a communication system comprising one or more transceiver units of a first type and one or more transceiver units of a second type capable of communicating with the transceiver units of the first type ; each transceiver unit of the first type comprising : a frequency comparison unit for comparing the frequency of a signal received from a transceiver unit of the second type with a reference frequency ; a feedback signal generator for generating a feedback signal dependent on the result of that comparison ; and a transmitter for transmitting that signal to the transceiver unit of the second type ; and each transceiver unit of the second type comprising : a local frequency reference unit on which the frequency of signals transmitted by it are dependent ; and a frequency adjustment unit for receiving the feedback signal and adjusting the local frequency reference unit in dependence on the feedback signal .