Lowe Electronics

TETRA stands for ‘Terrestrial Trunked Radio’.

The channel access procedure used for TETRA is the TDMA (Time Division Multiple Access) method.

TETRA provides four independent communication channels within one 25kHz radio channel. One individual channel is occupied only for one quarter of the time for the duration of a time slot. During the remaining period, the radio channel can be used by other users. On a call, the transmitter is therefore switched on and off approximately 18 times per second. If necessary for high capacity data transmission, one individual user can also occupy multiple time slots. Both voice and data can be transmitted simultaneously by a single user.

As for GSM, in the TETRA system the control channel is also transmitted on a specific carrier in the first slot in each frame. This control channel is used to transfer the network's system data to the handsets and to allow synchronisation between handsets and the base station.

The transmitting power of the mobile equipment is continuously regulated to the minimum which the base station needs to detect the data from the mobile equipment (power control). By means of this power control, interference can be minimised and the battery life of the mobile equipment can be maximised. The transmitting power of the base station is not regulated in TETRA.

The modulation method used is p/4-DQPSK (Differential Quaternary Phase Shift Keying). Though this linear modulation method is extremely spectrum-efficient, it requires very linear and therefore expensive transmitter final stages in order to minimise undesirable out-of-band transmissions.

TETRA's maximum data rate is up to 28.8 kbit/s (non-protected) for a 25kHz channel bandwidth. This relatively high data rate makes TETRA especially suitable for use in mobile data transmission.

In principle, TETRA can be used on all frequencies below 1 GHz. In practice, however, only the typical frequencies authorised for PMR in the 160, 400 and 870 MHz band are used.

The TETRA specifications cover three quite different areas of application, namely:

Voice plus Data (V+D);
Packet Data Optimised (PDO);
Direct Mode.

Different standards have been developed for these three applications. However, these are based on the same physical radio platform (modulation, RF channel spacing, frequencies, etc.).

Equipment complying with the V+D specification offers a wide range of carrier services,
teleservices and supplementary services for hybrid voice and data transmission.

Direct mode allows a direct link from mobile station to mobile station without involving a base station. This mode is used when users are outside the coverage area.

As with most radio systems, TETRA also uses the frequency duplex method (apart from
direct mode, see below). The up-link and down-link are handled on two different frequencies, which are separated from each other by so-called duplex separation. The size of this duplex separation depends on the frequency band in which the system is operated.

In the case of direct mode, only simplex operation is possible. The mobile stations involved in a call work on the same frequency. In the case of direct mode, up to two independent simplex calls can be maintained simultaneously on one carrier.

TETRA standardisation has reached a mature state. The major current activities are the maintenance of existing TETRA standards, and the standardisation of the next generation of services in TETRA Release 2. The objective of TETRA Release 2 is that EP TETRA produces an additional set of ETSI deliverables (and maintenance thereafter) in order to enhance TETRA in accordance with the following requirements:

Evolution of TETRA to provide packet data at much higher speeds than are available in the current standard. This is to support multi-media and other high speed data applications required by existing and future TETRA users.
Selection and standardisation of additional speech codec(s) for TETRA, to enable inter-communication between TETRA and other 3G networks without transcoding, and to provide enhanced voice quality for TETRA by using the latest low bit-rate voice codec technology.
Further enhancements of the TETRA air interface standard in order to provide increased benefits and optimisation in terms of spectrum efficiency, network capacity, system performance, quality of service, size and cost of terminals, battery life, and other relevant parameters.
Production and/or adoption of standards to provide improved interworking and roaming between TETRA and public mobile networks such as GSM, GPRS and UMTS.

Evolution of the TETRA SIM (Subscriber Identity Module), with the aim of convergence with USIM, to meet the needs for TETRA specific services whilst gaining the benefits of inter-working and roaming with public mobile networks such as GSM, GPRS and UMTS.
Extension of the operating range of TETRA, to provide increased coverage and low cost deployments for applications such as airborne public safety, maritime, rural telephony and ’linear utilities’ (eg pipelines).
Provision of new ETSI deliverables in order to support further user/market driven requirements that may be identified during study work in the early stages of the work programme.
Ensure full backward compatibility and integration of the new services with existing TETRA standards, in order to future-proof existing and future investments by TETRA users.

These requirements are in addition to the user requirements for PMR/PAMR that are already satisfied by existing TETRA standards. The work will include completion and formal approval of outstanding work related to these existing requirements. The work will build upon the unique combination of services and facilities already included within existing standards.