What type of radio system allows field units to communicate on the same frequency?

The simplex system is designed for communication where only one station transmits at a time while others receive, but all parties can communicate on the same frequency. This means that field units can send and receive messages on that same frequency, making it efficient for scenarios where quick and straightforward communication is necessary, such as in emergencies.

In contrast, the duplex system allows simultaneous communication, meaning two units can talk and listen at the same time, which requires a more complex setup with at least two different frequencies. The half-duplex system, while allowing communication in both directions, only permits one party to send a message at a time. This means that at any given moment, one party is talking while the other is listening, which can slow down communication if both parties need to respond.

The multicast system is oriented toward sending messages to multiple recipients simultaneously; it does not facilitate direct two-way communication on the same frequency as effectively as the simplex system does. Thus, when looking for a setup where field units can communicate on the same frequency, the simplex system is the ideal choice.