MU-MIMO also brings a heavy signal processing burden on the transmitter in order to multiplex the wireless frames across the spatial streams.
802.11af—Allows unlicensed 802.11 operations in the spectrum known historically as the
Television White Space (TVWS), between 54 and 790 MHz. Many of 802.11ac’s features are leveraged, such as OFDM, 10 MCS choices, channel aggregation, multiple spatial
streams, and MU-MIMO. The lower frequencies used tend to penetrate physical objects better,
improving the effective range. 802.11ah—Allows devices to communicate on frequencies
below 1 GHz. The emphasis is on a greater range (1 km), lower power consumption, and
connectivity to a large number of devices dispersed over the coverage area.
Tip
IEEE 802.11 WLANs are always half duplex because transmissions between stations use the same
frequency. Only one station can transmit at any time; otherwise, collisions occur.
To achieve full-duplex mode, one station’s transmission would have to
occur on one frequency while it receives over a different frequency—much like full-duplex Ethernet
links work. Although this is certainly possible and practical, the 802.11-2012 standard does not permit full-duplex operation. The 802.11ac amendment will somewhat ease that restriction in its “Wave 2” implementation, through the use of downstream multi-user MIMO (MU-MIMO).
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