it, provide access for outgoing calls, and also route any incoming calls to it in the most efficient manner. Registration also only allows those mobiles that have permission to access the network to communicate with it.

Handover and handoff

- a summary or tutorial about the basics handover or handoff, where a cellular telecommunications call is transferred from one cell to another. Hard, soft and softer hand over are all covered.

The concept of a cellular phone system is that it has a large number base stations covering a small area (cells), and as a result frequencies are able to be re-used. Cell phone systems also provide mobility. As a result it is a very basic requirement of the system that as the mobile handset moves out of one cell to the next, it must be possible to hand the call over from the base station of the first cell, to that of the next with no discernable disruption to the call. There are two terms for this process: handover is used within Europe, whereas handoff is the term used in North America.

The handover or handoff process is of major importance within any cellular telecommunications network. It is necessary to ensure it can be performed reliably and without disruption to any calls. Failure for it to perform reliably can result in dropped calls, and this is one of the key factors that can lead to customer dissatisfaction, which in turn may lead to them changing to another cellular network provider. Accordingly handover or handoff is one of the key performance indicators monitored so that a robust handover / handoff regime is maintained on the cellular network.

Handover basics
Although the concept of handover or handoff is relatively straightforward, it is not an easy process to implement in reality. The cellular network needs to decide when handover or handoff is necessary, and to which cell. Also when the handover occurs it is necessary to re-route the call to the relevant base station along with changing the communication between the mobile and the base station to a new channel. All of this needs to be undertaken without any noticeable interruption to the call. The process is quite complicated, and in early systems calls were often lost if the process did not work correctly.

Different cellular standards handle hand over / handoff in slightly different ways. Therefore for the sake of an explanation the example of the way that GSM handles handover is given.

There are a number of parameters that need to be known to determine whether a handover is required. The signal strength of the base station with which communication is being made, along with the signal strengths of the surrounding stations. Additionally the availability of channels also needs to be known. The mobile is obviously best suited to monitor the strength of the base stations, but only the cellular network knows the status of channel availability and the network makes the decision about when the handover is to take place and to which channel of which cell.

Accordingly the mobile continually monitors the signal strengths of the base stations it can hear, including the one it is currently using, and it feeds this information back. When the strength of the signal from the base station that the mobile is using starts to fall to a level where action needs to be taken the cellular network looks at the reported strength of the signals from other cells reported by the mobile. It then checks for channel availability, and if one is available it informs this new cell to reserve a channel for the incoming mobile. When ready, the current base station passes the information for the new channel to the mobile, which then makes the change. Once there the mobile sends a message on the new channel to inform the network it has arrived. If this message is successfully sent and received then the network shuts down communication with the mobile on the old channel, freeing it up for other users, and all communication takes place on the new channel.

Under some circumstances such as when one base transceiver station is nearing its capacity, the network may decide to hand some mobiles over to another base transceiver station they are receiving that has more capacity, and in this way reduce the load on the base transceiver station that is nearly running to capacity. In this way access can be opened to the maximum number of users. In fact channel usage and capacity are very important factors in the design of a cellular network.

Types of handover / handoff
With the advent of CDMA systems where the same channels can be used by several mobiles, and where it is possible to adjacent cells or cell sectors to use the same frequency channel there are a number of different types of handover that can be performed:

• Hard handover
• Soft handover
• Softer handover

Although all of these forms of handover or handoff enable the cellular phone to be connected to a different cell or different cell sector, they are performed in slightly different ways and are available under different conditions.

Hard handover
The definition of a hard handover or handoff is one where an existing connection must be broken before the new one is established. One example of hard handover is when frequencies are changed. As the mobile will normally only be able to transmit on one frequency at a time, the connection must be broken before it can move to the new channel where the connection is re-established. This is often termed and inter-frequency hard handover. While this is the most common form of hard handoff, it is not the only one. It is also possible to have intra-frequency hard handovers where the frequency channel remains the same.

Although there is generally a short break in transmission, this is normally short enough not to be noticed by the user.

Soft hand over
The new 3G technologies use CDMA where it is possible to have neighbouring cells on the same frequency and this opens the possibility of having a form of handover or handoff where it is not necessary to break the connection. This is called soft handover or soft handoff, and it is defined as a handover where a new connection is established before the old one is released. In UMTS most of the handovers that are performed are intra-frequency soft handovers.

Softer handover
The third type of hand over is termed a softer handover, or handoff. In this instance a new signal is either added to or deleted from the active set of signals. It may also occur when a signal is replaced by a stronger signal from a different sector under the same base station. This type of handover or handoff is available within UMTS as well as CDMA2000.

Summary
Handover and handoff are performed by all cellular telecommunications networks, and they are a core element of the whole concept of cellular telecommunications. There are a number of requirements for the process. The first is that it occurs reliably and if it does not, users soon become dissatisfied and choose another network provider in a process known as "churn". However it needs to be accomplished in the most efficient manner. Although softer handoff is the most reliable, it also uses more network capacity. The reason for this is that it is communicating with more than one sector or base station at any given instance. Soft handover is also less efficient than hard handover, but again more reliable as the connection is never lost.

It is therefore necessary for the cellular telecommunications network provider to arrange the network to operate in the most efficient manner, while still providing the most reliable service.

CDMA basics tutorial

- an overview or tutorial describing the way in which CDMA code division multiple access is used for cellular phone systems

CDMA or Code Division Multiple Access is now in widespread use for mobile or cell phone (cellular telecommunications) systems around the world. It was first used for the IS-95 mobile phone system also known by the trade name cdmaOne, and in its later 3G developments as CDMA2000. CDMA is also being used in the other major 3G cell phone system, Wideband-CDMA system originally called UMTS.

CDMA technology is based on a form of transmission known as Direct Sequence Spread Spectrum (DSSS). This form of transmission originally used for military and police communications because the transmissions were difficult to detect in many instances, and even if they were received they were very difficult to decipher without the correct codes. However the possibilities of using this technology to provide a multiple access scheme for mobile telecommunications and have now been exploited in a major way.

Previous cellular telecommunications technologies used either frequency division multiple access (FDMA) where different users were allocated different frequencies, or time division multiple access (TDMA) where they were allotted different time slots on a channel. CDMA is different. Using the CDMA system, different users are allocated different codes to provide access to the system. It can be likened to many different people standing in a room talking to others in many different languages. Although the ambient noise level is very high, it is nevertheless still possible to pick out someone speaking in the same language as yourself.

