Trunked Simulcast System Overview. Continuous Coverage Simulcast provides continuous coverage over a large geographic area (wide area) using a single set of frequencies. In a simulcast system, all sites have the same set of frequencies. The same channel is the control channel at every transmit site. Overlap Zone The challenge of a simulcast system is to provide high quality signaling to radios in the overlap zone. An overlap zone is the geographic region where two or more signals of comparable power are received by a single radio. To understand the overlap zone and its significance, an understanding of the concept of capture and non-capture zones is necessary. The simulcast coverage area contains two types of areas: those captured by a single site and those hearing two or more sites of approximately equal RF signal level. Capture Zones – A radio (receiver) is captured by an RF signal when the mean signal level is higher than other RF signal sources by a margin strong enough to suppress the weaker signal’s modulation product so that data errors (missing data/voice frames) created by the mixing of the two signals do not disrupt the intended data stream from the mean signal. Non-Capture Zones – In the "non-capture" or "overlap" zone, the mobile receiver accepts two or more signals. These signals mix randomly producing stronger or weaker signals. If one site’s received mean signal power level is within several decibels of other sites’ on-frequency modulated signals, audio intermodulation and distortion may occur. Audio distortion increases to a maximum when the received carrier signals are equal. In digital communications the distortion can cause data errors and result in distortion or missing voice (data) frames. Good communication in these areas is maintained by precision system synchronization and equalization. Control Point Basics The Control Point is the central point for system control. The Control Point provides the necessary interfacing between two or more transmit sites and dispatcher locations. Timing references for outbound (transmit) data are generated here and sent to each transmit site and selection of the best quality inbound (receive) data from each site is managed by the Control Point equipment. Outbound (Transmitter) Signaling Timing (or delay) for the outbound transmissions is adjusted by each transmit site to make the timing reference signals from the Control Point and transmit site match. Control of the exact time the common outbound signal is transmitted from each ...
