TETRA

Terrestrial Enhanced(changed from European) Trunked Radio (TETRA) is a specialist Professional Mobile Radio and walkie talkie standard used by police, fire departments, ambulance and military. Its main advantage over technologies such as GSM are:
the much lower frequency used, which permits very high levels of geographic coverage with a smaller number of transmitters, cutting infrastructure cost.
fast call set-up - a one to many group call is generally set-up within 0.5 seconds compared with the many seconds that are required for a GSM network.
the fact that its infrastructure can be separated from that of the public cellphone network, and made substantially more diverse and resilient by the fact that base stations can be some distance from the area served.
unlike most cellular technologies, TETRA networks typically provide a number of fall-back modes such as the ability for a base station to process local calls in the absence of the rest of the network, and for 'direct mode' where mobiles can continue to share channels directly if the infrastructure fails or is out-of-reach.
gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of contact with the infrastructure.
TETRA also provides a point-to-point function that traditional analogue emergency services radio systems didn't provide. This enables users to have a one-to-one trunked 'radio' link between sets without the need for the direct involvement of a control room operator/dispatcher.
unlike the cellular technologies, which connects one subscriber to one other subscriber (one-to-one) then Tetra is built to do one-to-one, one-to-many and many-to-many. These operational modes are directly relevant to the public safety and professional users.
Radio aspects
TETRA uses a digital modulation scheme known as PI/4 DQPSK which is a form of phase shift keying. TETRA uses TDMA (see above). The symbol rate is 18,000 symbols per second, and each symbol maps to 2 bits. A single slot consists of 255 symbols, a single frame constist of 4 slots, and a multiframe (whose duration is approximately 1 second) consists of 18 frames. As a form of phase shift keying the downlink power is constant. The downlink (i.e. the output of the basestation) is a continuous transmission consisting of either specific communications with mobiles, synchronisation or other general broadcasts. Although the system uses 18 frames per second only 17 of these are used for traffic channel, with the 18th frame reserved for signalling or synchronisation. TETRA does not employ amplitude modulation. However, TETRA has 17.65 slots per second (18000 symbols/sec / 255 symbols/slot / 4 slots/frame), which is the cause of the PERCEIVED 'amplitude modulation' at 17Hz.