Also known as: simulcast, simulcast distortion
Simulcast is a coverage technique in which several transmitters radiate the same modulated signal on the same frequency at nearly the same instant, so that a wide area is served by what looks to a radio like one big transmitter.1 The transmitters at each trunking site are locked to a common time and frequency reference — typically GPS-disciplined — and fed identical bits, so their carriers add constructively over most of the region instead of interfering as separate signals would.
How it works
Away from the boundaries a listener is dominated by one transmitter and hears a clean signal. In the overlap region, though, two or more copies arrive at slightly different times because the path lengths differ. The delay spread between them acts exactly like multipath: the symbols smear into one another, producing intersymbol interference — the effect operators call simulcast distortion. To keep this manageable, all sites must share a common clock so the launch times differ by only microseconds, and system planners tune per-site “launch time” delays so the overlap zones fall where few users are.
Simulcast is a property of one wide simulcast cell, distinct from multisite trunking, where separate sites use separate frequencies and radios roam between them. A simulcast system can itself be one of many sites in a larger multisite network.
Relevance to SDR
Simulcast is one of the harder demodulation cases for a software receiver: the composite waveform in the overlap zone has deep frequency-selective fades and a spread delay that a simple detector cannot equalise, so decode quality can collapse even though signal strength is high. C4FM and other narrowband trunking modulations are especially sensitive because their symbol period is short relative to the delay spread.
GopherTrunk decodes the control channel of simulcast systems where the signal is clean, but in the worst overlap zones the self-interference can defeat the demodulator — moving the antenna, or favouring one dominant site, is often the only fix, since the distortion is baked into the received samples rather than being something the DSP can undo. GopherTrunk treats a simulcast cell as a single site: it does not try to separate the contributing transmitters.