The other communications revolution As GSM continues its advance through the global market for mobile .

telephones, the TETRA standard is set to launch a comparable revolution in private mobile radio, reports Philip Whitehead

1 Many emergency situations demand

mobile-to-mobile operation

rivate mobile radio (PMR) is the unsung workhorse of mobile com- P munications. Once the only option

for people needing to communicate on the move, PMR has a far lower public profile than the cellular telephones to be heard trilling from briefcases world-wide. But of course, PMR was never aimed at the general public. It was and is a prof- essional tool, indispensable to large num- bers of industrial and commercial users. Its primary use is for group-orientated communications in organisations such as the police, fire and ambulance services, the utilities and fleet vehicle operators such as transport companies.

Superficially similar in appearance to a mobile telephone, the PMR hand-held radio is in practice used very d~erently For example, the average PMR call lasts just a few seconds. Calls are often urgent and require effectively instantaneous set- up times. Many PMR systems include a control room, which may need to contact large groups of users simultaneously Most networks are privately owned and

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operated, but recent years have seen a steady growth in public-access mobile radio (PAMR) networks which offer rented access in similar fashion to the public telephone networks. For users who do not need private control of their network, PAMR offers a cost-effective approach avoidmg direct investment in infrastructure.

For the great majority of PMR users, cel- lular telephony is not an equivalent alter- native, even with the enhanced capabil- ities offered by GSM However, it is equally true that existing PMR systems fall short of providing everything today's users might need and want. In particular, the lack of a widely accepted standard for PMR has been a major limitation on its growth.

One standard Every country in Europe currently has

its own variant of PMR, based on its own signalling protocols, frequency alloc- ations and regulatory framework. The lesson of GSM is that standardisation is the key to market success. Standardis-

ation brings consumer confidence, high- volume production and lower prices. The TETRA standard was developed primar- ily to bring the benefits of standardis- ation to PR/IR.

TETRA stands for Trans European Trunked Fadio, and compiises a com- prehensive suite of standards for digital mobile radio. Developed in considerably less time than the GSM standard - despite its (arguably more taxing technical challenges - TETRA is a fine example of European technical coopera tion. Stand- ardisation is never easy, and TETRA has required an enormous joint effort on the part of manufacturers, regulators and user groups. The project is coordinated by the European Telecommunications Standards Institute (ETSI), and TETRA is one of the most compreheinsive stand- ards the Institute has ever produced.

Performance and services TETRA'S most obvious benefit is that,.

being based on digitally encoded trans- mission, it provides improved perform-

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ance. Voice quality is at least as good as the best of today's analogue systems. Also, thanks to the use of sophisticated digtal signal processing, a TETRA radio will continue to transmit good, intelligible speech even in signal conditions where an analogue system would fail altogether.

TETRA systems will include the full range of basic services PMR users typ- ically require, including:

one-way or two-way calls (simp- lex/duplex)

0 calls to individual radios e group or broadcast calls e direct calls (mobile to mobile).

The last of these is particularly sig- nificant for users in the public safety organisations. At incident sites, such as fires or other emergencies, operation in and around buildings frequently rules out communication via the main network and officers have to rely on direct-mode calls between mobile and hand-held radios (Fig. 1).

In general, direct-mode operation necessitates the use of radio equipment entirely separate from the main PMR network, and lmking the two systems will normally involve installing dual radios and a converter in the control vehicle. TETRA provides a far easier and more cost-effective solution by allowing a standard mobile radio to act as a gateway into the main PMR network and as a

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repeater for mobile-to-mobile calls. A firefighter can thus call nearby colleagues in direct mode whilst remaining in contact with the main PMR network via a vehicle-mounted radio. It is even possible to dial directly into the public telephone system.

