A method of retaining channel use by a secondary system is described. A secondary channel used during periods of normal communication between a secondary transmitter and receiver is split into sub-bands for use during a quiet period containing quiet phases. During the quiet period, while the transmitter communicates with the receiver on one sub-band, the receiver simultaneously senses for incumbent devices on another sub-band, as well as vice-versa. Transmission and sensing do not occur simultaneously on the same sub-band and transmission is limited to only one of the sub-bands. Transmission and sensing are performed by both the transmitter and the receiver on every sub-band over the course of the quiet period. The locations of secondary receivers are determined and employed during scheduling such that devices sufficiently separated from each other transmit during different quiet phases and transmit and sense in sub-bands separated by another sub-band.