Page 440 - From GMS to LTE

426 From GSM to LTE-Advanced Pro and 5G

full channel bandwidth or in situations in which an 11 ac network is used on the same
spectrum as a network using a legacy PHY with a 20 or 40 MHz channel.
It should be noted at this point that the 2.4 GHz band is too small for 80 or 160 MHz
channels. As a consequence, 802.11ac is specified only for the 5 GHz band. In practice,
802.11ac‐compatible chips also support 802.11b, g and n in the 2.4 GHz band and
802.11a, n and ac in the 5 GHz band.
In addition to larger bands, a new modulation scheme has been added for situations
with exceptionally good channel conditions. Although 802.11n supports the transmis-
sion of up to 6 bits per transmission step with 64‐QAM modulation, 802.11ac now
allows 8 bits per transmission step, that is, 256‐QAM modulation. Together with a code
rate of 5/6, which is the number of overall bits to the number of user data bits plus error
correction bits, the new scheme requires a 5 dB better receiver performance compared
to that of 64‐QAM [12]. This requires less noise and a stronger received signal, which
can be partly achieved with an improved receiver performance as chips get more sensitive
and sophisticated over time. Adding 2 bits per transmission step increases the data
transmission speed by about 30% compared to a 64‐QAM transmission. Table 6.6 gives
an overview of the MCS that have been specified for 802.11ac. MCS0 is the combina-
tion of very conservative modulation (BPSK) that transmits only 1 bit per step and a
coding of 1/2, that is, the same number of data and error correction bits.
802.11ac has also retained the use of a short guard interval of 400 nanoseconds
between OFDM symbols that was introduced in 802.11n. In practice, it can be observed
that this feature is widely used in networks, resulting in a performance increase of about
10% when compared to 802.11a and g.
Another way to increase theoretical maximum data transmission speed is to increase
the number of MIMO streams. In 802.11ac up to 8 × 8 MIMO is supported, which goes
significantly beyond the 4 × 4 MIMO mode of 802.11n. In practice, however, it is already
difficult to realize the benefits of 4 × 4 MIMO, and the combination of 256‐QAM and
several MIMO streams requires an even more robust channel to a single subscriber
than is needed for 802.11n today. In practice, even 3 × 3 MIMO with 64‐QAM, typically
used in 802.11n networks today, provides only a small gain over a 2 × 2 MIMO in most

Table 6.6 Modulation and coding schemes in 802.11ac.

MCS Modulation Code rate
0 BPSK 1/2
1 QPSK 1/2
2 QPSK 3/4
3 16‐QAM 1/2
4 16‐QAM 3/4
5 64‐QAM 2/3
6 64‐QAM 3/4
7 64‐QAM 5/6
8 256‐QAM 3/4
9 256‐QAM 5/6