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The digital mobile radio (DMR) standard appears very attractive for the professional mobile radio applications. It is a powerful digital solution that meets the major needs of users: digital encryption, good and constant audio quality, reliable data, GPS positioning without any disturb in audio, text messaging, telemetry, two TDMA channels in 12.5KHz (6.25KHz/ch equivalent), soft migration from analog to digital, low power consumption, etc.. DMR has all the advantages of a digital radio at the cost of an analog (conventional) one. These benefits may be partially missed when the coverage requested by the radio system exceeds one repeater. Some “key points” have to be understood to design an affordable, stable and user-friendly radio system. The multisite system approach allows a radio to be in coverage area changing position and to participate in a call originating at any site. In multisite system configuration, repeaters communicate among themselves using a back-end network. A call originating at a repeater is transmitted by all the repeaters of the system.
In multicast approach, the geographically adjacent repeaters should use different frequencies. Their color code can be either same or different. It is not advisable to keep the same frequencies because in areas of overlap, without special precautions (see simulcast paragraph), there will be destructive interference. The main problem is the cell handover (site coverage changing) and roaming (research of desired group of users) in non trunking systems. These functions are not real time performed by DMR (tier II non trunking) over the air protocol: the available mobiles/portables should scan frequencies to look at the best “home” base station. Due to the scanning and validation time during handover, a mobile may be “missed” for relative long time (minutes). This problem may be unacceptable in an urban environment in which the cells are changed frequently
A simulcast radio network is a very powerful radio network in which all the repeaters are active on the same frequency and at the same time. To obtain this result it is necessary to implement a number of algorithms to assure perfect alignment between the signals broadcasted by different base stations. Main advantages are: no terminal scanning needed, automatic and continuous roaming and hand-over, all stations directly connected to the network, the same RF channel over all Network. The coverage area of every single simulcast channel could be expanded easily by adding some simulcast base stations. These simulcast base stations will be integrated in the network with few operations at network level only (nothing is requested on mobile terminals).
