TETRA 2 provides a major enhancement in capabilities of the TETRA radio system. The highlight addition provided by TETRA 2 is the TETRA TEDS or TETRA Enhanced Data service which provides more flexibility and greater levels of data capability for the TETRA radio system.
The TETRA Release 2 with TETRA TEDS has enabled a significant increase in performance level to meet the growing needs of users.
TETRA Release 2
Although the original TETRA 1 release provided many of the features needed by users, TETRA Release 2 provided additional functions and improvements to enable the system to keep pace with the growing requirements and taking account of what could be realised easily as technology advances.
The main areas where TETRA Release 2 has provided improvements are summarised below:
- Trunked Mode Operation, TMO, Range Extension: As with the GSM cellular telecommunications system, the TDMA technology used meant that there were maximum time delays that could be tolerated. This translated into a range limitation. The reason for this relates to the fact that uplink and downlink transmissions are allocated a time slot, and guard times must be allocated to account for the transmission travel time. If the transmission arrives after the guard time has passed, data from the transmission will be missed and the burst will not be accepted.
For TETRA radio Release 1, the maximum range was 58km. However for a number of applications including air to ground, i.e. air-ground-air, AGA, greater distances were needed. To achieve this, the uplink and downlink bursts and guard times have been modified in TETRA Release 2. These changes have enabled the TMO range of TETRA Release 2 to be extended up to 83 km for AGA applications.
Note: DMO has no TDMA structure range limitation as synchronisation takes place in DMO at the start of each transmission.
- TETRA Enhanced Data Service, TEDS: TEDS is the new TETRA High Speed Data, HSD, service that has been introduced. To achieve the higher data rates, TETRA TEDS uses different RF channel bandwidths and modulation schemes.
TEDS is fully backwards compatibility with TETRA Release 1 and as a result it enables easy migration from Release 1 to Release 2. Further details for the TETRA TEDS technology is given below.
- Adaptive Multiple Rate, AMR, Voice Codec: The AMR codec has been very successfully used within GSM and UMTS for example. It has been identified as being able to provide significant advantages for some TETRA radio applications. For TETRA radio, the AMR codec operating in the 4.75 kbits/s only mode, has been chosen for possible future applications. However, completion of the Air Interface Standard to accommodate the AMR codec is being delayed until a sufficient market need is identified.
- Mixed Excitation Liner Predictive, enhanced, MELPe, Voice Codec: Improvement of the codec system was another area of improvement. To this end one of the codecs implemented was the Mixed Excitation Liner Predictive, enhanced (MELPe) Voice Codec. This is also defined under the military designation STANAG 4591 (MELPe codec). This has been standardised by NATO for its own military communication applications because of its low bit rate - 2400 bps - immunity to high background noise and acceptable voice quality performance. As a result of the many applications where TETRA radio is required to interface with military networks, this choice provides many advantages. Although not yet implemented, feasibility studies are underway to achieve this.
TETRA TEDS within Release 2
The TETRA Enhanced Data Service, TEDS achieves much higher data rates than was previously achievable with the TETRA Release 1 radio system. It achieves this by using wider bandwidths and adaptive modulation schemes.
TETRA TEDS four different RF channel bandwidths:
- 25 kHz
- 50 kHz
- 100 kHz
- 150 kHz
It also allows for adaptive modulation, choosing the optimum modulation format for the prevailing conditions and requirements. The modulation schemes supported in TEDS are:
- π/4 DQPSK - used for common TETRA V+D and TEDS control channel.
- π/8 D8PSK - for early migration requiring modest increase in speed.
- 4QAM - used for efficient links at edge of coverage.
- 16QAM - used for moderate data rates.
- 64QAM - for high data rates.
The different combinations of modulation and channel bandwidth enable a variety of different data rates to be achieved.
|25 kHz||50 kHz||100 kHz||150 kHz|
Using the adaptive modulation schemes and variable channel bandwidths user data rates of between 10 and 500 kbps can be expected. This is sufficient for many new applications, allowing video to be transmitted over some links as required.
With the move towards more rich media services such as video, TEDS also allows up to 8 multimedia applications and QoS negotiation for real-time class data applications, such as voice and video and telemetry, with the QoS attributes negotiated being; throughput, delay, priority and reliability. In this way the system is able to provide the optimum service for the available bandwidth, user priorities and channel conditions.
A further advantage of TEDS is that there is support for sectored cells enabling the use of existing TETRA Release 1 Base Sites for TEDS without the need for additional sites.
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