GSM Digital Mobile Communications
© Mercury Communications Ltd - June 1993

GSM pico cell's moment of fame - March 2007
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Cellular mobile communications has been growing throughout Europe, especially in the UK, at a rate faster than even the most optimistic market research company forecasted a few years ago. Although the onset of recession has slowed the growth rate somewhat in the 1990s (Figure 1) it is likely to accelerate upwards again as we move towards the mid 1990s. Based as it is on analogue technology, this explosion of usage has led to inevitable problems as any regular user will know. Analogue technology means that speech and data are transmitted over the radio path as a frequency-modulated (FM) analogue radio signal, which is exactly the same as used for FM radio. Drop outs, fading and interference plague users. Also, much recent press has focused everyone on how open cellular calls are to eavesdroppers equipped with low-cost hand held scanner radios. To combat these problems, open the way for a broader spectrum of user services, and provide the basis of personal communications networks (PCN), the replacement of these analogue networks by digital ones is well underway. The adoption of a digital network standard originally known as Groupe Spéciale Mobile (GSM) but now in an anglicised form global system for mobile communications will enable European-wide compatibility for the first time. DCS 1800 is a derivative of GSM based on the 1800MHz band. This issue of Technology Watch provides an overview of GSM and its personal persona, DCS 1800.

UK Cellular History

Throughout Europe there are around ten different analogue standards including NMT450, NMT900. The analogue cellular radio system in the UK is known as total access communications systems (TACS) and consists of a modification of the US AMPS cellular system. Since its inception it has suffered from an underestimation of popularity. For example, the mid-1980s forecast for the number of subscribers in 1990 was 20,000, in fact it turned out to be nearer one million. This led directly to the industry seeing itself as being reactive rather than pro-active.

Figure 1 - Growth of UK Cellular Subscribers in Millions

Another result was that insufficient spectrum (50MHz) was allocated to accommodate this number of users thus creating problems with quality of service in one or more of the following areas:

  • First time connection. Subscribers expect to be connected on first call. Sufficient radio channels must be available to achieve this and there should not be any blocking within terrestrial fixed networks.
  • Receive first time. Similar to first time connection.
  • Interference. Two types of interference affect analogue cellular call quality, man-made and co-channel. As the mobile moves further away from the base station the signal gets weaker. This is perceived by the user as an increase in the background noise level and a greater susceptibility to interference in the form of bangs and crackles and co-channel interference. Co-channel interference is analogous to a crossed line in a terrestrial network.
  • Drop out. A drop out may occur when the radio link to the base station is impaired or if there is insufficient capacity in the network. If the event lasts longer than three or four seconds an affected call is automatically terminated.

The drive to improve quality and security and to unify the highly fractured European mobile scene has led to the development of a new digital technology called GSM. A 'subset' of GSM known as DCS 1800 is also being used as the technology for new PCN services as exemplified with C&W's One-2-One service due to be launched later this year.

GSM Background

The first GSM system specification was published in July 1991 and was immediately followed by several false starts. This was brought about by a combination over-optimism, difficulties in type approval testing, and inevitable changes to the GSM specification. The first terminals appeared on the market in June 1992.

A combination of high demand for mobile services and a lack of capacity in the installed analogue network, has made Germany the most advanced country for GSM deployment. In the UK, Vodafone have said that they now cover 60-70% of the UK population with their GSM service and expect 90% coverage by mid 1993.

GSM has also been accepted for use by over seventeen European countries and several others including New Zealand and Hong Kong ending a period of diverse and proprietary standards. Eventually users will be able to roam between countries using whatever communications variants they need in the form of voice, data or facsimile with all usage being charged to the user through the use of a personal telephone number. It is currently forecast that GSM terminal sales will not overtake analogue sales until at least 1955/6.

