Article

Contemplating Digital

Article published in Land Mobile, February 2003

The business and communications environment of the non-emergency public service users has changed significantly in the last ten to twenty years.

These users are the organisations in industry sectors such as the electricity, water, gas and transport - the majority of which have now been privatised or are likely to undergo some form of privatisation in the future. Currently the majority of these organisations use analogue FM PMR (Private Mobile Radio) systems. Since they installed their mobile radio communication systems there has been explosive growth in public mobile communications systems and other radio communications systems. The most significant has been the growth of GSM (Global System for Mobiles) networks

From my knowledge of the market sector, coupled with a few phone and face-to-face interviews, it becomes very apparent that there is no single self-evident solution available to meet the future needs of public service organisations. The way ahead is unclear for operational, regulatory and business reasons, the most significant being the increasing requirement for data services and the pressure on spectrum availability.

The choice of technology will depend on operational issues such as data requirements and the degree to which group calling and dispatch capability must be maintained. In addition, there are separate issues involving trade-offs between business and operational factors, such as network ownership and network sharing.

Supporting mobile workforce needs

Data transmission is becoming more in demand by mobile workforces, but is currently hindered by the existing systems data transfer rate of only 1.2 kbit/s. Mobile workforces need to be able to access databases, pictures, drawings, maps, status displays and more from central sources. Field workers also need to send details of what they find back to their headquarters. In particular, being able to send pictures back quickly would be of significant benefit to an organisation's operational efficiency.

Emerging digital systems

There are many emerging digital systems, and companies must unravel the merits of each looking at the various advantages and drawbacks.

TETRA (Terrestrial Trunked Radio), the European standard digital trunked system, allows four voice channels in 25 kHz of spectrum (twice as many as analogue FM). It also allows 4.8 kbit/s error-protected data transfer per channel and allows up to 19.2 kbit/s by aggregating four timeslots. This is better than circuit switched GSM, which only allows 9.6 kbit/s in one timeslot (nevertheless enhanced by the GPRS adaptation). In addition, TETRA wins out against GSM because calls can be set up almost instantly. Group voice calls can be set up on a single radio channel pair per coverage cell to all intended recipients - a major factor determining the amount of radio spectrum a network requires.

In a bid to keep TETRA attractive against the upcoming UMTS, the "TETRA Release 2" proposals promises system enhancements, offering far higher data bit rates. There are a number of proposals for higher-level modulation schemes, including a proposal to use 200 kHz-spaced channels and 8PSK, giving a data rate of 200 kbit/s and one to use 16QAM and COFDM, and another to use 25 kHz channels with lower data rates. However, additional spectrum blocks will be very difficult to find and such systems will be incompatible with current TETRA equipment.

There are two other PMR technologies which fit single radio carriers into existing 12.5 kHz spaced FM spectrum - Tetrapol (a system not compatible with TETRA, and not adopted by ETSI) and Discus. ETSI is also currently developing another wide-area high data rate service standard - until recently known as DAWS (Digital Advanced Wireless Service), but no DAWS systems are likely to be available in the near future.

Enhancing and adapting GSM

There are two new adaptations that are providing enhancements to GSM networks that will allow higher data rates by aggregating timeslots. These are HSCSD (High-Speed Circuit Switched Data) for services requiring a continuous connection, such as video, and GPRS (General Packet Radio Service) that enables use of the Internet Protocol, suitable for bursts of data and fixed images downloaded from the Internet or an Intranet. The data rate, of course, depends on the error protection required and the loading of the network, but usable rates of 30 or 40 kbit/s will be possible in HSCSD and GPRS mode. Finally there is the much-publicised next-generation cellular system called UMTS (Universal Mobile Telecommunication System).

Other adaptations of GSM are now available - GSM-R and GSM-Pro, which do allow group voice calls. GSM-R requires a new private network separate from the public GSM networks as it was specifically aimed at European Railways. GSM-Pro uses standard GSM networks, namely not HSCSD or GPRS, and simply dials-up groups of subscriber numbers simultaneously. Being slow, wasteful of channels and only supported by Ericsson, it is not a serious contender.

The European Railways have chosen to use GSM-R systems. Based upon the ETSI GSM Phase 2+ standard for enhanced calls these private GSM-R systems will allow trains to pass over international borders on a common system and also provide secure communications for train movements and track staff. GSM-R allows group calls and dispatcher operation, and data passes via GPRS. Data rates using all eight slots are possible giving a peak rate of over 100 kbit/s. European spectrum has been already been allotted as two (uplink and downlink) blocks of 4 MHz near the GSM 900 MHz bands. The West Coast Main Line is the first line to be equipped with a GSM-R system.

