TETRA - Terrestrial Trunked Radio - Online Article

TETRA - Terrestrial Trunked Radio for Professional Cellular System

TETRA, the European standard for Trunked Radio Systems, is primarily intended for users in the public security sphere, transport, utilities and operators of public Trunked Radio networks. As fully digital radio system, TETRA is hallmarked by improved transmission quality and higher frequency economy. The comprehensive functionality additionally ensures high flexibility in professional mobile communications.The European Institute for Telecommunications Standards (ETSI) has developed this standard in co-operation with the leading manufacturers, system operators and users. TETRA has been conceived for high frequency economy, reliable voice and data transmission and versatile operating features. In addition to the central standard for speech and data (TETRA “Voice and Data”) there is also a special variant which optimally supports the packet oriented transmission of data via the radio channel (TETRA PDO = “Packet Data Optimized”). Thus the system platform is able to combine the two radio services, mobile data transmission and paging, which were up to now only offered by separate infrastructures. This improves the flexibility and economic viability of the system for the network operator, service providers, and not least for the end users.

TETRA, the digital standard for professional communications TETRA (Terrestrial Trunked Radio) is the first real open standard for the digital professional mobile radio system, contributing to open the international market in the professional radio communications. The Professional Mobile Radio market, which includes Privateand Public Access Mobile Radio ( PMR and PAMR ), has traditionally been scattered in many dimensions in terms of technologies, frequency allocation etc. The first clear change towards international standardization was the introduction of the analogue MPT1327 trunked radio standard, that lead to a market success in most parts of the world. Terrestrial Trunked Radio (TETRA) is the first truly open digital private mobile radio standard. TETRA is opening an even more international Professional Mobile Radio market. The standard is defined by the European Telecommunications Standards Institute (ETSI), that joins the forces of network operators, national administrations, equipment manufacturers and users. What is notable in the status of ETSI is that it publishes telecommunications standards that are mandatory for use in Europe, but also widely applicable outside Europe. The Global Standard for Mobile telecommunication (GSM) is a good example of this. The essential parts of the TETRA standard have been approved in national voting's at the end of 1995 (22countries voted for the approval and none against it). New features and services will continue to be standardised to TETRA for years to come, just has been the case with GSM. The standard has gone through a thorough approval procedure with its vast number of commenting that as such ensure high quality compared with the development of proprietary solutions. User impact can be seen clearly on the development of TETRA, especially the emergency service users have contributed strongly in the creation of the standard. As a result, the TETRA standard contains high functionality for emergency services and is also very well suited for commercial trunked radiousers. The high level of user involvement in the creation of the standard ensures that it will meet the needs of the demanding users. To ensure an open multivendor market, TETRA specifies the following essential interfaces:

     
  1. Air Interface ensures the interoperability of terminal equipment from different manufacturers.
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  3. Terminal Equipment Interface (TEI) facilitates the independent development of mobile data applications.
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  5. Inter-System Interface (ISI) allows the interconnection of TETRA networks from different manufacturers.
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  7. Direct Mode Operation (DMO) guarantees communication between terminals also beyond network coverage.

Also a line station interface is standardised. It should be noted that the interfaces inside the Switching and Management Infrastructure are not standardised. This provides the essential benefits of an open market, but leaves the manufacturers the freedom to implement the most cost-efficient network solutions. Major user organisations, network operators, system manufacturers, regulators, test houses and application software developers have all signed the TETRA Memorandum of Understanding (MoU), a joint effort to support and promote fast and consistent implementation of TETRA systems in the member countries to ensure a wide open market,the tetra MoU strives for the maximum interoperability of equipment from different manufacturers. The TETRA MoU publishes a quarterly newsletter called TETRA News and has an informative website in the Internet at www.tetramou.com TETRA is thus the first and only approved digital trunking standard in Europeor in any other part of the world.

TETRA is a fully digital system providing consistent voice quality and low bit error rate for data accordingly. TETRA supports voice, circuit switched data and packet switched data services with a wide selection of data transmission rates and error protection levels. TETRA uses TDMA (Time Division Multiple Access) technology with four user channels interleaved into one carrier with 25 kHz carrier spacing. This means excellent efficiency of frequency spectrum Costsavings are also achieved in base stations were only one radio unit is needed for every four user channels.

