TERRESTRIAL TRUNKED RADIO - TETRA

The common mode of operation for them is in a group calling mode in which a single button push will connect the user to the users in a selected callgroup 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. If enabled by the Subscriber Management TETRA terminals can act as mobile phones (cell 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.

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 (Switching and Management Infrastructure or SwMI) made of TETRA Base Stations (TBS). As well as allowing direct communications in situations where network coverage is not available, Direct Mode or DMO also includes the possibility of using one (or a chain) of TETRA terminals as relays for a signal. This functionality is called DMO gateway (from DMO to TMO) or DMO Repeater (DMO to DMO). 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 (SDS) are provided over the system's main control channel, while Packet Data or Circuit switched data communication uses specifically assigned traffic channels.

TETRA provides Authentication Mechanisms of Terminals towards Infrastructure and vice versa. For protection against eavesdropping, over the air encryption and end-to-end encryption is available.

2. Advantages of TETRA

The main advantages of TETRA over other technologies (such as GSM) are:

Ø  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.

Ø  High spectral efficiency - 4 channels in 25 kHz and no guard bands, compared to GSM with 8 channels in 200 kHz and guard bands.

Ø  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.

Ø  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.

Ø  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.

Ø  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).

Ø  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.

Ø  in the absence of a network mobiles/portables can use 'direct mode' whereby they share channels directly (walkie-talkie mode).

Ø  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.

Ø  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.

Ø  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.

Ø  TETRA supports both air-interface encryption and End-to-end encryption

Ø  Rapid deployment (transportable) network solutions are available for disaster relief and temporary capacity provision.

Ø  Equipment is available from many suppliers around the world, thus providing the benefits of competition

Ø  Network solutions are available in both the older circuit-switched (telephone like) architectures and flat, IP architectures with soft (software) switches.

Ø  Further information is available from the TETRA Association (formerly TETRA MoU) and the standards can be downloaded for free from ETSI.

3. Disadvantages Of TETRA

Its main disadvantages are:

v  requires a linear amplifier to meet the stringent RF specifications that allow it to exist alongside other radio services.

v  handsets are more expensive than cellular (about 750 EUR in 2003, about 600 EUR in 2006). This is due to the more difficult technology, smaller economies of scale, and different business model (eg: need for security, high powers and robustness). However cheaper than main (PMR) competitor technology APCO where prices are >$3000 per handset. TETRA prices expected to fall further as far eastern manufacturers start production in 2007.

v  data transfer is efficient and long range (many km), but slow by modern standards at 7.2 kbit/s per timeslot (3.5 kbit/slot net packet data throughput), although up to 4 timeslots can be combined into a single data channel to achieve higher rates whilst still fitting into a single 25 kHz bandwidth channel. 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.

v  due to the pulsed nature of TDMA employed by the protocol and higher powers than cellular, handsets (only) can sometimes interfere with badly designed (usually old) or sensitive electronic devices such as broadcast (TV) receivers. This has prompted some concerns from users of sensitive devices such as heart pacemakers but there are no documented reports of problems even though used regularly by many ambulance crews. As a precaution, users of these equipments should take care with any RF transmitting equipment when used in close proximity (e.g. < 1 metre distance.)

4. Propagation

The TETRA-system is in use by the public sector in the following countries

Continent	Country	Name	Agency	Status
Asia	Pakistan	Ministry of Interior	Police	In use Nationwide
	India	AD100.	Kerala Police.	In use by police - Trivandrum city.
		Military College of Telecommunication Engineering (MCTE)	Indian Army	In use at Mhow, Indore, Madhya Pradesh
	Israel	Mountain Rose.	Israel Defense Forces (IDF).	In use by IDF, country-wide.
	Hong Kong	Hong Kong Police	Police, Fire Service, Ambulance	In use.
		Kowloon Canton Railway Corporation (KCRC) / Mass Transit Railway (MTR)	Transport	In use.
	Macao	Forças de Segurança de Macau	All Emergency Services	In use.
Africa	Sudan	Ministry of Interior	Police	In use Nationwide
Europe	Belgium	A.S.T.R.I.D
	Austria		Police, Fire, Ambulance, Local Train Company	In use in all states except Tyrol and Vorarlberg (still roll-out)
	Britain	Airwave	Police, Fire, Ambulance, Armed Forces, Highways Agency	In use by police all over the country.
	Ireland	TETRA Ireland	Police, Fire, Ambulance, Defence Forces, Civil Defence, Security of State services	Nation-wide roll-out network
	Finland	VIRVE	Police, Fire, Ambulance, Customs, Defence Forces, Security guards, Border guard	Nation-wide network
	Germany	BOS-Netz	Police, Fire, Ambulance, Customs	Roll-out
	Greece	OTE	Police, Fire, Coast Guard	Attica region
	Netherlands	C2000	Police, Fire, Ambulance	Nation-wide network
	Norway	Nødnett	Police, Fire, Ambulance	Roll-out
	Sweden	RAKEL		Roll-out
	Portugal	SIRESP	Police, Fire, Ambulance	Nation-wide Roll-out in use since 2007
	Romania	STS (Special Telecommunications Service)	Police, Fire, Search&Rescue	Nation-wide

Table 4.1

5. Radio Frequencies

In South America, TETRA uses frequencies:

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

Table 5.1

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

Table 5.2

6. Radio aspects

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 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 (rms) power is 3.65 dB. If non-linear (or not-linear enough) 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 (simultaneously) receive continuously from various mobiles - hence they are Frequency Division Duplex. TETRA also uses TDMA (see above). The mobiles normally only transmit on 1 slot/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 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 (whose duration is 1.02 seconds) consists of 18 frames. Hyperframes also exist, but are mostly used for providing synchronisation to encryption algorithms.

The downlink (i.e. the output of the base station) is normally a continuous transmission consisting of either specific communications with mobile(s), synchronisation or other 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 1.02 seconds), consists of 18000 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 (TDMA).

 15. References

[1] BOUCHER, N. The trunked radio and enhanced PMR radio handbook. John Wiley & sons, 2000.

[2] A Signalling Standard for Trunked Private Land Mob. Standard MPT 1327,

Radiocommunications Agency, 1991.

[3] Terrestrial Trunked Radio (TETRA). ETSI EN 300 392-2 V2.5.2, European

Telecommunications Standards Institute, 2005.

[4]Terrestrial Trunked Radio(TETRA). ETSI EN 300 392-2 v3.2.1 ,European Telecommunications Standards Institute, 2005.

[5] Internet protocol data communication service - IP packet transfer and availability performance parameters. Recommendation Y.1540, International Telecommunication Union, 1999.

[6] K. P. Ho, H. Dai, C. Lin, S.-K. Liaw, H. Gysel, and M. Ramachandran, Hybrid wavelength-division-multiplexing systems for high-capacity digital and analog trunking applications, IEEE Photon. Technol. Lett., vol. 10, pp. 297–299, Feb. 1998.

[7]William C. Y. Lee, Mobile Cellular Telecommunication System, McGraw-Hill, 1995.

[8] ETSI TC-RES. Radio Equipement and Systems (RES); Trans-European Trunked Radio

(TETRA); Voice plus Data (V+D) Part 2: Air Interface (AI). European Telecommunications

Standard, 1995. ETS 300 392-2.

[9] European Telecommunications Standards Institute. Radio Equipment and Systems

(RES); Trans-European Trunked Radio (TETRA); Voice plus Data (V+D); Part 10: Supplementary Services Stage 1; Part 10-04: Call Diversion. ETS 300 392-10-04.