Page 433 - From GMS to LTE

Wireless Local Area Network (WLAN) 419

Table 6.5 Feature combinations and resulting transmission speeds.

20 MHz, two MIMO 40 MHz, two MIMO
20 MHz, no MIMO (Mbit/s) streams (Mbit/s) streams (Mbit/s)

802.11b 1, 2, 5.5, 11 – –
802.11g 1, 2, 6, 9, 12, 18, 24, 36, 48, 54 – –
802.11n, GI 800 ns 6.5, 13, 19.5, 26, 39, 52, 58.5, 13, 26, 39, 52, 78, 104, 27, 54, 81, 108, 162,
65 117, 130 216, 243, 270
802.11n, GI 400 ns 7.2, 14.4, 21.7, 28.9, 43.3, 14.4, 28.9, 43.3, 57.8, 30, 60, 90, 120, 180,
57.8, 65, 72.2 86.7, 115.6, 130, 144.4 240, 270, 300

Owing to the many variables, such as the number of MIMO channels, long or short
guard time, modulation, coding, etc., there are up to 77 possible combinations resulting
in different transmission speeds. Table 6.5 shows a number of examples.
Channel bundling and the shorter GI, for example, more than double the transmis-
sion speed because of the reduced time per symbol, the use of additional subcarriers
and the reduced number of pilot channels. The effect of a shorter GI is most profound
with a 40 MHz channel and two MIMO streams as the PHY transmission speed
increases from 270 to 300 Mbit/s.
Together with the previously discussed enhancements, 2 × 2 MIMO (two transmitter
antennas and two receiver antennas) can achieve a 5× improvement over 802.11g and a
maximum transfer speed of 300 Mbit/s on the PHY. In a 4 × 4 MIMO system that uses
four antennas on each side, the theoretical peak datarate is 600 Mbit/s. In practice,
speeds of around 80–110 Mbit/s can be reached at the application layer under ideal
conditions, that is, within distances of a few meters without walls between the transmit-
ter and receiver, and by disabling backward compatibility (Greenfield mode). In addi-
tion, the AP needs to be equipped with gigabit Ethernet ports to be able to forward data
at speeds exceeding 100 Mbit/s. Under less ideal conditions, devices automatically
select a more robust modulation (16‐QAM, QPSK or BPSK) and a more conservative
coding such as 3/4, 2/3 or 1/2.
Another important property of 801.11n‐certified devices is the support of QoS mech-
anisms on the air interface as specified in 802.11e. With this extension, data packets of
applications such as VoIP programs can be preferred. This way, voice telephony and
other delay‐sensitive applications can be used over the air interface even in heavily
loaded networks, as the packet delay is deterministic.
To announce the new capabilities introduced with 802.11n, a number of new param-
eters have been defined, which are broadcast in beacon frames. The most important is
the ‘HT Capabilities’ parameter (element ID 45), which describes which HT options are
supported by the AP. The following list gives an overview of the options:
indication of 40 MHz mode support;
●
the number of supported MIMO streams modulation and coding schemes (MCS);
●
support of the short guard time (400 nanoseconds);
●
support of the optional MCS feedback mode. If supported, the receiver can inform
●
the transmitter about current reception conditions. This helps the transmitter to
adapt the modulation and coding rates accordingly;