Implementing a brand-new technology from scratch across Europe does raise some interesting problems that lie close to the hearts of product marketers and engineering managers:

  • In many countries there is no overt demand or need for GSM. Analogue services are available and under employed.
  • GSM coverage needs to be as wide as analogue before users will swap over.
  • The current generation of GSM hand portables are not as small or as light as analogue variants. This will limit the interest of many users, even though a better service may be provided by GSM technology.
  • Terminal prices for digital technologies are high compared to analogue. Although in time integration of GSM chip-sets will reduce costs it is likely that operators will need to subsidise the sale of terminals by selling them at below cost in the near term. Of course, once volumes really start to increase GSM handset prices will tumble.
  • It is likely that it will be very difficult to get users to pay higher call charges for an improved service so GSM cannot be positioned as a higher quality/higher price service.

Figure 2 - The GSM Cellular Transceiver Network

GSM Technical Overview

The GSM standard was developed on the back of collaboration between all the Telecommunications Administrations of Europe (including our own Radiocommunications Agency) under the administration of Conférence Européenne des Administrations des Postes et Télécommunications (CEPT). At a later date it was sponsored by ETSI which included a number of European manufacturers. GSM operates in the 900MHz frequency band where it co-exists with other cellular services. The system, like all other cellular networks, is based on a contiguous set of cells providing complete coverage of the service area (Figure 2) .

Radio

There are three fundamental ways of splitting spectrum between a number of users as required in a cellular radio system:

  • TDMA. With time division multiple access simultaneous conversations are supported by users transmitting in short bursts at different times or 'slots'.
  • FDMA. In frequency division multiple access, the total band is split into narrow frequency subbands and a channel is allocated exclusively to each user during the course of a call. One is used for transmission and one for reception.
  • CDMA. Code division multiple access allows all users access to all frequencies with the allocated band. A single user is extracted from the mayhem by looking for each user's individual code using a correlator. Although not selected for the current generation of mobile digital technologies, CDMA holds much promise as the future technology of choice for GSM replacement in the next century.

900MHz GSM uses a combination of TDMA and FDMA. It uses eight time slots, hence one carrier can support eight full rate or sixteen half rate channels. Channel separation is 200kHz with mobile transmit channels in the range 890 to 915MHz and mobile receive channels in the range 935 to 960MHz. Peak output power of the transmitters depends on the class of the mobile station and can be 0.8, 2, 5, 8, or 20 watts.

In-built functions to prevent interference are programmable transmit power control and power ramping at the beginning and end of the time slot. Frequency hopping minimises the effects of interference and helps to prevent dead spots in coverage caused by multipath fading. The overall data rate used is 270kbit/s split between eight channels. To minimise transmission errors the output of each speech encoder (digitiser) is encrypted and interleaved to allow forward error correction (FEC) to be used. The data is then sent in bursts of length 577µs, each containing 116 encrypted bits. Transmit and receive slots are staggered to give time for the frequency synthesisers to change channel.

System Components

In GSM-speak, each cell has a base transceiver station (BTS) (Figure 3) operating on fixed frequencies that are different to any of its neighbours. A cluster of base stations are controlled by a base station controller (BSC) and a group of BSCs is controlled by a Mobile Service Switching Centre (MSC). The MSC is the heart of the GSM cellular radio system and is responsible for routing, switching of calls from the originator to their destination. The prime function of the BSC is call maintenance. As a subscriber moves round he is likely to move between cells so the BSC controls the handover to minimise the break time.

Databases

There are two important databases that store information about subscribers, the home location register (HLR) which contains information about subscription levels, supplementary services, and current location. The authentication centre (AUC) works closely with the HLR to prevent fraud, stolen SIM cards or unpaid bills. The visitor location register (VLR) stores information about subscription levels, supplementary services, current location in the visited region. These databases also keep track of whether a subscriber is active i.e. whether his telephone is switched on or not. The equipment identity register (EIR) stores information about the type of mobile station in use and can bar calls if it finds a piece of equipment has been stolen.

