A communication system is designed that reduces co-channel interference when heterogeneous users are sharing spectrum. The system design enables a plurality of pairs of users to communicate their channel utilization time periods by using the durations and start or stop times of certain transmissions to signal the start times and durations of later transmissions. Other system users using possibly different communications protocols and different modulation technologies can measure the durations of received transmissions and to infer channel usage based on these durations.