Since we omit the case of using 160 MHz channel for simplicity, there are four primary channel choices of 20 MHz channel and three bandwidth choices as shown in Fig. 12. In Fig. 12a, the bandwidths of 1, 2 and 4 represent 20 MHz,40 MHz, and 80 MHz channels, respectively, and each number in bracket represents the primary channel index among four channels. Fig. 12b shows all possible combinations of the bandwidth and primary channel choices. Here, each filled square represents a chosen primary channel, shaded square a secondary channel, and empty square an unused channel