As a result, 802.11ac mandates only a single spatial stream instead of one or two spatial streams as specified in 802.11n. One reason for such change is that increasing the number of antennas often relates to higher cost. Hence,the modes that utilize more than one spatial streams are now optional in 802.11ac. Accordingly, the 80 MHz mode is seen as the lower cost alternative to increase PHY data rate as compared to the 40 MHz mode with two spatial streams, for example. In terms of optional features, 802.11ac defines the support of 160 MHz channel bandwidth for yet another twofold increase in data rate over the mandatory 80 MHz channel bandwidth. In addition, two new MCSs 8 and 9 are introduced based on 256-QAM with code rates of 3/4 and 5/6 for a further 20% and 33% improvements in data rate,respectively, when comparing to the highest MCS specified in 802.11n based on 64-QAM with a code rate of 5/6.