Systems and methods for determining primary channel availability are disclosed. One aspect is a method in a wireless communications system including a first primary channel having first and second frequency spectrum bandwidths. The second frequency spectrum bandwidth includes the first frequency spectrum bandwidth. The method includes determining whether a first preamble has been detected on the second primary channel during a first threshold period of time, determining whether a second preamble has been detected on the first primary channel during a second threshold period of time, determining whether a guard interval has been detected on the second primary channel during a third threshold period of time, determining whether the second primary channel is idle based at least in part on detection of the first preamble, the second preamble and the guard interval, and transmitting a wireless message based at least in part on whether the second primary channel is idle.