802.11ac: A Survival Guide for Free
For a limited time O’reilly is offering free online access over 50 books as part of their Atlas Beta program. Atlas is an attempt to create an interactive experience around O’reilly titles.
One of the titles that is included in this beta access program is Matthew Gast’s 802.11ac: A Survival Guide. If you are new to WiFi or well versed in 802.11 this is a must read if you are looking to get updated on 802.11ac. One of the most useful sections of this book for me is the “802.11ac Data Rate Matrix” that explains why 802.11ac has fewer MCS index values compared to 802.11n.
One of the reasons that 802.11ac has many fewer options for MCS values than 802.11n is that the MCS value is no longer tied to the number of spatial streams. In 802.11n, MCS 0 and MCS 8 both use BPSK with R=1/2. In 802.11ac, the MCS value is defined only as a modulation and code set, and no longer includes the number of spatial streams. The second way that 802.11ac simplified the MCS selection is that it dropped the unequal modulation option (802.11n MCS values from 33 to 76).
Unequal modulation is specified in 802.11n to support beamforming. Transmit beamforming in the form used by 802.11 results in each spatial stream having a different signal-to-noise ratio (SNR). Unequal modulation was designed so that high-SNR streams could use high-data-rate modulation options, and low-SNR streams could use low-data-rate modulation options. As an example, 802.11n MCS 42 modulates one stream at 64-QAM, one stream at 16-QAM, and one using QPSK; this modulation was intended for use with one high-SNR stream, one medium-SNR stream, and one low-SNR stream.
802.11ac eliminated unequal modulation as part of its simplification of data rates, and as a result, 802.11ac transmit beamforming requires that all spatial streams be modulated identically.
Read the entire 802.11ac: A Survival Guide for free online before the beta program ends.
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