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Simulcast is an abbreviation for simultaneous broadcast, and in the context of land mobile radio, it means simultaneous transmission from two or more repeater sites on the same radio frequency and with the same modulation. Simulcast has been around since the 1960s, but its popularity has increased recently - in part because modern technology has made simulcast more reliable.
Simulcast networks offer several advantages:
Overlapping coverage without handoff complexity.
Better trunking efficiency.
Simpler operation, especially for conventional (non-trunked) repeater systems.
Simulcast networks also employ voting receivers, which offer macro-diversity improvement on the inbound path.
Disadvantages of simulcast include high cost and signal distortion in simulcast-overlap regions. High cost results from more complex timing and control hardware, but also from the simple fact that each site must be populated with one repeater for each frequency used in the network. In contrast, a multisite system incorporates a handoff algorithm that chooses the site with the best signal. With multisite, the number of repeaters at each site is driven by traffic engineering considerations to ensure a low blocking probability.
Signal distortion in simulcast overlap regions also is referred to as time delay interference. TDI occurs when two or more simulcast signals arrive at the user's radio antenna with comparable amplitude and relatively large delay. Note that both conditions must occur for TDI to be a problem. If one site dominates, the carrier-to-interference ratio (C/I) is sufficient to overcome the delayed signal no matter how long the delay. Conversely, if the two signal amplitudes are comparable and the C/I is low, short delays are still tolerable. Two simultaneous signals with short delays appear to the receiver as one signal and no noticeable distortion occurs. In fact, if the two signals are of equal amplitude, the receiver sees a signal that is 3 dB stronger than it would appear otherwise.
When to use simulcast
Because simulcast systems are pricey, the vendor may propose simulcast even when conditions don't warrant it. The "better coverage" advantage often is oversold. True, seamless overlapping coverage is an advantage of simulcast, but multisite systems also have seamless overlapping coverage, provided an effective handoff algorithm is used. To control TDI in simulcast networks, it often is necessary to use...