TETRA means Terrestrial Enhanced (changed from european) Trunked Radio is a specialist Professional Mobile Radio & Walkie talkie standard used by police, fire departments, ambulance & military. Its main advantage over technologies such as GSM : the much lower frequency used , which permits very high levels of geographic coverage with a smaller no. of transmitters, cutting infrastructure cost.

Fast Call Set-Up a one to many group call is generally setup within 0.5 sec compared with the many seconds that r required for a GSM network.

The fact that its infrastructure can b separated from that of the public cell phone network & made substantially more diverse & resilient by the fact that base stations can b some distance from the area served.

Unlike most cellular technologies ,TETRA networks typically provide a no. of fallback modes such as the ability for a base station to process local calls in the absence of the rest networks & for direct mode whre mobiles can continue to share channels directly if yhe infrastructure fails or is out of reach.

Gateway Mode where a single mobile with connection to the networks can act as a relay for other nearby mobile that r out of contact with the infrastructure.

TETRA also provides a point to point function that traditional analogue emergency services radio system didnt provide. This enables users to have a one to one truncked radio link b/w sets without the need for the direct involvement of a control room operator / dispatcher.

Unlike the cellular technologies , which connects one subscribers to one other subscriber one to one then TETRA is built to do one to one ,one to many & many to many.Yhese operational modes r directly relevant to the public safety & professional users.

Radio Aspects :

TETRA uses a digital modulation scheme know as PI/4 DQPSK which is a form of phase shift keying. TETRA uses TDMA. The symbol rate is 18000 symbols per second & each symbol maps to 2 bits. A single slot consists of 255 symbols, a single frame consists of 4 slots & a multiframe(whose duration is approx. 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 base station) is a continuous transmission consisting of either specific communications with mobiles, synchronization or other general broadcasts. Although the system uses 18 frames per sec. only 17 of these r used for traffic channel, with the 18th frame reserved for signaling or synchronisation. TETRA doesnt employ amplitude modulation. However, TETRA has 17.65 slots per sec.(18000 symbols /sec 255 symbols/slot/4 slot/frame), which is the cause of the PERCEIVED amplitude modulation at 17Hz.

Conclusion is that To combat the traditional analogue emergency services flaws & brickbats, TETRA is the technology to watch out for.On demand requirement has pushed the researchers to improvise on the front of this digital modulation technique.

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