TETRA - Terrestrial Trunked Radio

Applications

TETRA (TErrestrial Trunked RAdio) has been designed to fulfil the requirements of users in Private Mobile Radio (PMR), Land Mobile Radio (LMR), Public-Access Mobile Radio (PAMR) and public safety and security applications such as police, border patrol and coast guard, fire departments and ambulances.

The main TETRA features supporting the requirements from all these user groups are

• low bandwidth (in Europe, ideally fitting to the existing 25 kHz channel spacing in use by analogue systems)
• support of both voice and data transmission at various data rates
• possibility of encrypting channels to prevent eavesdropping
• fast call setup time
• full and semi-duplex calls (push-to-talk operation)
• individual and group calls
• broadcast channels
• support of direct mode in emergency cases where a network is not accessible (e.g. in tunnel rescue cases)
• support of packet (PDO) and high speed data transmission (BRAN)

The acronym PMR stands for applications where the user group (usually one company) owns the radio network.

PAMR (Public Access Mobile Radio) networks are operated by companies that are not affiliated with the user groups. These networks are comparable to public cellular telephone networks, except that they offer their services to organisations such as truck and courier services, taxis and other car fleets.

The web pages of the TETRA MoU group (www.tetramou.com) list a long number of applications and existing networks. TETRA competes against a proprietary technology from EADS called Tetrapol.

Following up the way to the European Union and the Schengen Agreements, the need for a radio communication system covering all requirements of the European security administrations (Police, Border Patrol, Fire Department, Ambulance etc.) was raised in the late 80’s. In 1989, administrations, manufacturers and users of PMR¹  radios joined together within ETSI to establish a binding standard called TETRA, (former “Trans European Trunked RAdio”, today “TErrestrial Trunked RAdio").

Already some years before that, in the mid 80’s, other PMR system suppliers had developed some own proprietary standards for digital trunked radio because of customer demands for spectrum efficiency and interception safety. One of these standards is TETRAPOL, which was invented by Matra and is now the main competitor in European PMR/PAMR²  networks.

In Europe, all EC states have committed within the Schengen agreement to set up a digital trunked network supporting interoperation with the neighbor states. Unfortunately, some states decided for a TETRAPOL network (France, Switzerland, Spain etc.) and others for a TETRA network (UK, Poland, BeNeLux etc.). So the intention of the standardization, to create a common network with roaming capabilities, was not achieved. Germany has still raised no decision, which standard will be implemented.

¹PMR is the abbreviation for Private Mobile Radio, which means the market of radio systems, which are operated by the user himself. Those networks are mainly simple two-way radio networks.
²PAMR (Public Access Mobile Radio) networks are PMR networks operated not by the user organisation or the user himself, but by a independent public or private operator comparable to public cellular telephone networks.

Technology in brief

TETRA is a TDMA standard, similar to the GSM standard. It uses four timeslots per carrier, the carrier bandwidth is 25 kHz. Similar to GSM, the first timeslot on the first carrier transmits the BCCH, a logical channel that bears synchronisation and control data.

The connection between mobile radio and base station is separated into two bands for uplink and downlink (frequency division duplex).

TETRA uses π/4 DQPSK (Differential Quaternary Phase Shift Keying). This modulation is highly efficient with spectrum resources, but requires high linearity of all RF components, especially the RF power amplifiers in the radios.

The TETRA services are based on three major service classes with different air interfaces, all specified by ETSI:

• Voice plus Data (V+D), circuit switched speech and data transmission, (ETS 300 392)
• Packet Data Optimized (PDO), data traffic based on packet switching, (ETS 300 393)
• Direct Mode (DMO), a simplex voice transmission between two mobiles without using a network. On a physical channel two simultaneous DMO calls can be established. (ETS 300 396)

Features

Voice and data services within TETRA were specially standardised to meet the requirements of all security administrations. The users of former PMR and public safety and security radio systems do not need to change their communication behaviour, as nearly all features of those systems are now available in TETRA, too. Moreover, many additional services are available. TETRA services are divided in tele services, bearer services and additional services. Here is an overview of the most important TETRA features:

Tele Services:

• Individual Call connects one user of the network with one other user, comparable to a public telephone system.
• Group Call connects one user with a group of other users; groups do not have to be fixed but can be formed dynamically. It is possible to configure the group call, that it is necessary for every user to confirm reception of the call. This enables the calling station to be sure, that all users have received the call.
• Broadcast Call: A broadcast call is sent out of a control centre to inform all users. The users do not confirm the call.
• Emergency Calls are handled by the TETRA MSC with a high priority to enable a quick connection to a dispatcher or a group.
• Direct Mode (DMO): Two users connect themselves directly in simplex mode without using a TETRA network.
• Open Channel is a service that behaves very similar to an analogue two-way radio channel, every participant can talk or listen free.
• Call Include allows adding additional users to a group call that is already established and running.

Bearer Services:

• User Status Transmission comparable to FSK data-based status message systems, used to transmit short, predefined status messages (like “Patrol car on duty”, “Work order received” or “Fire engine back in fire station”) from the user towards the dispatching control room or vice versa.
• Short Data Service transmits short text messages between the users (cp. SMS in GSM)
• Circuit Switched Data Services in unprotected mode (7.2 kbit/s per timeslot), standard encryption (4.8 kbit/s per timeslot) and high encryption (2.4 kbit/s per timeslot)
• Packet Switched Data Services based on TCP/IP or X.25 protocol, depending on application, with a maximum data rate of 28.8 kbit/s

Additional Services:

• Priority and preemption services: Access priority, discrete listening, pre-emptive priority call, area selection, call authorised by dispatcher, late entry, ambience listening, dynamic group number assignment, CLIR, CLIP, call retention, call waiting, include call, transfer of control, barring and forwarding services.

Typical measurements

Public safety and the health of fire, ambulance and police officers are depending on the safe and reliable performance of TETRA mobiles and base stations. That is why Willtek has developed tailor-made test applications, which fit to the requirements of TETRA users in terms of test precision and test speed, to enable a short service outage together with the highest operational security for TETRA mobiles and networks:

TETRA MS/BS-Test:

• Registration, Attach
• Call from mobile station
• Call from base station
• Peak and RMS vector error magnitude
• Timing error (alignment)
• π/4 DQPSK graphical constellation display
• Paging sensitivity
• RF power
• Frequency error
• Residual carrier power
• Phase error
• Power over time
• Modulation spectrum

TETRA Air Interface Test:

• Air interface protocol analysis

Willtek Test Instruments