In addtion to these basic services, TETRA provides for a wide range of sup- plementary services such as call auth- orisation by dispatcher, dynamic group assignment, priority c h g and late entry to calls in progress. A dscreet- listening facility allows the control room to monitor calls. If a vehicle is stolen or hijacked, its TETRA ra&o can be remotely switched into ambient-listening mode to monitor verbal exchanges.

Call encryption is an important requirement for the police and security services. W s t analogue transmissions can be encrypted, the highest levels of security are difficult to achieve, and users have to accept certam operahng con- straints and a slightly strange voice qual- ity. Encryption is inherently easier in a digital system, and all the necessary mechanisms have been designed into TETRA. Calls are automatically encryp- ted across the air interface, and security can easily be extended up to the highest levels of end-to-end encryption.

Images of the future From the outset, TETRA'S develop-

ment partners reahsed that the new

2 How the bits add up

standard must provide advanced facilit- ies for data transmission. PMR users already routinely exchange factual in- formation in the form of short numeric or alphanumeric messages. In the multi- media world of the near future, fast and efficient data exchange is likely to be an increasingly important requirement, and there is little doubt that, in time, TETRA'S data capabilities will prove even more sig- nificant than its advanced voice services.

Market research indicates that one of TEIRA's most popular features is its ability to transmit data simultaneously with a voice call, on the same radio channel. It achieves th~s impressive feat by converting all calls, whether voice or data, into digtal packets and trans- mitting them in sequential time slots. Up to four calls can be interleaved (multi- plexed) on each 25 kHz TETRA channel. The receiving radios combine each call's digtal packets seamlessly and convert them back to voice or data information.

A single call can in fact occupy any- thing from one time slot to all four, according to the bit rate required - an extremely useful feature known as 'bandwidth on demand. digital transmissions are sent at 36 kbit/s, equiv- alent to a gross bit rate of 9 kbit/s per user if four users are sharing a channel. In practice, signalling and other overheads reduce this figure to a net data rate of 7.2 kbit/s (Fig. 2). TETRA'S highly efficient codec (coder/decoder) operates at 456 kbit/s. The balance of the 7.2 kbit/s data rate is devoted to error protection to give a high-quality speech signal.

A data call can be allocated to one, two, three or all four time slots, giving data rates of 7.2, 14~4~21.6 or 28.8 kbit/s -more than twice the fastest rate available on GSM. TETRA also provides error cor- rection options if required, the degree of error correctlon employed depenctmg on the nature and sensitivity of the trans- mitted material (Fig. 3). A special packet data optimised (PDO) version uses, as far as possible, a common core design, and a study has been commissioned to exam- ine the possibility of sigruficantly higher data rates in this mode (E - PDQ).

TETRA'S high data rates open the door to entirely new types of application for PMR. Obvious examples include fax transmissions, and over-the-air file ex- changes between portable computers

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and office networks. Facilities of ths kind would be invaluable to engineers work- ing out in the field, not to mention sales teams and roving reporters. Radio telemetry already a fast-growing market, will also benefit greatly from TETRA'S data capabilities.

More intriguing are the prospects of image transmission systems based on TETRA. It is not hard to envisage devel- opments such as fingerprint readers cap- able of transmitting pictures instantly to the police database for identification. Image transmission systems of this kind have already appeared in prototype. TETRA'S video capabilities are not lim- ited to the transmission of stdl images, and manufacturers have already used the latest data compression techniques to stage demonstrations of TETRA-based equipment transmitting live video images in full colour. -

Where this kind of cap- ability might lead to is any- one's guess, but a popular favourite is the future police officer equipped with a port- able datacomms centre, in- cluding a drop-down video visor like a virtual reality headset (Fig. 4). At the scene of a bank raid, this 'technocop' would be able to receive live video pictures relayed from

(TDMA) - using time slots to mulitiplex several channels on a single carrier - in preference to frequenc y-division multiple access (FDMA). In FDMA the allocated radio spectrum is divided into narrow channels, usually in uplink/downlink pairs, with just one channel per carrier. Typically these are 12.5kHi for a two- way conversation. All current analogue, non-trunked PMR systems use FDMA.