DSSS basics
The key element of code division multiple access CDMA is its use of DSSS. In essence the required data signal is multiplied with what is known as a spreading or chip code data stream. This has a higher data rate than the data itself and it enables the overall signal to be spread over a much wider bandwidth. Signals with high data rates occupy wider signal bandwidths than those with low data rates.

To decode the signal and receive the original data, the CDMA signal is multiplied with the spreading code to regenerate the original data. When this is done, then only the data with that was generated with the same spreading code is regenerated, all the other data that is generated from different spreading code streams is ignored

This is a powerful principle and using code division multiple access technique, it is possible to transmit several sets of data independently on the same carrier and then reconstitute them at the receiver without mutual interference. In this way a base station can communicate with several mobiles on a single channel. Similarly several mobiles can communicate with a single base station, provided that in each case an independent spreading code is used.

The CDMA spreading codes can either be a random number (or pseudo random), or more usually orthogonal codes are used. Two codes are said to be orthogonal if when they are multiplied together and then the result is added over a period of time they sum to zero. For example a codes 1 -1 -1 1 and 1 -1 1 -1 when multiplied together give 1 1 -1 -1 which gives the sum zero. Although pseudo random number codes can be used there is possibility of data errors being introduced into the system.

Advantages
There are several advantages to using code division multiple access CDMA. The main reason for its acceptance is that it enables more users to use a given amount of spectrum. Its use also enables adjacent base stations to operate on the same channel, allowing more efficient use of the spectrum and it provides for an easier handover.

In view of these advantages CDMA has been adopted for all the 3G technologies and will be around for many years to come.

OFDM Tutorial

- OFDM - Orthogonal Frequency Division Multiplex, the modulation concept being used for many radio and wireless applications from DAB, DVB, Wi-Fi and Mobile Video.

Orthogonal Frequency Division Multiplex or OFDM is a modulation format that is finding increasing levels of use in today's communications scene. OFDM has been adopted in the Wi-Fi arena where the 802.11a standard uses it to provide data rates up to 54 Mbps in the 5 GHz ISM (Industrial, Scientific and Medical) band. In addition to this the recently ratified 802.11g standard has it in the 2.4 GHz ISM band. If this was not enough it is also being used for digital terrestrial television transmissions as well as DAB digital radio. A new form of broadcasting called Digital Radio Mondiale for the long medium and short wave bands is being launched and this has also adopted COFDM. Then for the future it is being proposed as the modulation technique for fourth generation cell phone systems that are in their early stages of development and OFDM is also being used for many of the proposed mobile phone video systems.

OFDM concept
An OFDM signal consists of a number of closely spaced modulated carriers. When modulation of any form - voice, data, etc. is applied to a carrier, then sidebands spread out either side. It is necessary for a receiver to be able to receive the whole signal to be able to successfully demodulate the data. As a result when signals are transmitted close to one another they must be spaced so that the receiver can separate them using a filter and there must be a guard band between them. This is not the case with OFDM. Although the sidebands from each carrier overlap, they can still be received without the interference that might be expected because they are orthogonal to each another. This is achieved by having the carrier spacing equal to the reciprocal of the symbol period.

Traditional view of receiving signals carrying modulation

To see how OFDM works, it is necessary to look at the receiver. This acts as a bank of demodulators, translating each carrier down to DC. The resulting signal is integrated over the symbol period to regenerate the data from that carrier. The same demodulator also demodulates the other carriers. As the carrier spacing equal to the reciprocal of the symbol period means that they will have a whole number of cycles in the symbol period and their contribution will sum to zero - in other words there is no interference contribution.

OFDM Spectrum

One requirement of the OFDM transmitting and receiving systems is that they must be linear. Any non-linearity will cause interference between the carriers as a result of inter-modulation distortion. This will introduce unwanted signals that would cause interference and impair the orthogonality of the transmission.

In terms of the equipment to be used the high peak to average ratio of multi-carrier systems such as OFDM requires the RF final amplifier on the output of the transmitter to be able to handle the peaks whilst the average power is much lower and this leads to inefficiency. In some systems the peaks are limited. Although this introduces distortion that results in a higher level of data errors, the system can rely on the error correction to remove them.

Data
The data to be transmitted on an OFDM signal is spread across the carriers of the signal, each carrier taking part of the payload. This reduces the data rate taken by each carrier. The lower data rate has the advantage that interference from reflections is much less critical. This is achieved by adding a guard band time or guard interval into the system. This ensures that the data is only sampled when the signal is stable and no new delayed signals arrive that would alter the timing and phase of the signal.

Guard Interval

The distribution of the data across a large number of carriers in the OFDM signal has some further advantages. Nulls caused by multi-path effects or interference on a given frequency only affect a small number of the carriers, the remaining ones being received correctly. By using error-coding techniques, which does mean adding further data to the transmitted signal, it enables many or all of the corrupted data to be reconstructed within the receiver. This can be done because the error correction code is transmitted in a different part of the signal. It is this error coding which is referred to in the "Coded" word in the title of COFDM which is often seen.

Other variants
Flash OFDM - This is a variant that was developed by Flarion and it is a fast hopped form of OFDM. It uses multiple tones and fast hopping to spread signals over a given spectrum band.

VOFDM - Vector OFDM. This form of OFDM uses the concept of MIMO technology. It is being developed by CISCO Systems. MIMO stands for Multiple Input Multiple output and it uses multiple antennas to transmit and receive the signals so that multi-path effects can be utilised to enhance the signal reception and improve the transmission speeds that can be supported.

WOFDM - Wideband OFDM. The concept of this form of OFDM is that it uses a degree of spacing between the channels that is large enough that any frequency errors between transmitter and receiver do not affect the performance. It is particularly applicable to Wi-Fi systems.

Summary
OFDM and COFDM have gained a significant presence in the wireless market place. The combination of high data capacity, high spectral efficiency, and its resilience to interference as a result of multi-path effects means that it is ideal for the highdata applications that are becoming a common factor in today's communications scene.

MIMO Wireless Technology

- key points, overview or tutorial about the basics of MIMO - Multiple Input Multiple Output, a wireless or radio technology normally used with OFDM for many wireless and cellular telecommunications applications.

Multiple-input multiple-output, or MIMO, is a technology that is being mentioned and used in many new technologies these days. Wi-Fi, LTE (3G long term evolution) and many other radio and wireless technologies are using the new MIMO wireless technology to provide increased link capacity and spectral efficiency combined with improved link reliability using what were previously seen as interference paths.

Even now many there are many MIMO wireless routers on the market, and as the technology is becoming more widespread, more MIMO routers and other items of wireless MIMO equipment will be seen.

