The IEEE 802.11ac is actually a better version of the 802.11n. It is built with a 5GHz ISM band but only for a minimum interference and maximum bandwidth. It continues the use of MIMO and OFDM just like its predecessors. However, it also has some primary changes such as it now has a wider channel from 80 and 160 MHz than the usual 40 MHz channel. Since the bandwidth has greatly increased, so do the number of OFDM subcarriers to a maximum of 512 at 160 MHz bandwidth.
