12.6 TRUNKED RADIO SYSTEMS

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Radio systems used in cities and towns to provide mobile communication for closed user groups such as taxi operators, ambulance service operators, police, etc. are called trunked radio systems. Trunked radio systems can also be installed by organizations that need communication among their employees in their area of operation such as construction sites. The trunked radio system consists of a base station at a central location in a city/town. The user terminals are mobile devices that communicate with the operator at the base station or with another mobile device using FDMA/TDMA. The operator of the trunked radio system will be assigned a set of frequencies that are shared by all the mobile devices. As compared to cellular mobile communication systems, trunked radio systems are low-cost systems because the entire service area is just a single cell.

Trunked radio systems are used to provide low-cost mobile communication services to closed user groups such as taxi operators, ambulance service operators, and police.

Note

In trunked radio systems, a city is generally covered by a single base station. The base station and the mobile terminals communicate using FDMA/TDMA.

12.6.1 TETRA

ETSI developed a standard for trunked radio system called TETRA (Terrestrial Trunked Radio). The configuration of TETRA is shown in Figure 12.11.

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Figure 12.11: TETRA.

A TETRA system consists of TETRA nodes, which are radio base stations, to provide mobile communication facilities for users in a service area. A trunked radio operator can interconnect a number of TETRA nodes using cable or microwave radio. Each node can cater to a portion of a large city.

The TETRA standard has been formulated by ETSI. TETRA uses TDMA/FDD mechanism with 380–390 MHz uplink frequency and 390–400 MHz downlink frequency. Both circuit switching and packet switching are supported.

The salient features of TETRA are as follows:

Frequency of operation: The uplink frequency is 380 to 390 MHz, and the downlink frequency is 390 to 400 MHz. Each channel has a bandwidth of 25kHz.

Multiple access: The system operates in TDMA/ FDD. The TDMA frame is divided into four time slots that are dynamically assigned.

Types of services: Two types of services are supported. Voice and data services use circuit switched operation. This service uses the TDMA scheme with four slots per carrier. Packet data optimized (PDO) services use packet switching for data communication. Data rates up to 36kbps can be achieved.

TETRA-based trunked systems are used extensively in Europe.

Summary

In this chapter, we discussed the details of representative terrestrial radio communication systems. Radio systems provide many advantages—easy installation and maintenance and support for mobility. The radio as the transmission medium is used in audio and video broadcasting, in wireless local loops, in cordless telephony applications, and for trunked radio systems. The radio broadcasting systems are mostly analog, though in the next few years, digital broadcasting systems are likely to increase. Wireless local loops are now being deployed extensively in both urban and rural areas. Analog wireless local loops are shared radio systems in which a few radio channels (2 or 4 or 8) are shared by a number of remote stations (16 or 32 or 64). Digital wireless local loops use TDMA and CDMA technologies. Low-cost local loops can be provided using these technologies.

The digital enhanced cordless telecommunications (DECT) standard developed by ETSI can be used for cordless telephony applications at home or office or at public places such as shopping complexes. DECT operates in the 1880–1900 MHz band and uses the TDMA-TDD scheme to support up to 120 voice channels using the ADPCM coding scheme for voice.

Trunked radio systems provide mobile communication facilities for closed user groups such as police, taxi operators, and ambulances. The Terrestrial Trunked Radio (TETRA) standard developed by ETSI supports both voice and data services. TETRA offers a low-cost solution for mobile communication as compared to cellular mobile communication systems.

References

G. Karlsosson. "Asynchronous Transfer of Video", Vol. 34, No. 8, August 1996. Proceedings of the IEE Conference on Rural Telecommunications, May 1988, London.
http://www.etsi.org The Web site of European Telecommunications Standards Institute. You can obtain the ETSI standards referred to in this chapter from this site.
http://www.motorola.com Motorola is a leading manufacturer of trunked radio systems. You can obtain the details from this site.

Questions

Draw the block diagram of a radio communication system and explain the various blocks.
What is a wireless local loop? Explain the architecture of analog wireless local loop systems.
Explain the architecture of digital wireless local loop systems based on TDMA.
Explain the various configurations of a DECT system. List the salient features of the DECT standard.
Describe the operation of a trunked radio system. What are the salient features of the TETRA standard?

Exercises

1.	Prepare a technical report on digital broadcasting using HDTV.
2.	Prepare a technical report on a wireless local loop using CDMA technology.
3.	Prepare a technical report on the various commercial products available for trunked radio.
4.	Study the details of path loss calculations for terrestrial radio systems.
5.	Instead of air, water is used as the transmission medium for underwater communication. What are the frequency bands used for this type of communication? As compared to terrestrial radio systems, what are the major differences in underwater communication systems?

Answers

1.	Digital TV transmission has many advantages: improved signal quality, bandwidth efficiency through use of compression techniques, and effective control and management. Two standards HDTV (High Definition TV) and SDTV (Standard Definition TV) have been developed for digital video broadcasting. In HDTV, the aspect ratio is 16:9. Twenty-four or 30 frames are sent per second. Each frame is divided into 1080 × 1920 pixels or 720 × 1280 pixels. In SDTV, the aspect ratio is 4:3, with 24 or 30 frames per second, and each frame with 480 × 640 pixels.
2.	To provide wireless local loops, small base stations are installed at various locations. The subscriber terminal (telephone instrument) communicates with the base station using CDMA access mechanism. You can get the details of wireless local loop using CDMA from the Web site http://www.qualcomm.com.
3.	Though most of the trunked radio systems are analog, the latest standard developed by European Standards Telecommunications Institute (ETSI) is based on digital technology—it is called TETRA (Terrestrial Trunked Radio). You can get the details from http://www.etsi.org. Motorola is a leading supplier of trunked radio systems. You can get product details from http://www.motorola.com.
4.	Path loss calculation involves finding out the loss/gain of different network elements. The network elements are filters, amplifiers, antennas, and the cable connecting the RF equipment with the antenna. A major contributor to path loss is the propagation loss in the medium.
5.	Underwater communication systems are used in sports, search and rescue operations, and military applications. As the attenuation of the electrical signal is very high in water, high-power transmitters are required. VLF band is used in underwater communication. For instance, the phones used by divers operate in the frequency band 30 to 35 kHz. If the transmit power is 1/2 watt, the range is about 400 yards, and if the transmit power is 30 watts, the range is about 5 miles. You can get the details of some commercial underwater communication systems from the site http://www.oceantechnologysystems.com.

Projects

Study the various propagation models for modeling the radio wave propagation in free space (such as the Okumura-Hata model). Develop software to calculate the path loss in a radio communication system and to carry out link analysis.
You are asked to design a cordless PBX for your office. Work out the details of location of base stations, if a DECT-based system has to be installed.

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