Radio transceivers 5 in a communication system periodically broadcast signal pulses 6 to enable a radio transceiver 5 to determine their proximity. Each radio transceiver 5 has a clock 16 for maintaining a time synchronised by a synchronisation means 17 via the Global Positioning System (GPS). Each radio transceiver 5 transmits a signal pulse 6 at the same point in the synchronised time. A signal detector 19 detects the start of a signal pulse 6 received from the closest radio transceiver 5 and a proximity estimator 20 estimates the proximity of the closest radio transceiver 5 by determining the difference between the point in the synchronised time at which the signal pulses 6 are broadcast and the point in the synchronised time at which the start of a signal pulse 6 received from another radio transceiver 5 is detected. A controller 21 compares the estimated proximity to a threshold and, if the proximity estimate is less than a given threshold, it prevents a transmission means 7 of the radio transceiver 5 from transmitting a communication signal.