MIMO overview
MIMO is effectively an antenna technology as it uses multiple antennas at the transmitter and receiver to enable a variety of signal paths to carry the data, choosing separate paths for each antenna to enable multiple signal paths to be used.

It is found between a transmitter and a receiver, the signal can take many paths. Additionally by moving the antennas even a small distance the paths used will change. The variety of paths available occurs as a result of the number of objects that appear to the side or even in the direct path between the transmitter and receiver. Previously these multiple paths only served to introduce interference. By using MIMO, these additional paths can be used to increase the capacity of a link.

Shannon's Law and MIMO
As with many areas of science, there a theoretical boundaries, beyond which it is not possible to proceed. This is true for the amount of data that can be passed along a specific channel in the presence of noise. The law that governs this is called Shannon's Law, named after the man who formulated it. This is particularly important because MIMO wireless technology provides a method not of breaking the law, but increasing data rates beyond those possible on a single channel without its use.

Shannon's law defines the maximum rate at which error free data can be transmitted over a given bandwidth in the presence of noise. It is usually expressed in the form:

C = W log₂(1 + S/N )

Where C is the channel capacity in bits per second, W is the bandwidth in Hertz, and S/N is the SNR (Signal to Noise Ratio).

From this it can be seen that there is an ultimate limit on the capacity of a channel with a given bandwidth. However before this point is reached, the capacity is also limited by the signal to noise ratio of the received signal.

In view of these limits many decisions need to be made about the way in which a transmission is made. The modulation scheme can play a major part in this. The channel capacity can be increased by using higher order modulation schemes, but these require a better signal to noise ratio than the lower order modulation schemes. Thus a balance exists between the data rate and the allowable error rate, signal to noise ratio and power that can be transmitted.

While some improvements can be made in terms of optimising the modulation scheme and improving the signal to noise ratio, these improvements are not always easy or cheap and they are invariably a compromise, balancing the various factors involved. It is therefore necessary to look at other ways of improving the data throughput for individual channels. MIMO is one way in which wireless communications can be improved and as a result it is receiving a considerable degree of interest.

Basic concept of MIMO wireless schemes
One of the core ideas behind MIMO wireless systems space-time signal processing in which time (the natural dimension of digital communication data) is complemented with the spatial dimension inherent in the use of multiple spatially distributed antennas, i.e. the use of multiple antennas located at different points. Accordingly MIMO wireless systems can be viewed as a logical extension to the smart antennas that have been used for many years to improve wireless.

To take advantage of this in a MIMO wireless system, the transmitted data must be encoded using what is termed a space-time code to allow the receiver to extract the fundamental transmitted data from the received signals.

The codes used for MIMO wireless systems vary according to a number of parameters. Some codes, known as "space-time diversity codes" are optimised for what is termed the diversity order. These optimise the signal to noise ratio, and the codes used define the performance gain that can be achieved and obviously the more gain that is achieved, the more processing power is required.

Other MIMO codes are used for spatial multiplexing and improve the channel capacity. Although both schemes are of considerable interest, it is the spatial multiplexing that is of considerable interest in many applications where bandwidth is limited.

MIMO spatial multiplexing
To take advantage of the additional throughput capability, MIMO utilises several sets of antennas. In many MIMO systems, just two are used, but there is no reason why further antennas cannot be employed and this increases the throughput. In any case for MIMO spatial multiplexing the number of receive antennas must be equal to or greater than the number of transmit antennas.

To take advantage of the additional throughput offered, MIMO wireless systems utilise a matrix mathematical approach. Data streams t1, t2, … tn can be transmitted from antennas 1, 2, …n. Then there are a variety of paths that can be used with each path having different channel properties. To enable the receiver to be able to differentiate between the different data streams it is necessary to use. These can be represented by the properties h12, travelling from transmit antenna one to receive antenna 2 and so forth. In this way for a three transmit, three receive antenna system a matrix can be set up:

r1 = h11 t1 + h21 t2 + h31 t3

r2 = h12 t1 + h22 t2 + h32 t3

r3 = h13 t1 + h23 t2 + h33 t3

Where r1 = signal received at antenna 1, r2 is the signal received at antenna 2 and so forth.

In matrix format this can be represented as:

[R] = [H] x [T]

To recover the transmitted data-stream at the receiver it is necessary to perform a considerable amount of signal processing. First the MIMO system decoder must estimate the individual channel transfer characteristic hij to determine the channel transfer matrix. Once all of this has been estimated, then the matrix [H] has been produced and the transmitted data streams can be reconstructed by multiplying the received vector with the inverse of the transfer matrix.

[T] = [H]^-1 x [R]

This process can be likened to the solving of a set of N linear simultaneous equations to reveal the values of N variables.

In reality the situation is a little more difficult than this as propagation is never quite this straightforward, and in addition to this each variable consists of an ongoing data stream, this nevertheless demonstrates the basic principle behind MIMO wireless systems.

MIMO OFDM
The spatial multiplexing techniques used in MIMO wireless systems makes any receivers that are used very complicated. As a result, MIMO wireless systems combine the use of MIMO with OFDM (Orthogonal Frequency Division Multiple). The reason is that the problems created by multi-path channel are handled efficiently using OFDM. The IEEE 802.16e standard incorporates MIMO-OFDM and the IEEE 802.11n standard also uses MIMO-OFDM. In addition to this the new 3G LTE (Long Term Evolution) format for cellular telecommunications also uses MIMO-OFDM.

Summary
As a result of the use multiple antennas, MIMO wireless technology is able to considerably increase the capacity of a given channel while still obeying Shannon's law. By increasing the number of receive and transmit antennas it is possible to linearly increase the throughput of the channel with every pair of antennas added to the system. This makes MIMO wireless technology one of the most important wireless techniques to be employed in recent years. As spectral bandwidth is becoming an ever more valuable commodity, techniques are needed to use the available bandwidth more effectively. MIMO wireless technology is one of these techniques.

Cellular Phone Conformance Testing

An overview or tutorial about the basics of cellular phone conformance test and the way that GSM and UMTS testing is handled by organisations such as GCF, PTCRB, TIA, etc.

Today, vast numbers of mobile phones are in use around the globe. 2006 saw in well excess of 2 billion subscribers connected and over 1 billion phones manufactured. When users buy phones they expect that the system will work. They are not interested in the reasons why there may be problems, often blaming the phone itself. Poor service, for whatever reason will result in users choosing a different network, and adding to the rate of churn.