Figure 3 - GSM Main System Components

GSM's Smartcard

The GSM network is underpinned by the use of a smartcard known as the subscriber identity module (SIM). The major task of the SIM is to support voice encryption and to manage user authentication. The SIM also supports other intelligent services such as:

The SIM card can hold up to fifty abbreviated dialling codes.

Advice of charge meter. Based on tariffs held in the SIM, downloaded from the network, the subscriber can display the real cost of the call in real time.

The GSM system supports short message transmission. Messages are downloaded to the subscriber's current location where it is stored in the SIM card. Up to five, 160 character long, messages can be stored at any one time. The subscriber can access these messages at any time.

GSM User Services

From a user's perspective, GSM consists of set of mobile services, some of which are:

  • High quality voice connections through vehicle, portable and hand-held telephones.
  • Data services including short message service, facsimile transmission and data communications at rates of up to 9600Bits/s with full duplex capability.
  • Full European roaming capability. Switch on in any area covered by GSM and the home network will be notified as to where the portable is. Thus it will be possible to receive and make calls without the recipients knowing that the subscriber is abroad.
  • Subscriptions are recorded on a subscriber information module (SIM) and when this is inserted into any GSM telephone it immediately becomes the subscribers. The network checks that the subscription is valid and that the card is not stolen by authenticating it back to the home database. For further information on smartcards see Technology Watch #14.
  • Automatic routing of calls to local emergency services.

Phase 1 of GSM supports call forwarding if the mobile is busy or not reachable, call barring for such things as international calls and incoming calls when roaming, call waiting and call hold options, and immediate advice of charge in any currency! Phase 2 plans to support more advanced services.

The Benefits of Going Digital

What are some of the benefits of introducing a digital cellular network?

  • The benefit of a standard in that pan-European coverage will permit cross-border roaming for the first time.
  • Greater spectrum usage efficiency compared to analogue approaches.
  • Improved service quality for users in the form of improved speech quality, improved security through inbuilt encryption (there is none at present), and higher connection reliability.
  • Larger number of advanced user services and easier linkage to private and public ISDN networks as they become available.

At this time the general consensus is that GSM based digital cellular telephones will overtake that of analogue telephones by 1996.

GSM Issues

There are many issues that are currently being evaluated:

  • Intellectual property rights (IPR). Most of the early research undertaken to define the GSM standard was carried out by the large multinationals who could afford to indulge in the early research and development. As a result, most patents are held by these manufacturers who can work together through cross licensing. The problem with this is that it makes it very difficult for a small player who does not hold patents to negotiate with to move into the market without paying significant royalties to a combination of large manufacturers. The blame can be laid at the door of the early players and the standards bodies although it does look like that this issue has now been resolved.
  • GSM Encryption and the A5 algorithm. The GSM specification utilised a very secure data encryption algorithm called 'A5'. The use of this algorithm is limited to the original signatories only because of national security considerations. This has led to significant problems when exporting GSM technology to non Co-Com countries. Exportation is possible as long as certain, country specific, conditions are met. These include (1) irreversibly disabling encryption, (2) message encryption is disabled but subscriber identity (IMSI) encryption is allowed, and (3) the use of simplified (i.e. crackable) algorithms.
  • Inter-operator billing. In countries where roaming is allowed inter-operator billing agreements need to be agreed and data passed between countries on a daily basis.
  • In recent months there has been considerable media coverage concerning interference to other electronic systems from GSM phones. For example, it has been reported that in certain circumstances hearing aid users can hear a buzzing sound at a distance of three to five metres from a GSM phone. Also, engineers at Volkswagen are reported as having detected interference with automatic brake systems (ABS) and other electronic car subsystems. This interference is caused by the higher peak output power of bursty GSM transmissions when compared to the steady continuous transmission nature of analogue systems. Although work has started at ETSI to solve the problem, effective solutions will require effort both on the part of GSM operators and designers of electronic subsystems to reduce the susceptibility of their equipment to outside interference.

