ETSI Standard

Nowadays, practically everything electronic uses digital technology and wireless communications are no exception.  Even though analogue FM PMR communications will remain a viable option for several years, digital radio provides relative advantages and disadvantages in the important performance areas of:

• Voice Quality
• RF Coverage
• Non-Voice Services
• Security
• Cost

Trunking techniques have been used for many years in switched telephone networks.  The first trunked mobile radio communication systems were deployed as early the 70’s in North America with proprietary signalling protocols and shortly afterwards in Europe using analogue MPT1327 technology. The main benefit of trunking is normally seen as spectrum efficiency, or more radio users per RF channel compared with a conventional radio channel for a given Grade of Service (GoS), brought about by the automatic and dynamic assignment of a small number of communication channels shared amongst a relatively large number of users.Because trunking systems support more radio users than conventional systems, national administrations actively support the deployment of trunking systems as this helps reduce pressure on meeting PMR spectrum demands.  However, from a radio users operational point of view, spectrum efficiency does not really mean anything. What users want is to solve all the operational problems associated with conventional PMR, yet still retain the simplicity of conventional open channel ‘all informed net’ operation.  The fundamental element of trunking that solves these conventional PMR problems is the use of a control channel. Table 1 below lists the operational problems of conventional PMR and also lists how the use of trunking solves these problems. It is important to note that the operational simplicity of conventional PMR ‘all informed net’ talk group communications is still retained by employing fast call set-up “Push To Talk” (PTT) operation on radio terminals.

As the control channel acts as a signalling communications link between the Trunking Controller and all mobile radio terminals operating on the system, the Trunking Controller knows the status of the system at any moment in time as well as its historic usage, which is stored in its memory.  For example, the Trunking Controller knows:

• The individual and group identity of all radio units registered on the system
• The individual identity and time radio units registered on the system
• The individual identity and time radio units de-registered from the system
• The individual and group identity, time and duration of all messages

With additional intelligence in both the radio terminals and the trunking controller the advantages and benefits of trunking can be increased.  For example, the length of the control channel signalling messages can be increased by a set amount to accommodate a variety of new services and facilities.  Also, the trunking controller can be programmed to handle calls in a variety of ways as required by the operator of the system.

A four time slot TDMA technology was adopted in TETRA as it offered the optimum solution to balance the cost of equipment with that of supporting the services and facilities required by user organisations for a medium to high capacity network providing single site local RF coverage and/or multiple site wide area RF coverage.

RF Spectrum efficiency is a combination of three main factors being the occupied bandwidth per communication channel, the frequency re-use factor determined by the Carrier to Interference protection ratio C/I in dB’s and the trunking technology used.  As previously mentioned TETRA utilises the latest in trunking technology.  Also, the TDMA technology used in TETRA provides 4 independent communications channels in a 25 kHz RF bandwidth Channel, making it twice as efficient in occupied bandwidth terms as a traditional 12.5 kHz RF bandwidth FDMA channel.  Although FDMA technologies tend to have a better C/I performance than TDMA TETRA, the overall spectrum efficiency advantage lies with TETRA, especially for medium to high capacity networks.

Because of using TDMA technology, the cost and equipment space at base station sites can be significantly reduced compared with traditional FDMA technology trunking solutions. Another advantage of TDMA technology is that it enables new services and facilities to be supported with minimum cost.  Some examples are:

• Higher Data Rates
  • The ‘laws of physics’ limits the maximum data rate in a given RF channel bandwidth.  Assuming the same modulation scheme, the wider the channel bandwidth the higher the data rate.  Because TDMA uses wider channels than FDMA, the combined data rate on a single RF carrier is greater.
• Improved Data Throughput in Poor RF Signal Conditions
  • The net data rate in TDMA is better than FDMA in poor RF propagation conditions.  This is because Automatic Repeat Requests (ARQ’s) are required when received data is corrupted as a result of RF fading.  As TDMA terminal devices effectively operate in full duplex ARQ’s can be sent efficiently after each time slot transmission instead of waiting until the end of each voice transmission, as is usually the case with FDMA.
• Bandwidth on Demand
  • In TDMA any number of time slots up to the maximum limit of the technology being employed can be combined to increase data throughput as required for specific applications.
• Concurrent Voice and Data
  • Because of the TDMA time slot structure it is possible to assign one time slot to support voice and the next time slot to support data in a two slot transmission from radio terminals.  This capability effectively allows a single radio terminal to concurrently transmit or receive voice and data at the same time.
• Full Duplex Voice Communications
  • TDMA technology inherently supports full duplex communications.  Although full duplex voice communications can be supported on FDMA systems the need for duplex operation requires RF screening between the transmitter and receiver and also a duplexer to allow single antenna working.  Because of this, duplex FDMA radio terminals are usually bulkier and more costly to produce than TDMA terminals, which do not need RF screening or antenna duplexers.