Ensuring that cell phones operate correctly when deployed is no easy task. Testing of the design is required at all stages of the development. It is necessary to check the hardware, and software. Once the cell phone is assembled, full testing of the complete "system" is required, testing it against its requirements and specification. Then prior to deployment it must undergo formal testing and be "approved" before it can be used.

Dependent upon the type of cellular network, i.e. GSM / UMTS, or CDMA (cdmaOne / cdma2000) this formal "approval" may take one of two forms:

• Conformance test
• Interoperability test

Once the formal approval has been gained, the cell phone can then be manufactured in quantity, sold and deployed on the cellular network.

In many respects cell phone conformance test and interoperability test perform the same function but they have some significant differences between them. A conformance test tests for conformance to a particular specification, whereas an interoperability test checks that the phone will work on a given network. Both conformance test and interoperability tests have the advantages and disadvantages. However a conformance test is required for GSM, UMTS cell phones and generally interoperability tests are required for phones for many of the CDMA networks.

Conformance and interoperability tests
To ensure that a mobile phone meets its required standards it has to undergo a variety of types of test. These are often categorised into different areas. In order to undertake these tests different test house may be required.

1. Basic safety testing This is a form of test that every piece of equipment, whether mobile phone or otherwise has to undergo to ensure that it is intrinsically safe to use and no injury will be inflicted for example from sharp edges, etc..
2. SAR, Specific Absorption Rate This test involvesmeasuring the amount of radio frequency power that a human head will absorb when the cell phone is transmitting. The test uses an anatomically correct model of the human head. Inside the model temperature sensors are set up to measure the temperature rises to ensure that the heating effects caused by the cell phone fall below acceptable limits.
3. Protocol testing One of the major areas of cellular conformance testing is the protocol testing of the cell phone. With the complicated protocols used in mobile phones this is a critical area. If the phone protocol software operates incorrectly then it could result not only in problems experienced by the phone, but also on the network. In view of the complexity of the protocols that are used this testing can be very involved. Specialised network simulators are used. These testers emulate a variety of network entities, i.e. base stations or Node B's (in the case of UMTS), RNCs (Radio Network Controller and the like. In this way a host of scenarios from registration to handover, and in fact any situation that can be encountered can be simulated.
4. RF testing Conformance testing also includes testing of the RF signal. Many measurements of the transmitter and receiver performance are undertaken in a variety of areas such as the out of band emissions. Measurements of the Radio Resource Management (RRM) are undertaken to ensure that the control capability of the phone is operating correctly. There are for instance very tight limits on the control of the transmitter output power to ensure that the cell phone radiates only as much as is needed under any given conditions and noise in the phone bands is reduced to the minimum level. To achieve this testing a protocol tester is often used to control the phone and set up the relevant scenarios. In addition to this an RF measurement and generation equipment is required. This is often in the form of additional signal generators, power meters, analysers, noise generators, etc. To check operation of the phone with multi-path and fading, special fading simulators are required.
5. SIM card testing Another very important area of cellular conformance or interoperability testing is the operation of the SIM card, or in the case of UMTS the USIM. As SIMs are interchangeable between phones it is necessary to rigorously check the interface. It is also vital to check the security aspects of the operation of the SIM, as lapses in security could compromise elements of the network security. To undertake this testing a SIM simulator (or USIM simulator) is required. This simulator emulates the operation of the SIM, and tests on the phone can then be run using a protocol tester to set up the variety of scenarios that are needed.
6. Audio tests Finally audio checks of the cell phone are undertaken. These check the correct operation of the audio aspects of the cellular phone, both in terms of the microphone and the earphone. Checks of audio levels, quality and much more are measured using a variety of audio equipment to ensure they conform to the requirements laid down.

Test cases
It is obviously necessary to ensure that each stage in the testing is repeatable regardless of the test equipment is used and the organisation performing the testing.

To achieve uniformity a large number of what are termed "test cases" are defined. These may be expressed in a number of ways, but typically they may be in prose, or they may be in a form of computer notation called TTCN (Tree and Tabular Combined Notation). In the latter format they can often be compiled directly into code that can be run on a given test system.

For any given standard, i.e. GSM UMTS, cdma2000, etc there are many hundreds of test cases that need to be run. They are prepared under the auspices of the governing body, e.g. 3GPP for GSM and UMTS. Manufacturers of conformance test equipment are then able to take these test cases and convert them to run on their test equipment. Often the test cases must be ratified on a given test system before they can be used towards approval of a phone.

Summary
The process of conformance testing and interoperability testing can be time consuming and expensive. However it is essential because the cost of releasing a phone that does not operate correctly is very much higher. As a result network operators and phone manufacturers alike see the importance of conformance testing and interoperability testing.

GSM / UMTS conformance testing

An overview or tutorial about the basics of GSM and UMTS cellular phone conformance testing and the way that GSM and UMTS testing is handled by organisations such as GCF, PTCRB, TIA, etc.

he process for conformance testing for GSM and UMTS is well defined and operates well. The conformance test approval process is run by the industry itself and is effectively self regulating. However the overall GSM / UMTS conformance testing process works well, and the fact that all phone designs have to pass the conformance test process means that the number of problems discovered with phones is very low. Additionally as all phones conform to the same standards, it is possible to roam from one network in one country to another network in another country with no problems, provided that there are agreements betweent he oerpators. This has been one of the major reasons fort he global success of GSM, and much of this can be attributed to the conformance test process.

For GSM these test cases were written in prose, and they described the test itself, the set-up conditions as well as the applied stimuli and of course the pass and fail criteria. It was then possible for each test equipment manufacturer to implement these tests on their equipment. However to ensure that the tests are a faithful implementation of the original intent, a validation and certification process has been set up. The conformance test cases were originally defined by ETSI (European Telecommunications Standards Institute) for GSM, but they are now controlled by 3GPP (Third Generation Partnership Project).

Once the manufacturer is satisfied that the test operates satisfactorily he provides it to an independent validation organisation that will test it and for conformance with the original test case. Assuming that the test case satisfactorily passes this process then it is presented to an industry body for certification. Once it has achieved this status then it can then be used for formal handset testing and certification.

The organisation that has overall control of the test cases for GSM and UMTS is 3GPP. However changes are handled by the GERAN (GSM Enhanced Radio Access Network) working Group for GSM and a group known as the T1 group for UMTS. The validation and approval of the implemented test cases is then handled by the GCF (Global Certification Forum).

The North American version of GSM running in the 1900 MHz band is often referred to as PCS (Personal Communications System) and there is another allocation at 850 MHz. A group known as PVG (PCS Validation Group) handles the approvals and their results are ratified by PTCRB (PCS Type Certification Review Board). Phones are then tested against the test cases which if successful are certified by CTIA. To achieve CTIA certification, it is necessary for phones to be tested in CTIA approved laboratories.

