TETRA
Terrestrial Trunked Radio, a European standard for a trunked radio system, is a professional mobile radio and two-way transceiver specification. TETRA was specifically designed for use by government agencies, emergency services, for public safety networks, rail transport staff for train radios, transport services and the military. TETRA is the European version of trunked radio, similar to Project 25.
TETRA is a European Telecommunications Standards Institute standard, first version published 1995; it is mentioned by the European Radiocommunications Committee.
Description
TETRA uses time-division multiple access 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 can communicate direct-mode operation or using trunked-mode operation using switching and management infrastructure made of TETRA base stations. As well as allowing direct communications in situations where network coverage is not available, DMO also includes the possibility of using a sequence of one or more TETRA terminals as relays. This functionality is called DMO gateway or DMO repeater. In emergency situations this feature allows 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 are provided over the system's main control channel, while packet-switched data or circuit-switched data communication uses specifically assigned channels.
TETRA provides for authentication of terminals towards infrastructure and vice versa. For protection against eavesdropping, air interface encryption and end-to-end encryption is available.
The common mode of operation is in a group calling mode in which a single button push will connect the user to the users in a selected call group and/or a dispatcher. It is also possible 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. TETRA terminals can act as mobile phones, with a full-duplex direct connection to other TETRA Users or the PSTN. 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.
Advantages
The main advantages of TETRA over other technologies are:- The much lower frequency used gives longer range, which in turn permits very high levels of geographic coverage with a smaller number of transmitters, thus cutting infrastructure costs.
- During a voice call, the communications are not interrupted when moving to another network site. This is a unique feature, which dPMR networks typically provide, that allows a number of fall-back modes such as the ability for a base station to process local calls. So called 'mission critical' networks can be built with TETRA where all aspects are fail-safe/multiple-redundant.
- In the absence of a network, mobiles/portables can use 'direct mode' whereby they share channels directly.
- Gateway mode - where a single mobile with connection to the network can act as a relay for other nearby mobiles that are out of range of the infrastructure. A dedicated transponder system isn't required in order to achieve this functionality, unlike with analogue radio systems.
- TETRA also provides a point-to-point function that traditional analogue emergency services radio systems did not 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 cellular technologies, which connect one subscriber to one other subscriber, 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.
- Security TETRA supports terminal registration, authentication, air-interface encryption and end-to-end encryption.
- Rapid deployment network solutions are available for disaster relief and temporary capacity provision.
- Network solutions are available in both reliable circuit-switched architectures and flat, IP architectures with soft switches.
Disadvantages
Its main disadvantages are:- Security issues have been identified, although mitigations were quickly introduced in the TETRA security standard. There was an intentional weakening of the TEA1 cipher to meet export control criteria, allowing a break within a minute on consumer hardware.
- Requires a linear amplifier to meet the stringent RF specifications that allow it to exist alongside other radio services.
- Data transfer is slow by modern standards.
Both options permit the use of between one and four timeslots.
Different implementations include one of the previous connectivity capabilities, both, or none, and one timeslot or more.
These rates are ostensibly faster than the competing technologies DMR, dPMR, and P25 are capable of.
Latest version of standard supports 115.2 kbit/s in 25 kHz or up to 691.2 kbit/s in an expanded 150 kHz channel. To overcome the limitations many software vendors have begun to consider hybrid solutions where TETRA is used for critical signalling while large data synchronization and transfer of images and video is done over 3G / LTE / 5G.
Technical details
Radio aspects
For its modulation TETRA, uses differential quadrature phase-shift keying. The symbol rate is 18,000 symbols per second, and each symbol maps to 2 bits, thus resulting in 36,000 bit/s gross.As a form of phase shift keying is used to transmit data during each burst, it would seem reasonable to expect the transmit power to be constant. However it is not. This is because the sidebands, which are essentially a repetition of the data in the main carrier's modulation, are filtered off with a sharp filter so that unnecessary spectrum is not used up. This results in an amplitude modulation and is why TETRA requires linear amplifiers. The resulting ratio of peak to mean power is 3.65 dB. If non-linear amplifiers are used, the sidebands re-appear and cause interference on adjacent channels. Commonly used techniques for achieving the necessary linearity include Cartesian loops, and adaptive predistortion.
The base stations normally transmit continuously and receive continuously from various mobiles on different carrier frequencies; hence the TETRA system is a frequency-division duplex system. TETRA also uses FDMA/TDMA like GSM. The mobiles normally only transmit on 1 slot/4 and receive on 1 slot/4.
Speech signals in TETRA are sampled at 8 kHz and then compressed with a vocoder using algebraic code-excited linear prediction. This creates a data stream of 4.567 kbit/s. This data stream is error-protection encoded before transmission to allow correct decoding even in noisy channels. The data rate after coding is 7.2 kbit/s. The capacity of a single traffic slot when used 17/18 frames.
A single slot consists of 255 usable symbols, the remaining time is used up with synchronisation sequences and turning on/off, etc. A single frame consists of 4 slots, and a multiframe consists of 18 frames. Hyperframes also exist, but are mostly used for providing synchronisation to encryption algorithms.
The downlink is normally a continuous transmission consisting of either specific communications with mobile, synchronisation or other general broadcasts. All slots are usually filled with a burst even if idle. Although the system uses 18 frames per second only 17 of these are used for traffic channels, with the 18th frame reserved for signalling, Short Data Service messages or synchronisation. The frame structure in TETRA, consists of 18,000 symbols/s; 255 symbols/slot; 4 slots/frame, and is the cause of the perceived "amplitude modulation" at 17 Hz and is especially apparent in mobiles/portables which only transmit on one slot/4. They use the remaining three slots to switch frequency to receive a burst from the base station two slots later and then return to their transmit frequency.
Radio frequencies
| Country | Allocation | Frequency pairs |
| France | Civilian/private | 410–430 |
| France | Emergency services | 380–400 |
| Belgium | Emergency services/civilian | 380–386.5, 390–396.5 |
| Belgium | Commercial | 410-420 |
| the Netherlands | Emergency services | 380–386.5, 390–396.5 |
| the Netherlands | Civil/Commercial | 410–430 |
| Germany | Emergency services | 380–385, 390–395, 406 - 410 for DMO |
| Ireland | Civilian/private | 385–389.9, 395–399.9 |
| Ireland | Emergency services | 380–385, 390–395 |
| Italy | Emergency services / armed forces | 380–390 |
| Italy | Civilian/private | 462 |
| Norway | Emergency services | 380–385, 390–395, 406.1–426, 870–876 |
| Slovenia | Emergency services | 380-385, 390-395 |
| South Africa | Emergency services, Public works | 420–423 |
| Sweden | Emergency services | 380-395 |
| Sweden | Civilian/airport/public transportation | 425-429 |
| UK | Airwave | 390.0125–394.9875, 380.0125–384.9875 |
| UK | Airwave | 420.0125-421.9875, 410.0125–412.9875 |
| UK | Prison service | 450, 460 / 452, 462 |
| UK | AirRadio | 454, 464 or 460 |
| UK | Offshore Oil platforms | 423, 413 |
| Hong Kong | Emergency services | 382.65–399.9, 410–430 |
| Hong Kong | Civil/Private | 806–818, 851–863 |
| Portugal | SIRESP - Public Safety | 380–395 |
| Portugal | Commercial/Private | 420–430 |
| Saudi Arabia | 350–370, 380–395, 385–399.99, 410–430, 450–470, 870–921 |