GSM-R is a better choice for railways than UMTS, since UTMS high data rates are not possible at very high speeds in multipath fading environments. Clearly, public safety is another critical factor and the railways wished to adopt a standard based upon proven GSM technology. The public utilities (electricity, gas and water) could consider having a GSM-R system too, particularly if they feel that TETRA would not provide sufficient data transmission capacity. However, spectrum may be difficult to obtain, particularly since GSM uses 200 kHz-spaced carriers.

If UMTS networks will provide suitable call services for private network users, such as closed user group, broadcast and multicast, priority and pre-emption services. Private companies would require access to the UMTS system as a VPN (Virtual Private Network) user. If UMTS will provide such services in future, it may prove to be the single wide-area system solution that has been sought by the public utility and road transport organisations for many years. However, even the basic services of UMTS may take far longer than was expected when the five UK operators paid over £20 billion for their licences in 2000. They are bound by their conditions of licence to achieve only 80 percent coverage of the population by 2007, so rural areas may be far less well covered then than GSM/GPRS is now.

Group calls, broadcast calls

Until now, a substantial proportion of voice traffic on private systems has been of a group nature. It's not yet entirely clear to what degree workers will require broadcast and group, or multipoint/multicast, communication at high data rates. Large transfers may only be required between individuals or from a database to a single individual and some organisations appear to believe that workers only need small data downloads over the air. While for others, mass information can be replicated on CDs and issued to all workers to use on laptop computers.

The problem of group data transactions is ensuring all members receive the data at the same time and without significant errors. Not only are return channels required for error control and acknowledgement but intended recipients may be engaged on other calls or may be out of coverage. Except for data which is not required to be stored, such as live pictures, the use of single radio channel pairs for group and broadcast calls becomes impracticable.

Major manufacturers have recently adapted even TETRA - specifically designed for group and broadcast transmissions - for IP (Internet Protocol) mode communication of data as well as circuit. Since in IP mode, it cannot send data simultaneously to multiple subscribers using single pairs of radio frequencies, one of the key benefits of its technology is lost. If data sent to groups of workers require multiple channels there may be serious repercussions on the amount of spectrum and base station transceiver infrastructure that a system requires.

Sharing radio spectrum

There is no guarantee that licences for current, and less spectrum-efficient, analogue FM systems will be renewed indefinitely. The radio spectrum is subject to increasing pressure by all types of existing and potential radio users within the UK, Europe and the rest of the world. Companies may be forced by commercial factors to make the greatest use of sharing with other companies or even enter into service contracts with the public cellular operators.

Obtaining TETRA spectrum from the RA (Radio Communications Agency) is a significant problem in making a new TETRA system difficult to procure. The RA have so far only reserved spectrum for the public safety radio service network (Airwave) and for the public access network (Dolphin). However, the RA is currently also considering the allocation of the paired bands 872 to 876 and 917 to 921MHz to TETRA private networks users (bands also available for TETRA in Europe). Any privately-owned TETRA system replacing the analogue systems will be considerably more expensive since the numbers of base station sites required will be much greater at frequencies around 400 or 900 MHz the number of base station sites required will be much greater.

The TETRA system, Airwave, being built by O2 for use by the public safety organisations is impossible because the spectrum (in the 380 to 400 MHz range) has been allotted solely for public safety use in Europe.

What are the technology options?

Companies must first decide which system will meet their communication needs over the life of the system, and also whether they, or another party, should own it. This may have business advantages, but has operational drawbacks.

Many organisations attempting to resolve these problems are retaining their current analogue trunked systems and may consider adopting GSM/GPRS or UMTS services, offered by the national operators, to supplement their communication needs, in particular for long individual voice calls and for high-speed packet data transfer.

Switching to TETRA may only be an option in the future if spectrum is available and the replacement costs are favourable. And if TETRA spectrum is indeed not available, their current systems may become unserviceable, and they will face a significant problem. Even so, once a TETRA system is obtained it may be overtaken in the future in many aspects of data rate functionality by GPRS and UMTS. Some organisations are even contemplating re-equipping their 10-year old analogue trunked systems with new analogue software and hardware, while others are considering scrapping their systems and relying in the future on GSM services provided by one of the national operators.

In the meantime, for those companies not able to acquire spectrum or justify a TETRA system, GPRS appears to be the best prospect to try out applications that make use of its relatively high data rates and Internet/Intranet connection capability. It will then be possible for the companies to make their data connections to and from their mobile staff via their Intranet.

Buses, coaches and super-trams currently use local area analogue non-trunked systems, mobile data systems, GSM and also trunked systems in major cities. Provided that spectrum is available, they may benefit from self-provided local TETRA systems in the future, particularly for trams. Since their mobile data requirements are regular but modest, and include location and other status information; they may consider using one of the four national GSM/GPRS systems.