As might be expected, experience gained on GSM has been reflected into improvements for the new third generation UMTS system. One of the main changes is that the protocol test cases, are initially written in prose and then converted into TTCN (Tree and Tabular Combined Notation) to be made available to the industry. This language enables the test cases to be compiled into a format that can be run directly on the target test equipment. This approach saves time for the industry as a whole and reduces costs because generating the test cases is far easier to achieve. The main advantage is that it gives far more consistency across the industry as tests are no longer open to the same level of interpretation that they were before. This also saves time in the validation process.

The TTCN test cases have been prepared by a team of industry experts based at ETSI working on behalf of 3GPP. They prepared the basic TTCN code that was reviewed within the 3GPP community using email reflectors. By reviewing thesoftware in this way at the beginning of the process, the individual test cases do not need to be reviewed each time they are submitted by a test equipment manufacturer for validation.

To test a new handset, the manufacturer generally approaches a qualified test house who will possess a variety of different test systems. They will then run the required certified test cases and be able to present a case for a handset being suitable for use on the available networks.

CDMA interoperability testing

- an overview or tutorial about the basics of cdmaOne / cdma2000 cell phone interoperability testing.

The approach for approvals testing for cdmaOne / CDMA2000 cell phones is that of an interoperability test. Here the emphasis of testing is for interoperability with the network rather than conformance with a specification - the approach that is adopted by the GSM / UMTS fraternity.

For CDMA2000 the standards are written by 3GPP2. This is the equivalent of 3GPP, but for the CDMA standard IS-2000. These standards are then published by the relevant standards body for a particular area. In North America this is TIA (Telecommunications Industry Association), for Japan it is ARIB (Association of Radio Industries and Businesses) etc..

In North America, testing is carried out under the auspices of either CDG (CDMA Development Group) or CTIA.

Documents written by CDG reference the 3GPP2 and TIA standards. Testing is then conducted in three stages, namely Stages 1, 2, and 3. Stage 1 testing verifies the RF performance of a system, checking parameters such as receiver sensitivity, performance under fading conditions and the like. Stage 2 is what is known as the Cabled Interoperability Testing. Here the protocols of the mobile handset are checked against a base station to ensure the mobile performs in the correct manner under a variety of conditions such as call set-up, handoff, and the like. Finally there is the Stage 3 testing. This is undertaken by an operator and consists of running the mobile on a live network to ensure that it works over the air. This may include what is termed a drive test where the mobile is driven around a live network and its performance checked under real conditions.

The testing carried out under CTIA auspices is undertaken by approved laboratories. They use test equipment to simulate the network, and tests similar to those in CDG Stages 1 and 2 are employed. Once mobile phones successfully pass the CTIA tests they are given an approval certificate.

Different test requirements are placed upon handset manufacturers by the various network operators, and therefore phones may undergo a variety of tests under either or both the CDG and CTIA auspices.

GSM Cell Phone System

- a short history of the development of the GSM cellular phone system
Today the GSM cell or mobile phone system is the most popular in the world. GSM handsets are widely available at good prices and the networks are robust and reliable. The GSM system is also feature-rich with applications such as SMS textmessaging, international roaming, SIM cards and the like. It is also being enhanced with technologies including GPRS and EDGE. To achieve this level of success has taken many years and is the result of both technical development and international cooperation.
The first cell phone systems that were developed were analogue systems. Typically they used frequency-modulated carriers for the voice channels and data was carried on a separate shared control channel. When compared to the systems employed today these systems were comparatively straightforward and as a result a vast number of systems appeared. Two of the major systems that were in existence were the AMPS (Advanced Mobile Phone System) that was used in the USA and many other countries and TACS (Total Access Communications System) that was used in the UK as well as many other countries around the world.
Another system that was employed, and was in fact the first system to be commercially deployed was the Nordic MobileTelephone system (NMT). This was developed by a consortium of companies in Scandinavia and proved that international cooperation was possible.
The success of these systems proved to be their downfall. The use of all the systems installed around the globe increased dramatically and the effects of the limited frequency allocations were soon noticed. To overcome these a number of actions were taken. A system known as E-TACS or Extended-TACS was introduced giving the TACS system further channels. In the USA another system known as Narrowband AMPS (NAMPS) was developed.
New approaches
Neither of these approaches proved to be the long-term solution as more efficient systems were required. With the experience gained from the NMT system, showing that it was possible to develop a system across national boundaries, and with the political situation in Europe lending itself to international cooperation it was decided to develop a new Pan-European System. Furthermore it was realized that economies of scale would bring significant benefits. This was the beginnings of the GSM system.
To achieve the basic definition of a new system a meeting was held in 1982 under the auspices of the Conference of European Posts and Telegraphs (CEPT). They formed a study group called the Groupe Special Mobile ( GSM ) to study and develop a pan-European public land mobile system. Several basic criteria that the new system would have to meet were set down for the new GSM system to meet. These included: good subjective speech quality, low terminal and service cost, support for international roaming, ability to support handheld terminals, support for range of new services and facilities, spectral efficiency, and finally ISDN compatibility.
With the levels of under-capacity being projected for the analogue systems, this gave a real sense of urgency to the GSM development. Although decisions about the system were not taken at an early stage, all had been working toward a digital system. This decision was finally made in February 1987. This gave a variety of advantages. Greater levels of spectral efficiency could be gained, and in addition to this the use of digital circuitry would allow for higher levels of integration in the circuitry. This in turn would result in cheaper handsets with more features. Nevertheless significant hurdles still needed to be overcome. For example, many of the methods for encoding the speech within a sufficiently narrow bandwidth needed to be developed, and this posed a significant risk to the project. Nevertheless the GSM system had been started.
Launch dates
Work continued and a launch date for the new GSM system of 1991 was set for an initial launch of a service with limited coverage and capability to be followed by a complete roll out of the service in major European cities by 1993 and linking of the areas by 1995.
Meanwhile technical development was taking place. Initial trials had shown that time division multiple access techniques offered the best performance with the technology that would be available. This approach had the support of the major manufacturing companies which would ensure that with them on board sufficient equipment both in terms of handsets, base stations and the network infrastructure for GSM would be available.
Further impetus was given to the GSM project when in 1989 the responsibility was passed to the newly formed EuropeanTelecommunications Standards Institute (ETSI). Under the auspices of ETSI the specification took place. It provided functional and interface descriptions for each of the functional entities defined in the system. The aim was to provide sufficient guidance for manufacturers that equipment from different manufacturers would be interoperable, while not stopping innovation. The result of the specification work was a set of documents extending to more than 6000 pages. Nevertheless the resultant phone system provided a robust, feature-rich system. The first roaming agreement was signed between Telecom Finland and Vodafone in the UK. Thus the vision of a pan-European network was fast becoming a reality. However this took place before any networks went live.
The aim to launch GSM by 1991 proved to be a target that was too tough to meet. Terminals started to become available in mid 1992 and the real launch took place in the latter part of that year. With such a new service many were sceptical as the analogue systems were still in widespread use. Nevertheless by the end of 1993 GSM had attracted over a million subscribers and there were 25 roaming agreements in place. The growth continued and the next million subscribers were soon attracted.
Global usage
Originally GSM had been planned as a European system. However the first indication that the success of GSM was spreading further a field occurred when the Australian network provider, Telstra signed the GSM Memorandum of Understanding.
Frequencies
Originally it had been intended that GSM would operate on frequencies in the 900 MHz cellular band. In September 1993, the British operator Mercury One-to-One launched a network. Termed DCS 1800 it operated at frequencies in a new 1800 MHz band. By adopting new frequencies new operators and further competition was introduced into the market apart from allowing additional spectrum to be used and further increasing the overall capacity. This trend was followed in many countries, and soon the term DCS 1800 was dropped in favour of calling it GSM as it was purely the same system but operating on a different frequency band. In view of the higher frequency used the distances the signals travelled was slightly shorter but this was compensated for by additional base stations.
In the USA as well a portion of spectrum at 1900 MHz was allocated for cellular usage in 1994. The licensing body, the FCC, did not legislate which technology should be used, and accordingly this enabled GSM to gain a foothold in the US market. This system was known as PCS 1900 (Personal Communication System).
A great success
With GSM being used in many countries outside Europe this reflected the true nature of the name which had been changed from Groupe Special Mobile to Global System for Mobile communications. The number of subscribers grew rapidly and by the beginning of 2004 the total number of GSM subscribers reached 1 billion. Attaining this figure was celebrated at the Cannes 3GSM conference held that year.