Beyond GSM - PCN

GSM has been complimented in the UK by an additional cellular service called the digital cellular service (DCS 1800) based on GSM technology.

In January 1989 the DTI published a consultative document outlining its ideas about a personal communication network (PCN) . Initially three licenses were issued to consortia:

  • Microtel - British Aerospace, Pacific Telesis, Millicom and Sony.
  • Unitel - STC, Thorn EMI, US West, and Deutsche Bundespost Telecom.
  • Mercury PCN

In 1993 there are only two potential PCN operators left following many changes which included the merger of Unitel and Mercury Personal Communications (MPC). These are Mercury One-2-One (formally MPC) and Hutchison Microtel.

Because of the major costs in setting up a new PCN network the DTI introduced two measures to reduce costs, (1) PCN operators can provide their own radio links between radio sites and mobile switching centres and (2) PCN operators can share infrastructure.

As a modified form of GSM it operates in the 1.8GHz band with smaller cell sizes and lower power. DCS 1800 has also been allocated 150MHz of spectrum compared to the 50MHz of GSM. The standard finalised in January 1991 stipulates a number of differences to its forebear at the station/handset level. As the standard is aimed at PCN applications it has been optimised for higher-density traffic that would be seen in smaller PCN cells.

There are several areas where DCS 1800 differs from GSM. Table 2 shows the principal differences which are concerned with spectrum, transmit powers, and cell sizes. In all other aspects DCS 1800 and GSM are the identical except for a few minor specification differences.

Frequency                GSM           DCS 1800         
      Mobile TX   890-915MHz     1,710-1,785MHz 
        base TX   935-960MHz     1,805-1,880MHz     
Handset power                          
    peak output    800mW-20W           250mW-1W   
    mean output   100mW-2.5W         40mW-125mW  
     Cell Sizes   1km - 35km         <1km - 8km

Table 2 - GSM and DCS 1800 Compared

Mercury One-2-One based in Borehamwood it a joint venture between Cable & Wireless and US West and plans to launch the UK's first PCN network within the bounds of the M25 later this year.

The One-2-One personal communications network will rely for success on clearly differentiating itself from other mobile cellular operators in the eyes of potential users. This is to be achieved through several means:

  • It is phone service for a person rather a car. The phone can be used at home, when out and about walking, and in the office.
  • It will provide high quality communication that cannot be overheard.
  • One-2-One will provide subscribers with advanced services such as:

Personal SmartCard for customised services

VoiceMail answer phone service

Fully itemised billing

Call waiting, divert, and barring

Current call charges enquiry

Optional services:

Monthly call limit

Insurance cover

VoiceMail Plus

And, most importantly, lower monthly standing charges combined with lower call charges.

The Future - FPLMTS?

The CCITT is currently working on a universal standard for the next decade called future public land mobile telecommunication system (FPLMTS). This standard ultimately aims to fully merge cellular and cordless standards.

The FPLMTS standard is planned to support the following all-encompassing voice and data services:

  • Universal personal telecommunication (UPT) voice services
  • Message handling
  • Teletex
  • Paging
  • Point-to-multipoint communication
  • Data services 300-9,600 baud and under favourable circumstances up to 20Mbit/s in connection and connectionless modes can be provided.
  • Videotex, Video telephone
  • Program video
  • location services (via GPS)
  • multimedia - voice, video and data simultaneously.

Conclusions

It is still early days for GSM and although a long term perspective indicates that a digital solution for mobile communications will the norm, the near term will be clouded by the commercial and technical issues caused by the replacement of clearly outdated analogue cellular technology.

Like all digital standards that are aimed at replacing older analogue technology, GSM has taken many years to come to fruition with many heartaches on the way. But the benefits to the user of pan-European roaming, the stream of new mobile services, and the improvement in security and quality will ensure that GSM has a bright future.

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