Higher data transfer rates upto 28.8 Kbit/s are implemented by reserving up to four channels for the same user connection bandwidth is allocated by demand. TETRA has from the beginning been designed as a trunked system that effectively and economically supports shared usage of the network by several organisations, yet maintaining privacy and mutual security. Virtual networking inside the TETRA network enables each organisation to operate independently, but still enjoys the benefits of alarge, high-functionality system with efficient resource employment. TETRA is a high security technology that inherently includes encryption of voice, data, signalling and user identities. Two encryption mechanisms are defined:

 

Air interfaceencryption, which encrypts the radio path between the terminal and the base station.

 

End-to-End encryption for the most critical applications where encryption is required for the transmission throughout the system to the other terminal.

TETRA provides very fast call set-up time (300 ms), that is crucial for the public safety an emergency services. Naturally, TETRA supports both semi-duplex operations for efficient group communication and duplex operation for telephony type individual calls. The advanced group and announcement call features included in TETRA meet the needs of the most critical user applications. Multiple call priority schemes ensure effective resource allocation to the most urgent traffic in the network. The TETRA frame structure has four time slots per TDMA frame. This is further organised as 18 TDMA frames per multiframe. In circuit mode voice and data operation traffic from an 18 frame multiframe length of time is compressed and conveyed within 17 TDMA frames, thus allowing the 18th frame to be used for control signalling without interrupting the flow of data. This 18th frame is called the control frame and provides the basis for slow associated control channel (SACCH). The SACCH provides the background control channel signaling that is always present, even in minimum mode when all channels are allocated totraffic, one of the most powerful features of the TETRA protocol. The gross bitrate of one channel is 9 Kbit/s, into which speech is coded with 4.8 Kbit/s netbit rate using ACELP coding, one of the most efficient voice coding methods to date.The modulation method applied in TETRA is P/4-DQPSK – a linear modulation. TETRA includes direct mode operation between mobile radios without the need for network infrastructure Also repeater and gateway functions are defined to extend the coverage of hand portable radios in both direct mode and network operation. The defined power classes of TETRA radio equipment are 25W 10W, 3Wand 1W. TETRA radios can automatically adjust the output power according to the needed field strength.

Connectivity between networks of different type is becoming increasingly important. This has been taken into account in the development of TETRA technology. TETRA networks facilitate a wide range of connections to external networks. A TETRA network can be connected to, for example, public and private telephone networks different types of data networks as well as large commandand control systems. All these networks can be accessed from the mobile terminal. Connectivity combined with bandwidth-on-demand makes TETRA a superior platform for data application development.

TETRA technology is independent of frequency, but harmonized use of frequencies gives significant economies of scale. The North Atlantic Treaty Organisation (NATO) has given up 20 MHz of radio frequencies in Europe for emergency and public safety services. This new frequency band resides in between 380 - 400 MHz. National authorities have allocated two sets 5 MHz each in this 20 MHz band for Public Safety TETR Anetworks. Due to a common frequency, a large market for TETRA products will becreated guaranteeing product supply and competitive prices. European national authorities have also begun allocating frequencies for commercial TETRA. Various plans exist to implement commercial TETRA networks starting in the 410 - 430 frequency band. The UK has already awarded two licences to operate commercial TETRA networks in this band. Other frequencies for these commercial TETRA applications in Europe reside in the following bands:

 

450÷460 / 460÷470 MHz

 

870÷876 / 915÷921 MHz.

TETRA Radio Aspects & Modulation

To send information TETRA uses a digital modulation scheme known as π/4 DQPSK, this is a form of phase shift keying. The symbol (baud) rate is 18,000 symbols per second, and each symbol maps to 2bits, 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 side bands,which are essentially a repetition of the data in the main carrier's modulation, are filtered off with a sharp filter so that unnecessary spectrumis not used up. This results in an amplitude modulation and is why TETRA requires linear amplifiers. The resulting ratio of peak to mean (rms) power is3.65 dB. If non-linear (or not-linear enough) amplifiers are used, the side bands 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 (simultaneously) receive continuously from various mobiles - hence they are Frequency Division Duplex. TETRA also uses TDMA (see above). The mobiles normally only transmit on 1slot/4 and receive on 1 slot/4 so they are both Time Division and Frequency Division Duplex.