GSM tutorial [1]

The GSM system is the most widely used mobile telecommunications system in use in the world today. The letters GSM originally stood for the words Groupe Speciale Mobile, but as it became clear this standard was to be used world wide the meaning of GSM was changed to Global System for Mobile Communications. Since it was first deployed in 1991, the use of GSM has grown steadily, and it is now the most widely cell phone system in the world. GSM reached the 1 billion subscriber point in February 2004, and continued to grown in popularity.

System idea
The GSM system was designed as a second generation (2G) cellular communication system. One of the basic aims was to provide a system that would enable greater capacity to be achieved than the previous first generation analogue systems. GSM achieved this by using a digital TDMA (time division multiple access approach). By adopting this technique more users could be accommodated within the available bandwidth. In addition to this, ciphering of the digitally encoded speech was adopted to retain privacy. Using the earlier analogue systems it was possible for anyone with a scanner receiver to listen to calls and a number of famous personalities had been "eavesdropped" with embarrassing consequences.

Services provided
Speech or voice calls are obviously the primary function for the GSM system. To achieve this the speech is digitally encoded and later decoded using a vocoder. A variety of vocoders are available for use, being aimed at different scenarios.

In addition to the voice services, GSM supports a variety of other data services. Although their performance is nowhere near the level of those provided by 3G, they are nevertheless still important and useful. A variety of data services are supported with user data rates up to 9.6 kbps. Services including Group 3 facsimile, videotext and teletex can be supported.

One service that has grown enormously is the short message service. Developed as part of the GSM specification, it has also been incorporated into other cellular systems. It can be thought of as being similar to the paging service but is far more comprehensive allowing bi-directional messaging, store and forward delivery, and it also allows alphanumeric messages of a reasonable length. This service has become particularly popular, initially with the young as it provided a simple, low fixed cost.

Basic concept
The GSM system had a number of design aims when the development started. It should offer good subjective speech quality, have a low phone or terminal cost, terminals should be able to be handheld, the system should support international roaming, it should offer good spectral efficiency, and the system should offer ISDN compatibility.

The system that developed provided for all of these. The overall system definition for GSM describes not only the air interface but also the network. By adopting this approach it is possible to define the operation of the whole network to enable international roaming as well as enabling network elements from different manufacturers to operate alongside each other, although this last feature is not completely true, especially with older items.

GSM uses 200 kHz RF channels. These are time division multiplexed to enable up to eight users to access each carrier. In this way it is a TDMA / FDMA system.

The base transceiver stations (BTS) are organised into small groups, controlled by a base station controller (BSC) which is typically co-located with one of the BTSs. The BSC with its associated BTSs is termed the base station subsystem (BSS).

Further into the core network is the main switching area. This is known as the mobile switching centre (MSC). Associated with it is the location registers, namely the home location register (HLR) and the visitor location register (VLR) which track the location of mobiles and enable calls to be routed to them. Additionally there is the Authentication Centre (AuC), and the Equipment Identify Register (EIR) that are used in authenticating the mobile before it is allowed onto the network and for billing. The operation of these are explained in the following pages.

Last but not least is the mobile itself. Often termed the ME or mobile equipment, this is the item that the end user sees. One important feature that was first implemented on GSM was the use of a Subscriber Identity Module. This card carried with it the users identity and other information to allow the user to upgrade a phone very easily, while retaining the same identity on the network. It was also used to store other information such as "phone book" and other items. This item alone has allowed people to change phones very easily, and this has fuelled the phone manufacturing industry and enabled new phones with additional features to be launched. This has allowed mobile operators to increase their average revenue per user (ARPU) by ensuring that users are able to access any new features that may be launched on the network requiring more sophisticated phones.

Specification Summary of GSM Cell Phone System

		Multiple Access Technology	FDMA / TDMA
		Duplex Technique	FDD
		Uplink frequency band	933 - 960 MHz (basic 900 MHz band only)
		Downlink frequency band	890 - 915 MHz (basic 900 MHz band only)
		Channel spacing	200 kHz
		Modulation	GMSK
		Speech coding	Various - Original was RPE-LTP/13
		Speech channels per RF channel	8
		Channel data rate	270.833 kbps
		Frame duration	4.615 mS

GSM tutorial [2]

- an overview or tutorial of the network architecture and hardware used in the GSM system

The architecture of the GSM system with its hardware can broadly be grouped into three main areas: the mobile station, the base station subsystem, and the network subsystem. Each area performs its own functions and when used together they enable the full operational capability of the system to be realised.