In TDMA, the radio channels are normally wider - 25kHz in the case of TETRA. The number of time slots avail- able is limited only by the bandwidth required within each slot. TETRA'S choice of four slots per 25kHi channel allows adequate bandwidth within a single time slot to support high-quality voice or data.

TDMA makes efficient use of the radio spectrum, which is an important consid- eration in PMR where operators have

Technical challenges The E l R A project set ambitious objec-

tives which involved some substantial technical challenges. One of the greatest was the need for a speech codec more ad- vanced than anythmg curreritly available.

The codec is a signal processing device which uses a set of coding algorithms to convert and compress the analogue speech signal into digital form for transmission at a low bit rate. At the other end of the link, the codec in the receiving radio decodes the bit stream and converts it back into speech. The two primary requirements for TETRA'S codec were that it should provide speech commun- ication of high quality, yet be robust enough tlo survive the difficult radio environments in which PMR users frequently operate. A TETRA radio must continue to transmit and receive reliably

data rates applicatlons

user protection applied image files, video, 7.2 to 28.8 kbiffs fingerprint scanning

normal protection applied text files, computer files, 4.8 to 19.2 kbiffs map informaiton

high protection applied credit card validation and 2.4 to 9.6 kbiffs other high-security data

surveillance cameras inside the building. A camera in the officer's helmet would provide the control room with views of the action as it devel- oped, and relay the images to other officers speeding to the site in their vehicles.

TDMA advantages Digital transmission emerged as the

right way forward at quite an early stage in TETRA'S development. Existing anal- ogue PMR systems have been moving for some years in this direction, with the latest installations generally incorpor- ating &@tal switching and control. Em- ploying digital technology throughout would, potentially at least, allow im- proved performance, more efficient use of the radio spectrum and easier con- nection to fixed networks such as telephony and ISDN.

Another key decision was the choice of time-division multiple access

3 Bandwidth on demand

traditionally jostled for elbow room in the allocated bandwidth. However, spec- tral efficiency is far from being the sole advantage of TDMA. Other advantages include the ability to provide bandwidth on demand by concatenating time slots and support for simultaneous voice and data. For operators, TETRA's TDMA technology offers a range of significant economies. For example, a TETRA mob- ile radio can act as a repeater (using time- slot management) to relay calls to and from other radios whilst continuing to allow access for its own users, avoiding the need to buy separate repeater units. Also, for many smaller PMR systems four user channels are more than adequate, and these can be provided by a single TETRA base station (central trans- ceiver), without the need for a separate RF combiner unit to handle multiple channels.

even in fringe (and shadow areas where signal strength is poor. It must also ignore dis- tractions such as other voices close to the principal speaker.

In practice the require- ments of high performance and selectivity conflict with the need for roklustness, and the designers had to strike the right balance between these aims. The codec was selected from a number of competing designs after an extensive series of listening, tests.

In a TETIW transceiver, the speech signal is first converted into digital form at a data rate of 128 kbit/s. The codec then reduces this to 4.56 kbit/s - a compression ratio of more than 20. The technique used to achieve this degree of compression is known as CELP (code excited linear pre- diction). The basic principle is that of 'analysis by synthesis', in which the syn- thesised output is compared with the input, and the various parameters manip- ulated to nninimise the error between the two. CELI' also includes a perceptual weighting filter which takes into account the charaderistics of human lhearing.

For the future, TETRA'S, developers envisage producing an even more am- bitious half-rate codec which will double TETRA'S spectrd efficiency by squeezing eight calls into a single 25 kHz channel.

