TETRA Release 1 (Voice + Data) already provides a very comprehensive portfolio of services and facilities but as time progresses there is a need to evolve and enhance all technologies to better satisfy user requirements, future proof investments and ensure longevity. Like GSM moving to GPRS, EDGE and UMTS/3G, TETRA also needs to evolve to satisfy increasing user demand for new services and facilities as well as gleaning the benefits of new technology.
As early as 1999, interest groups comprising both users and manufacturers within Technical Committee (TC) TETRA and the TETRA Association identified the need to enhance TETRA in several areas. Although the initial number of areas identified were very comprehensive, significant events in the telecommunications industry, combined with changing market needs, resulted in the following services and facilities being standardised at the end of 2005 as part of TETRA Release 2.
- Trunked Mode Operation (TMO) Range Extension
- Adaptive Multiple Rate (AMR) Voice Codec
- Mixed Excitation Liner Predictive, enhanced (MELPe) Voice Codec
- TETRA Enhanced Data Service (TEDS)
Trunked Mode Operation (TMO) Range Extension
The ability for TETRA to operate beyond the 58 km range limit (a function of TETRA’s TDMA structure) was required by certain user organisations to allow efficient Air-Ground-Air (AGA) communications whilst operating on the main TMO network. By modifying uplink and downlink bursts, as well as guard times, the TMO range of TETRA is 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).
Adaptive Multiple Rate (AMR) Voice Codec
The AMR codec, operating in the 4.75 kbits/s only mode, has been chosen for possible future applications in TETRA. However, completion of the Air Interface Standard to accommodate the AMR codec is suspended in TC TETRA until sufficient market need is identified
Mixed Excitation Liner Predictive, enhanced (MELPe) Voice Codec
The STANAG 4591 (MELPe codec), to use its correct NATO reference, has been standardised by NATO for its own military communication applications because of its low bit rate (2400 bit/s), immunity to high background noise and acceptable voice quality performance. Because of TETRA’s suitability for certain military communication applications TC TETRA carried out a technical feasibility study to see if could be supported on TETRA. The results of this study indicated potential benefits such as
- Interworking with government systems (no tandem operation)
- Suppression of background noise
- Improved RF Coverage using spare bits available for extra FEC
- Simultaneous V+D using spare bits available for data
However, the way the MELPe codec needs to be implemented in TETRA increases “end to end” voice delay, which needs to be balanced against its possible benefits. Completion of the TETRA standard to accommodate the MELPe codec will be dependent on the outcome of cost/benefit comparisons with the existing TETRA codec, which will be carried out in TC TETRA.
TETRA Enhanced Data Service (TEDS)
TEDS is a new TETRA High Speed Data (HSD) service using different RF channel bandwidths and data rates for flexible use of PMR frequency bands. TEDS is fully compatibility with TETRA Release 1 and allows for ease of migration. It has been optimised for efficient use of PMR frequency bands and designed for all TETRA market segment applications. The RF channel bandwidths supported in TEDS are:
- 25 kHz
- 50 kHz
- 100 kHz
- 150 kHz
The modulation schemes supported in TEDS are:
- pi/4 DQPSK (for common TETRA V+D and TEDS control channel)
- pi/8 D8PSK (for early migration requiring modest increase in speed)
- 4 QAM (for efficient links at edge of coverage)
- 16 QAM (for moderate speeds)
- 64 QAM (for high speeds)
Figure 1 is a matrix showing the different RF channel bandwidths and data rates supported in TEDS. The figures are obtained after allowance is made for the synchronization and pilotsymbols, channel coding and lower layer protocol headers and functions. The throughput assessment is based on using MAC-U-BLCK PDU for the uplink and MAC-D-BLCK PDU, without slot granting element, for the downlink in an original advanced link.
|Channel Type (Bandwidth)|
Note: All channels are 4 slots
Figure 1: Packet Data Throughput Downlink [kbit/s]
With adaptive selection of modulation schemes, RF channel bandwidths and coding according to propagation conditions, user bit rates in the region of 10 to 500 kbits/s can be expected. For ease of evolution and migration from TETRA Release 1 reuse of the TETRA protocol stack and TDMA structure have been maximised. 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. Support for sectored cells is also provided enabling the use of existing TETRA Release 1 Base Sites for TEDS without the need for additional sites. Even though TEDS is capable of providing High Speed Data in 150 kHz RF channels, the current limitation caused by insufficient RF spectrum to support the growth of TETRA will probably limit early deployments to 50 kHz RF channel assignments only.
Now that the TETRA Release 2 standards are sufficiently complete for product development purposes, actual product availability will be dependent on the different manufacturers development plans.