Mobile station
Mobile stations (MS), mobile equipment (ME) or as they are most widely known, cell or mobile phones are the section of a GSM cellular network that the user sees and operates. In recent years their size has fallen dramatically while the level of functionality has greatly increased. A further advantage is that the time between charges has significantly increased.

There are a number of elements to the cell phone, although the two main elements are the main hardware and the SIM.

The hardware itself contains the main elements of the mobile phone including the display, case, battery, and the electronics used to generate the signal, and process the data receiver and to be transmitted. It also contains a number known as the International Mobile Equipment Identity (IMEI). This is installed in the phone at manufacture and "cannot" be changed. It is accessed by the network during registration to check whether the equipment has been reported as stolen.

The SIM or Subscriber Identity Module contains the information that provides the identity of the user to the network. It contains are variety of information including a number known as the International Mobile Subscriber Identity (IMSI).

Base station subsystem
The Base Station Subsystem (BSS) section of the GSM network is fundamentally associated with communicating with the mobiles on the network. It consists of two elements, namely the Base Transceiver Station (BTS) and the Base Station Controller (BSC).

The BTS used in a GSM network comprises the radio transmitter receivers, and their associated antennas that transmit and receive to directly communicate with the mobiles. The BTS is the defining element for each cell. The BTS communicates with the mobiles and the interface between the two is known as the Um interface with its associated protocols.

The BSC forms the next stage back into the GSM network. It controls a group of BTSs, and is often co-located with one of the BTSs in its group. It manages the radio resources and controls items such as handover within the group of BTSs, allocates channels and the like. It communicates with the BTSs over what is termed the Abis interface.

Network subsystem
The network subsystem contains a variety of different elements, and is often termed the core network. It provides the main control and interfacing for the whole mobile network. It includes elements including the MSC, HLR, VLR, Auc and more as described below:

The main element within the core network is the Mobile switching Services Centre (MSC). The MSC acts like a normal switching node within a PSTN or ISDN, but also provides additional functionality to enable the requirements of a mobile user to be supported. These include registration, authentication, call location, inter-MSC handovers and call routing to a mobile subscriber. It also provides an interface to the PSTN so that calls can be routed from the mobile network to a phone connected to a landline. Interfaces to other MSCs are provided to enable calls to be made to mobiles on different networks.

To enable the MSC to perform its functions it requires data from a number of databases. One is known as the Home Location Register (HLR). It contains all the administrative information about each subscriber along with their last known location.

When a user switches on their phone, the phone registers with the network and from this it is possible to determine which BTS it communicates with so that incoming calls can be routed appropriately. Even when the phone is not active (but switched on) it re-registers periodically to ensure that the network (HLR) is aware of its latest position.

There is one HLR per network, although it may be distributed across various sub-centres to for operational reasons.

Another of the databases is known as the Visitor Location Register (VLR). This contains selected information from the HLR that enables the selected services for the individual subscriber to be provided.

The VLR can be implemented as a separate entity, but it is commonly realised as an integral part of the MSC, rather than a separate entity. In this way access is made faster and more convenient.

The third register is the Equipment Identity Register (EIR). The EIR is the entity that decides whether a given mobile equipment may be allowed onto the network. Each mobile equipment has a number known as the International Mobile Equipment Identity. This number, as mentioned above, is installed in the equipment and is checked by the network during registration. Dependent upon the information held in the EIR, the mobile may be allocated one of three states - allowed onto the network, barred access, or monitored in case its problems.

The final register is the Authentication Centre (AuC). The AuC is a protected database that contains the secret key also contained in the user's SIM card. It is used for authentication and for ciphering on the radio channel.

Another element in the network is the Gateway Mobile Switching Centre (GMSC). The GMSC is the point to which a ME terminating call is initially routed, without any knowledge of the MS's location. The GMSC is thus in charge of obtaining the MSRN (Mobile Station Roaming Number) from the HLR based on the MSISDN (Mobile Station ISDN number, the "directory number" of a MS) and routing the call to the correct visited MSC. The "MSC" part of the term GMSC is misleading, since the gateway operation does not require any linking to an MSC.

The SMS-G or SMS gateway is the term that is used to collectively describe the two Short Message Services Gateways defined in the GSM standards. The two gateways handle messages directed in different directions. The SMS-GMSC (Short Message Service Gateway Mobile Switching Centre) is for short messages being sent to an ME. The SMS-IWMSC (Short Message Service Inter-Working Mobile Switching Centre) is used for short messages originated with a mobile on that network. The SMS-GMSC role is similar to that of the GMSC, whereas the SMS-IWMSC provides a fixed access point to the Short Message Service Centre.

GSM tutorial [3]

- an overview or tutorial of the air interface and channels for the GSM system

There are a number of elements to the GSM radio or air interface. There are the aspects of the physical power levels, channels and the like. Additionally there are the different data channels that are employed to carry the data and exchange the protocol messages that enable the radio subsystem to operate correctly.

Basic signal characteristics
The GSM system uses digital TDMA technology combined with a channel bandwidth of 200 kHz. Accordingly the system is able to offer a higher level of spectrum efficiency that that which was achieved with the previous generation of analogue systems. As there are many carrier frequencies that are available, one or more can be allocated to each base station. The system also operates using Frequency Division Duplex and as a result, paired bands are needed for the up and downlink transmissions. The frequency separation is dependent upon the band in use.

The carrier is modulated using Gaussian Minimum Shift Keying (GMSK). GMSK was used for the GSM system because it is able to provide features required for GSM. It is resilient to noise when compared to some other forms of modulation, it occupies a relatively narrow bandwidth, and it has a constant power level.

The data transported by the carrier serves up to eight different users under the basic system. Even though the full data rateon the carrier is approximately 270 kbps, some of this supports the management overhead, and therefore the data rate allotted to each time slot is only 24.8 kbps. In addition to this error correction is required to overcome the problems of interference, fading and the like. This means that the available data rate for transporting the digitally encoded speech is 13 kbps for the basic vocoders.

Power levels
A variety of power levels are allowed by the GSM standard, the lowest being only 800 mW (29 dBm). As mobiles may only transmit for one eighth of the time, i.e. for their allocated slot which is one of eight, the average power is an eighth of the maximum.

Additionally, to reduce the levels of transmitted power and hence the levels of interference, mobiles are able to step the power down in increments of 2 dB from the maximum to a minimum 13 dBm (20 milliwatts). The mobile station measures the signal strength or signal quality (based on the Bit Error Rate), and passes the information to the BTS and hence to the BSC, which ultimately decides if and when the power level should be changed.

