In order to support wider channel bandwidths, 802.11ac defines its channelization for 20, 40, 80, and 160 MHz channels
as shown in Fig. 2. For example, a 40 MHz transmission band is formed by two contiguous 20 MHz bands, whereas an 80 MHz transmission band is formed by two contiguous 40 MHz bands, in which one of the 20 MHz bands is the primary channel and the rest are secondary channels. However,a 160 MHz transmission band is formed by both lower and higher 80 MHz bands which may be either contiguous or noncontiguous.Note that such a channelization structure implies that only a specific set of primary and secondary channels can be used to construct wider channel bandwidths.