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AMATEUR RADIO BY GARRY CRATT, VK2YBX Trunked radio communications While trunked radio techniques are not yet used by the amateur radio fraternity, they are widely used by commercial organisations. In the years to come, trunked radio services may displace some the amateur bands as pressure on radio space increases. Throughout the world, the radio spectrum available for mobile radio services is becoming more congested. The major capital cities of Australia are no exception, where demand is growing at an alarming rate. Latest DOTAC licensing figures show that there are now more than 400,000 commercial operators in Australia, sharing VHF and UHF commercial frequency allocations, and these are increasing at a rate of 25% per year. low coverage of such cells, when compared with VHF and UHF systems. In addition, not all users require access to the PSTN telephone system , and some wish to deliberately avoid such access. Also, there are significant charges incurred every time a cellular call is made and this cost is intolerable to many commercial users such as taxis, couriers, etc. Fortunately, there is an alternative communications system which has "Standard mobile radio operates on a single channel and thus calls can only be made when the channel is free. Trunked radio uses a number of discrete frequencies within a small band and allows a call to be made whenever any channel is free" As the demand for spectrum increases, there is no doubt that the relatively uncongested and highly prized VHF and UHF amateur allocations will become valuable targets for commercial users. Some of the increased demand has been met by the Telecom Cellular system (see 1ast month's issue) but there are significant drawbacks in using this system for some commercial users. High hardware costs exist to service cells at 800MHz, due to the relatively 72 SILICON CHIP been in use in some of the more densely populated European countries for some time. Called "trunked radio", it goes a long way towards solving the problems of increasing spectrum congestion. There are several conflicting standards that are presently being evaluated for use in Australia. The most well known is "MPT1327", the standard adopted by the British government, which has been in use since 1988. Other standards claim to have various advantages over MPT132 7 but for the purpose of this article, those differences are immaterial. Multiple frequencies In essence, the basic difference between trunked mobile radio and standard mobile radio is this: standard mobile radio operates on a single channel and thus calls can only be made when the channel is free. Trunked radio uses a number of discrete frequencies within a small band, and allows a call to be made whenever any channel is free. Obviously not all users will wish to call at the same time and even if several do, there are multiple frequencies available. So more users can share the band of frequencies and achieve the same grade of service as a single channel would allow. One disadvantage of a trunked system it that one channel must be used to transmit call data to all mobiles in the system. Whilst this means that one channel in the system cannot be used for 2-way communications by a mobile, this disadvantage is overcome once three or more channels are trunked. In the UK, wide-area networks have been set up and now cover much of mainland Britain, allowing up to 10,000 subscribers. These networks allow secure voice and data communications with other mobiles and allow mobiles to connect to private telephone, PABX systems as well as the PSTN where requested. PSTN features include conference calls, transfers, automatic call holding until a channel becomes available, a call-back facility for unattended mobiles, roaming and priority calling. In addition, the system features include: a capacity of one million addresses per system code, automatic SPARE RECEIVER OUTPUTS RECEIVE ~ - - - - < DISTRIBUTION MODEM DIPLEXER How it works SYSTEM CONTROL UNIT TSC BASE RF 1 TSC ·BASE RF 2 TSC BASE RF3 TSC BASE RF 4 TSC BASE R~S CENTRAL COMPUTEII COMBINER& EXCHANGE LINES PABX SWITCH SPARE TRANSMITTER INPUTS COMBINER~ COMBINERS Fig.1: block diagram of a trunked radio system. The Trunk System Controller (TSC) connects callers, validates mobiles, records billing details where required, and manages the band of radio frequencies at its disposal. user location and registration, auto release of traffic channels at call termination, periodic checking of serial numbers for subscriber security, etc. The system is completely transparent to the end user. One of the main differences between a cellular telephone and trunked mobile radio is that TMR is usually a simplex system, meaning that the mobile cannot receive and transmit at the same time, as is the case with the cellular system. This has the advantage that it simplifies transceiver design and also means that two transceivers in close proximity can share the same pair of channels. Block diagram Fig.1 shows a block diagram of a trunked radio system. At the heart of any trunked radio system is the TSC (Trunking System Controller). This is the computer which connects callers, validates mobiles, records billing details where required, and manages the band of radio frequencies at its disposal. All data signalling which takes place on an MPT1327 system uses FFSK (fast frequency shift keying). Data is sent at 1200bps. Each bit is represented by either one cycle of 1200Hz tone, or one and a half cycles of 1800Hz, where 1200Hz = 1 and 1800Hz = 0. This data is always phase continuous and bit transitions always fer to another traffic channel. CLEAR - used by the base to clear a mobile call. occur at the zero crossing points. This reduces the required bandwidth compared to FSK. Data transmitted on the control channel consists of two types of data word sent alternately. These are the CCSC (control channel system codeword) and data codewords. The CCSC identifies the trunked radio system (useful where there are more than one in operation) and it identifies which type of mobile can access the system. In addition, it acts as a synchronisation pattern for radio modems. Data codewords are used to control and direct mobiles active on the system. Some common data codewords are: ALOHA (ALH) - this message is sent by the base station to indicate that it is free to accept messages at that time. Aloha messages can be used to address specific groups of mobiles. AHOY (AHY) - this word is used to call a mobile to see if it is available. REQUESTS (RQS) - this word is used by mobiles to request a call. These messages can also be used to call for data, to signify an emergency, to call status and to divert calls. ACKNOWLEDGE (ACK) - used to confirm a request or an ahoy. These messages can also indicate that the service request is accepted, denied or queued. GO TO TRAFFIC CHANNEL (GTC) - this message is used to send a mobile from control to a traffic channel, or to trans- In a trunked radio system, the available spectrum is shared by a number of mobiles. If there is no available channel, the call is placed on hold for a few seconds until a channel becomes free. In simple terms, the system operates like this: Each mobile transmits a control signal on a dedicated channel to the system computer. When the user wishes to make a call, the mobile transmits the request in the form of a data signal to the system computer. The computer locates the called party by means of the control channel and checks its ability to receive the call. When both called and calling parties are ready to communicate, the computer allocates the first available traffic channel. When the call is terminated by either party, the mobile sends a signal to the system computer, releasing the channel. When not in use, each transceiver is tuned to the control channel and the transceiver microprocessor can communicate with the system computer. Finally, when roaming across geographic boundaries, the mobile calls the system computer to register its presence at a new location, so that the system computer knows where to locate the user for the next incoming call. As can be seen from the above explanation, Trunked Radio Systems offer improved spectrum usage whilst maintaining a high standard of service. Presently, in Australia, there are several private and public trunked systems in operation, with others planned to come on line during the next few years. No doubt there will be further developments in this field, which will affect the spectrum allocation and operating efficiency of all mobile radio users, including amateur radio operators. Acknowledgments (1) "Trunking Test Solutions"; "What's New in Communications" Feb/March 1991. (2) "MPT1327 Trunked PMR"; What's New in Communications" Feb/March 1991. (3) "Discussion Paper on MPT1327"; Japanese public domain 1991. SC FEBRUARY1992 73