A further power saving and interference reducing facility is the discontinuous transmission (DTx) capability that is incorporated within the specification. It is particularly useful because there are long pauses in speech, for example when the person using the mobile is listening, and during these periods there is no need to transmit a signal. In fact it is found that a person speaks for less than 40% of the time during normal telephone conversations. The most important element of DTx is the Voice Activity Detector. It must correctly distinguish between voice and noise inputs, a task that is not trivial. If a voice signal is misinterpreted as noise, the transmitter is turned off an effect known as clipping results and this is particularly annoying to the person listening to the speech. However if noise is misinterpreted as a voice signal too often, the efficiency of DTX is dramatically decreased.

It is also necessary for the system to add background or comfort noise when the transmitter is turned off because complete silence can be very disconcerting for the listener. Accordingly this is added as appropriate. The noise is controlled by the SID (silence indication descriptor).

Multiple access and channel structure
GSM uses a combination of both TDMA and FDMA techniques. The FDMA element involves the division by frequency of the (maximum) 25 MHz bandwidth into 124 carrier frequencies spaced 200 kHz apart as already described.

The carriers are then divided in time, using a TDMA scheme. The fundamental unit of time is called a burst period and it lasts for approximately 0.577 mS (15/26 mS). Eight of these burst periods are grouped into what is known as a TDMA frame. This lasts for approximately 4.615 ms (i.e.120/26 ms) and it forms the basic unit for the definition of logical channels. One physical channel is one burst period allocated in each TDMA frame.

There are different types of frame that are transmitted to carry different data, and also the frames are organised into what are termed multiframes and superframes to provide overall synchronisation.

GSM tutorial [4]

- an overview or tutorial of the channels and their structure for the GSM system

GSM uses a variety of channels in which the data is carried. In GSM, these channels are separated into physical channelsand logical channels. The Physical channels are determined by the timeslot, whereas the logical channels are determined by the information carried within the physical channel. It can be further summarised by saying that several recurring timeslots on a carrier constitute a physical channel. These are then used by different logical channels to transfer information. These channels may either be used for user data (payload) or signalling to enable the system to operate correctly.

Common and dedicated channels
The channels may also be divided into common and dedicated channels. The forward common channels are used for paging to inform a mobile of an incoming call, responding to channel requests, and broadcasting bulletin board information. The return common channel is a random access channel used by the mobile to request channel resources before timing information is conveyed by the BSS.

The dedicated channels are of two main types: those used for signalling, and those used for traffic. The signalling channels are used for maintenance of the call and for enabling call set up, providing facilities such as handover when the call is in progress, and finally terminating the call. The traffic channels handle the actual payload.

The following logical channels are defined in GSM:

TCHf - Full rate traffic channel.

TCH h - Half rate traffic channel.

BCCH - Broadcast Network information, e.g. for describing the current control channel structure. The BCCH is a point-to-multipoint channel (BSS-to-MS).

SCH - Synchronisation of the MSs.

FCHMS - frequency correction.

AGCH - Acknowledge channel requests from MS and allocate a SDCCH.

PCHMS - terminating call announcement.

RACHMS - access requests, response to call announcement, location update, etc.

FACCHt - For time critical signalling over the TCH (e.g. for handover signalling). Traffic burst is stolen for a full signalling burst.

SACCHt - TCH in-band signalling, e.g. for link monitoring.

SDCCH - For signalling exchanges, e.g. during call setup, registration / location updates.

FACCHs - FACCH for the SDCCH. The SDCCH burst is stolen for a full signalling burst. Function not clear in the present version of GSM (could be used for e.g. handover of an eight-rate channel, i.e. using a "SDCCH-like" channel for other purposes than signalling).

SACCHs - SDCCH in-band signalling, e.g. for link monitoring.

GSM tutorial [5]

- an overview or tutorial of the speech coding techniques and vocoders used the GSM system
If digitised in a linear fashion, the speech would occupy a far greater bandwidth than any cellular system and in this case the GSM system would be able to accommodate. To overcome this, a variety of voice coding systems or vocoders are used. These systems involve analysing the incoming data that represents the speech and then performing a variety of actions upon it to reduce the data rate. At the receiving end the reverse process is undertaken to re-constitute the speech data so that it can be understood. In GSM a variety of vocoders are used, including LPC-RPE, EFR, etc as described in the following paragraphs.

The vocoder that was originally used in the GSM system was the LPC-RPE (Linear Prediction Coding with Regular Pulse Excitation) vocoder. This vocoder took each 20 mS block of speech and then represented it using just 260 bits. This actually equates to a data rate of 13 kbps.

In GSM it is recognised that some bits are more important than others. If some bits are missed or corrupted, it is more important to the voice quality than others. Accordingly the different bits are classified:

Class Ia 50 bits - most important and sensitive to bit errors
Class Ib 132 bits - moderately sensitive to bit errors
Class II 78 bits - least sensitive to bit errors

The 50 Class 1a bits are given a 3 bit Cyclic Redundancy Code (CRC) so that errors can be detected. This makes a total length of 53 bits. If there are any errors, the frame is not used, and it is discarded. In its place a version of the previously correctly received frame is used. These 53 bits, together with the 132 Class Ib bits with a 4 bit tail sequence, are entered into a 1/2 rate convolutional encoder. The total length is 189 bits. The encoder encodes each of the bits that enter as two bits, the output also being dependent upon a combination of the previous 4 input bits. As a result the output from the convolutional encoder consists of 378 bits. The remaining 78 Class II bits are considered the least sensitive to errors and they are not protected and simply added to the data. In this way every 20 ms speech sample generates a total of 456 bits. Accordingly the overall bit rate is 22.8 kbps. Once in this format the data is interleaved to add further protection against interference and noise.

The 456 bits output by the convolutional encoder are divided into 8 blocks of 57 bits, and these blocks are transmitted in eight consecutive time-slots, i.e. a total of four bursts as each burst takes two sets of data.

Later another vocoder called the Enhanced Full Rate (EFR) vocoder was added in response to the poor quality perceived by the users. This new vocoder gave much better sound quality and was adopted by GSM. Using the ACELP (Algebraic Code Excitation Linear Prediction) compression technology it gave a significant improvement in quality over the original LPC-RPE encoder. It became possible as the processing power that was available increased in mobile phones as a result of higher levels of processing power combined with their lower current consumption.

There is also a half rate vocoder. Although this gives much inferior voice quality, it does allow for an increase in networkcapacity. It is used in some instances when network loading is very high to accommodate all the calls.