TErrestrial Trunked RAdio (TETRA)

Overview

Terrestrial Trunked Radio is an open digital Private Mobile Radio (PMR) and Public Access Mobile Radio (PAMR) standard defined by the European Telecommunications Standardization Institute (ETSI) to meet the needs of the most demanding professional mobile radio users. TETRA is designed to maximize the use of radio spectrum by sharing available radio resources among users on demand and exploit the advantages of digital transmission. Potentially any set of users within the coverage area of a TETRA system may communicate with each other. This may range from a single base station system to one with national or even international coverage.

TETRA originated from a group of manufacturers, telecommunications operators, and end users signing the TETRA MoU (Memorandum of Understanding) in 1994. TETRA used to stand for Trans European Trunked RAdio but has been renamed to reflect growing world-wide interest in the standard. The fact that almost all major players signed TETRA MoU and that it is an open multi-vendor standard has made TETRA 1 a big success. TETRA 2 is basically to enhance the services and facilities and will be discussed later.

Features

TETRA offers fast call set-up time, addressing the critical needs of many user segments, excellent group communication support, Direct mode operation between radios, packet data and circuit data transfer services, frequency economy and excellent security features. A simple comparison between TETRA and GSM is shown below.

Trunking is a method of traffic channel organization where a traffic channel is allocated for each call transaction. TETRA trunking facility provides a pooling of all radio channels that are then allocated on demand to individual users, in both voice and data modes. By the provision of national and multi-national networks, national and international roaming can be supported, the user being in constant seamless communications with his colleagues. TETRA supports point-to-point, and point-to-multipoint communications both by the use of the TETRA infrastructure and by the use of Direct Mode without infrastructure.

Applications and Markets

The intended market areas are Police, ambulance and fire Services, Security Services, Utilities, Military, Public Access, Fleet Management, Transport Services, Closed User Groups (CUGs), Factory site services, Mining, etc. TETRA is now widely established with more than 170 contracts placed. A number of the operational network fleets contain terminals from different manufactures. The Digital Advanced Wireless Services (DAWS) (defined in Packet Data Optimized (PDO)) further target at applications includes Video on demand, Video-conferencing, www accesses, Packet Radio Voice, File Transfer and E-Mail, Messaging and Paging, and any other ‘bursty’ oriented sessions.

Technical  

Simplified system architecture and defined interfaces are shown below. Two major distinct services are Voice Plus Data (V+D) and Packet Data Optimized (PDO.) These two specifications are based on the same physical radio platform but implementations are not inter-operated at the physical layer. Full inter-operability is put at layer 3. V+D could be either in trunked TETRA mode or Direct Mode Operation (DMO.) A gateway terminal can provide trunked TETRA/DMO interoperability at layer 3. The specification ensures compatible communication channel operation at layer 1. For MS, ‘Dual Watch’ is the facility that requires the MS to monitor operation on the alternative transmission system. Interoperability at layer 3 is assured by use of common layer 3 protocols for trunked V+D, DMO and PDO.

There are about 300 TETRA standards and publications, ongoing or finalized. Core TETRA standards and standard interfaces consists of V+D, DMO, TETRA Speech Codec, Conformance Testing, PDO, Air Interface (AI), Inter-System Interface (ISI), Peripheral Equipment Interface (PEI). The standard defines the network protocols only up to layer 3 of the OSI model.

In the Physical Layer, PDO and V+D use the same RF platform. The modulation technique is p/4 DQPSK, with bit rate 36kbps at 25kHz carrier spacing both uplink and downlink. PDO uses Statistical Multiplexing/Multiple Access (STM/STMA) offering packet switching at 36kbps gross data rate; V+D uses TDMA with 4 channels on a single 25 kHz RF channel. Voice and Data can be transmitted simultaneously in up/downlink using different time slots.

Trunked TETRA AI supports both Individual Call and Group Call (including Broadcast Call.) Services include Speech (Half Duplex/Full Duplex, Clear/Encrypted), Data (Circuit mode, Packet mode (Connection Oriented/Connectionless (X.25), IP Packet Data), Short data service), Supplementary Services, Call Control, Mobility Management, and Security Services.

DMO AI also supports both Individual Call (Half Duplex) and Group Call/Broadcast Call. Typical operation modes are Back-to-Back (standard DMO terminal coverage), DMO Repeater (extended DMO terminal coverage,) Dual Watch (periodic “scanning” of ‘other’ mode), DMO Gateway (continuous relay between modes.) Normal DMO enables 1 conversation per carrier (i.e. 1 per 4 timeslots.) Transmitting radio acts as ‘Master’ on a single timeslot, while receiving radio is ‘Slave’ on a single timeslot. Therefore, in normal mode two time slots are used, two are free.

ISI provides connection to other TETRA Networks. ISI tasks contain Multi-sourcing of the Infrastructure, Interoperability between different TETRA Systems, and Cross-border Roaming. Individual call/Group Call, Mobility Management, Short Data Service, Supplementary Service, and Security are all supported by ISI. Services offered by PEI can be categorized into Set-up & Communication (Packet Data, Circuit Data, Short Data) and Set-up & Control (Speech Calls.)

TETRA Security features Authentication, AI encryption, Temporary / permanent disabling, Aliasing / User logon, Ambience listening, Discrete Listening, and Lawful Interception. Other issues such as TETRA as an IP Network (See below), PSTN / ISDN / PDN Gateways, Codec, PDO, etc. are specified in the standards. Details are not discussed here.

TETRA 2

The aim of the ongoing TETRA 2 suite of standards is to enhance the services and facilities of TETRA. ETSI Project (EP) TETRA has achieved one major step for the success of TETRA 2. It covers six work areas including high-speed data, speech coding, air interface enhancements, inter-working and roaming, SIM (Subscriber Identity Module), and security.

In order to meet the IP driven requirement for high speed packet data, Two-track approach adopted – TAPS (of PAMR Operators for Rapid market deployment ) and TEDS (backward compatibility.)

AI enhancements consist of Extending RF Range, Reducing Call set- up Time and End-to-end Delay, Increasing in Subscriber Capacity, Extending Battery Life, and Reducing Terminals Size & Weight. Other improvements include Roaming to GSM/ GPRS Systems, Retrieval of WAP Applications, Retrieval of SMS Messages, Use of UMTS/ 3G SIM Technology for Improved Inter-working, Use of UMTS/ 3G Codecs to Avoid Double Voice Coding, etc.

Conclusion

What makes TETRA unique? Important characteristics such as Instant group communication, Fast call set-up time, Direct Mode (DMO), Queuing of channel resources, Pre-emptive priorities, Uncompromising security (authentication of radios and networks, air interface encryption, static or dynamic keys, end-to-end encryption support, disabling of stolen radios,) and Access to data from the field with IP packet data are what make TETRA stands out. With TETRA bringing Internet access to mobile workforce, it incorporates itself very well into the future wireless communication technology.