The design of the circuitry and equip- ment to bring TETRA to life were equally

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miniature camera sending images to control centre

aerial of the GPS which provides the officer's location

a voice activated mini screen, attached to the brim, can be pulled down in front of the eye

maps, files and pictures of criminals can be viewed on the screen instantly

demanding. For example, the transceiver must hold on to its signal not only in dd- ficult terrain such as hdls or urban areas, but also in fast-moving vehicles where the Doppler effect causes frequency changes. It must maintain its switching and timing extremely precisely whilst absorbing errors introduced by signal loss, interfering signals and multiple signals from reflections and refractions.

Much of TETRA'S circuitry required innovative design. Its h e a r transmitter, for example, is a highly advanced piece of electronic design which includes a novel combination of analogue and digital filtering.

Will it succeed? The future of any radical new technol-

ogy in the marketplace can never be guaranteed, but TETRA has a number of powerful factors in its favour. First, it works, and works very well indeed. Man- ufacturers involved in TETRA product development are conducting field trials during the course of this year. In trials of a fully TETRA-compliant vehicle-mounted radio conducted by my own company the performance confirmed or exceeded pred- ictions in all respects. Driving along the M11 through Cambridgeshire, the equip- ment maintained excellent voice quality at all speeds up to the legal maximum, and showed no susceptibihty to fading under major obstacles such as bridges.

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Secondly TETRA has already won considerable international support. Ten European governments, including the UK, are now firmly backing TETRA for public safety applications, and major manufacturers such as Alcatel, Ericsson, GEC-Marconi, Motorola, Nokia and Philips have joined with large users and regulators in a memorandum of under- s t a n h g group to promote the estab- lishment of TETRA. An important accep- tance hurdle was cleared earlier ths year, when a vote amongst ETSI member nations yielded a 100% 'yes' in support of the core TETRA standards.

Frequencies are currently being alloc- ated to TETRA around Europe, the inten- tion being to agree common bands for major application areas. TETRA can oper- ate happily at frequencies from 60 MHz to over 1000 MHz, with the optimum some- where around the middle of ths range. Public safety services in Europe will use the 380-400MHz band. For commercial operators, the band is expected to be 410- 430 MHz.

The third factor in TETRA'S favour is that it has emerged from its primary development phase as a very solid suite of standards, which between them cover all the current types of application for PMR and open up a number of fascin- ating avenues to be explored in the fut- ure.

Finally one of TETRA'S most compel-

4 Technocop

h g attractions is that it fulfils the urgent need for standardisation in PMR - and does it rather better than anyone could have hoped when the ETSI committee first sat down to review the options. The first products are very close to moving from prototype to production, and man- ufacturers have been working for some time with their first clients for TETRA.

TETRA and GSM GSM is sometimes portrayed as a

competitive threat to PMR in general and TETRA in particular. It has the advantage of being first to market, and has already established a broad user base across the world. There are also proposals, GSM-R, to add some superficially PMR-hke feat- ures such as group calls to the standard.

However, these are aimed at an essen- tially dlfferent type of user. The fast, group-wide messaging facilities PMR users need require a specific design approach and a very high level of per- formance. In general, cellular character- istics are simply not suited to PMR-type applications. For example, call set-up times are too long. Even if the design objectives of the GSM-R development prove achievable, set-up times will still be no better than two seconds, compared with less than a third of a second in TETRA. There may well be certain over- lap areas where these two systems will compete, but most observers now accept that TETRA and GSM can prosper very well side by side, between them meeting the needs of the great majority of mobile communications users.

TETRA already shows signs of making an impact world-wide as well as in Eur- ope, with many government and com- mercial PMR users demonstrating a very active interest. It is not too fanciful to hope that it could blossom into a truly international standard, as GSM has done in its own field. And whether or not that happens, TETRA will certady spur the development of some novel applications for PMR.

Philip Whitehead is Strategies Manager (New Products and Services) at Philips Telecom-PMR, EO. Box 24, St Andrews Road, Cambridge CB4 lDP, and chair- man of the TETRA memorandum of understanding working party on valid- ation.

IEE REVIEW JULY 1996