a) Stronger modulation method
Wireless transmission is affected by a number of factors that can damage the RF signal quality and cause
the receiver fail to read. So, each data bit is encoded into multiple bits before transmission. This way of
adding redundant information to the data can improve the system's ability to resist data corruption, which
we call encoding. After the data is encoded, the radio transmitter performs carrier modulation on the data,
and the modulation method adopted by 802.11n/ac is QAM.
The densest modulation method of 802.11n is 64 order QAM. 802.11ac uses a new scheme to achieve 256
order QAM, which contains 33% more data than 64-QAM into each constellation mapping point.
256-QAM corresponds to two new Modulation Coding Scheme (MCS 8 and MCS 9) that achieve the higher
data connection rate than MCS 7 in the same air time. However, 256-QAM requires a more sensitive
detection on phases and amplitudes in the modulation. Therefore, it is more sensitive to the noise in the
environment and requires a cleaner signal with a higher signal-to-noise ratio (SNR).
The following figure 10 is the required SNR value in the 256QAM modulation mode. When the SNR is less
than 27dB, the bit mapping point on the constellation cannot be resolved at all, and the receiver cannot be
correctly parsed. And this failure transmission will lead to retransmissions.
Figure 10: Required SNR value in the 256QAM modulation mode
Therefore, in actual deployment, modulation high like 256QAM can only be achieved in a small distance
range. This requires a full visual line of sight between the wireless access point and the client, and it is
only possible to establish an MCS 8 or MCS 9 data connection rate within 10 meters. Otherwise it will
drop to the lower data connection rate, such as MCS7 by 64-QAM modulation.
b) Wider channel width
By combining channels into a wider channel, just like expanding highway lanes, the traffic volume per unit
time will increase exponentially. The wider channel width is used in the spectrum, the more data can be
transmitted.
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