Terrestrial Trunked Radio

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The TETRA logo

TErrestrial Trunked RAdio (TETRA) (changed from Trans European Trunked Radio) is a specialist Professional Mobile Radio and walkie talkie standard used by police, fire departments, ambulance and military.

TETRA is an ETSI standard, first version published 1995. TETRA is endorsed by the European Radio Communications Committee (ERC).

1 Description
2 Advantages
3 Disadvantages
4 Radio Frequencies
5 Radio aspects
6 See also
7 External links

[edit] Description

TETRA terminals can act as cell phones, with a direct connection to the PSTN. It is common also for them to operate in a group calling mode in which a single button push will connect the user to a dispatcher and all the other users in a group. Also possible is for the terminal to act as a one to one walkie talkie but without the normal range limitation since the call still uses the network. Emergency buttons, provided on the terminals, enable the users to transmit emergency signals, to the dispatcher, overriding any other activity taking place at the same time.

TETRA uses Time Division Multiple Access (TDMA) with four user channels on one radio carrier and 25 kHz spacing between carriers. Both point-to-point and point-to-multipoint transfer can be used. Digital data transmission is also included in the standard though at a low data rate.

TETRA Mobile Stations (MS) can communicate Direct Mode or using Trunked infrastructure made of TETRA Base Stations (TBS). As well as allowing direct communications in situations where network coverage has been lost, DMO also includes the possibility of using one (or a chain) of TETRA terminals as relays for a signal. In rescue situations this feature could allow direct communications underground or in areas of bad coverage.

In addition to voice and dispatch services, the TETRA system supports several types of data communication. Status messages and short data services (SDS) are provided over the system's main control channel, while Packet Data communication uses specifically assigned traffic channels.

All traffic is normally encrypted. TETRA provides both over the air encryption and end-to-end encryption.

[edit] Advantages

The main advantages of TETRA 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 connect 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.

[edit] Disadvantages

Its main disadvantages are:

it can only support a much lower teledensity in a given area, compared to GSM and similar technologies (which is not a problem in the applications for which it is used, but mostly limits it to these applications).
handsets are more expensive (about 750 EUR in 2003), due to the reduced economies of scale, different business model when compared to mass-market cellphones, need for security evaluation, and robustness.
data transfer is slow at 7.2 kbit/s per timeslot (4.8 kbit after error correction), although up to 4 timeslots can be combined into a single data channel to achieve higher rates, due to need to fit into 25kHz bandwidth channel plans.
due to the pulsed nature of TDMA employed by the protocol, handsets can interfere with sensitive electronic devices such as heart pacemakers and defibrillators, as can other RF transmitting equipment when used in close proximity (e.g. < 1 metre distance.)

[edit] Radio Frequencies

In Europe, TETRA uses frequencies:

Emergency Systems
Number	Frequency Pair (MHz)
	Band 1	Band 2
1	380-383	390-393
2	383-385	393-395

Civil Systems
Number	Frequency Pair (MHz)
	Band 1	Band 2
1	410-420	420-430
2	870-876	915-921
3	450-460	460-470
4	385-390	395-399.9

In Britain the public sector TETRA system operates under the name 'Airwave'. In Belgium, the 'A.S.T.R.I.D' system uses TETRA. In the Netherlands, the TETRA system is called 'C2000'. In Sweden the RAKEL system uses TETRA.

[edit] Radio aspects

TETRA uses a digital modulation scheme known as π/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 consist 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.