One aspect disclosed is a method in a wireless communications system including a first primary channel having a first frequency spectrum bandwidth and a second primary channel having a second frequency spectrum bandwidth, wherein the second frequency spectrum bandwidth includes the first frequency spectrum bandwidth. The method includes performing a first and a second back-off procedure at least partially in parallel, the first back-off procedure based on whether the first primary channel is idle, and the second back-off procedure based on whether the second primary channel is idle, and transmitting a wireless message based on whether the first or the second back-off procedure completes first.