Link: a Look at Time-Delay Interference
In a simulcast system there are several parameters that adversely can affect system performance, but one parameter that is sometimes misunderstood is time-delay interference, or TDI. In this article we begin a discussion of TDI and its significance in a simulcast radio system.
Simulcast radio systems are used when a limited number of RF channels are available and a large geographical area needs to be covered. As the name implies, simultaneous broadcast, or simulcast, occurs when two or more sites transmit at the same time on the same frequency; invariably the coverage from these multiple sites will overlap.
See the full article here: A look at time-delay interference — Urgent Communications article.