Speech signals in TETRA are sampled at 8 kbit/s and then compressed with a vocoder using a technique called ACELP (Adaptive 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 (erroneous) channels. The data rate after coding is 7.2 kbit/s - the capacity of a single traffic slot when used 17/18 frames.

A singleslot 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 (whose duration is 1.02 seconds) consists of 18 frames. Hyper frames also exist, but are mostly used for providing synchronisation to encryption algorithms.

The downlink (i.e. the output of the basestation) is normally a continuous transmission consisting of either specific communications with mobile(s), synchronisation orother general broadcasts. All slots are usually filled with a burst even if idle (continuous mode). 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 (like SMS in GSM) or synchronisation. The frame structure in TETRA (17.65 frames per second = 18 frames in 1.02seconds), consists of 18000 symbols/s / 255 symbols/slot / 4 slots/frame, and is the cause of the perceived " amplitude modulation" at17 Hz and is especially apparent in mobiles/portables which only transmiton one slot/4. They use the remaining three slots to switch frequency to receive a burst from the basestation two slots later and then return to their transmit frequency (TDMA).

About TETRA

Terrestrial Trunked Radio (TETRA) is a digitaltrunked mobile radio standard developed by the European Telecommunications Standards Institute (ETSI). The purpose of the TETRA standard was to meet the needs of traditional Professional Mobile Radio (PMR) user organisations such as those listed below. The air interfaces, network interfaces as well as the services and facilities are specified in sufficient detail to enable independent manufacturers develop infrastructure and radio terminal products that would fully inter operate with each other. For example, radio terminals from different manufacturers can operate on infrastructures from other manufacturers. The ability for full interoperability between different manufacturer's products is a distinct advantage of open standards developed by ETSI. As the TETRA standard is supported by several independent manufacturers this increases competition, provides second source security and allows a greater choice of terminal products for specific user applications.

  • Public Safety
  • Transportation
  • Utilities
  • Government
  • Military
  • PAMR
  • Commercial & Industry
  • Oil & Gas

Because the TETRA standard has been specifically developed to meet the needs of a wide variety of traditional PMR user organisations it has a scaleable architecture allowing economic network deployments ranging from single site local area coverage to multiple site wide area national coverage. Besides meeting the needs of traditional PMR user organisations, the TETRA standard has also been developed to meet the needs of Public Access Mobile Radio (PAMR) operators.

The main advantages of TETRA over other technologies (such as GSM) 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.
  • High spectral efficiency - 4 channels in 25 kHz and no guard bands, compared to GSM with 8 channels in 200 kHz and guard bands.
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  • Very fast call set-up - a one to many group call is generally set-up within 0.5 seconds (typical less than 250 msec for a single node call) compared with the many seconds (typically 7 to 10s) that are required for a GSM network.
  • Works at high speeds >400 km/h. TETRA was used during the French TGV train speed record on 3 April 2007 at 574.8 km/h.
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  • The system contains several mechanisms, designed into the protocols and radio parameters, to ensure communication success even during overload situations (e.g. during major public events or disaster situations), thus calls will always get through unlike in cellular systems. The system also supports a range of emergency calling modes.
  • TETRA infrastructure is usually separate from (but connected to) that of the public (mobile) phone networks, resulting in (normally) no call charges for the system owners, substantially more diverse and resilient communications and it is easy to customise and integrate with data applications (vehicle location, GIS databases, dispatch systems etc).
  • 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. 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 (walkie-talkie mode).
  • 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.
  • 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 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.
  • TETRA supports both air-interface encryption and End-to-end encryption
  • Rapid deployment (transportable) network solutions are available for disaster relief and temporary capacity provision.
  • Equipment is available from many suppliers around the world, thus providing the benefits of competition
  • Network solutions are available in both the older circuit-switched (telephone like) architectures and flat, IP architectures with soft (software) switches.

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