Page 439 - From GMS to LTE

Wireless Local Area Network (WLAN) 425

doubling of the channel bandwidth. In practice, however, even high‐end notebooks are
often only equipped with two antennas and hence a 160 MHz channel will double data
throughput in this scenario.
In most parts of the world, the spectrum available in the 5 GHz band is around
400 MHz, which allows four to five non‐overlapping 80 MHz networks. As with the
previous PHYs, it is also possible to operate many networks on the same channel, which
then share the available bandwidth. When several networks are used on the same chan-
nel, devices of one network can sense the transmission of devices of the other network
that are within range and can thus apply collision avoidance mechanisms as described
at the beginning of this chapter. In practice, however, several networks on the same
channel are usually not fully overlapping in a geographical sense. As a consequence,
some devices in one network may not be able to sense transmissions of devices com-
municating with another AP and hence, the collision avoidance mechanism is not as
effective as is the case when only one network is present on a single channel.
To improve the interworking between networks on the same channel, a number of
features have been introduced with 802.11ac. As discussed before, a device senses the
use of the air interface before it starts to transmit its own frame. If an ongoing transmis-
sion is detected, the device’s own transmission is deferred. This is known as collision
avoidance. In practice, this is achieved by two methods, namely, signal sensing and
energy sensing. Signal sensing means that a device properly receives the beginning of a
frame and is thus aware by decoding the header information how long the channel will
be busy for. Energy sensing on the other hand blocks the use of the channel if a certain
signal level is detected on the channel.
For backward compatibility reasons, 802.11ac divides the full channel bandwidth into
20 MHz chunks. An 80 MHz channel thus has four chunks. When the RTS/CTS scheme
is used to reserve the channel, individual packets are sent simultaneously on all 20 MHz
chunks of the channel, so 11 ac networks on the same channel as 11n networks that
support only 40 MHz channels and 11a networks that support only 20 MHz channels
can properly detect the channel reservation (Clear Channel Assessment, CCA). This
can also be used to reduce the channel bandwidth for a transmission if the CTS is, for
example, received on only two 20 MHz channels instead of four due to interference
from overlapping networks. This decision can be taken on a per‐frame basis, which
makes the system very flexible.
In practice, not all devices in a network may support 80 MHz transmissions. Therefore,
a method has been specified to coordinate the use of several independent networks on
the same channel to allow simultaneous transmissions in two networks. This is done by
splitting the channel bandwidth into a primary channel and a non‐primary channel.
The channel bandwidth is half of the full channel bandwidth, for example, 40 MHz in an
80 MHz network. Two fully overlapping 80 MHz networks thus configure themselves in
such a way that each network uses a different primary channel. If two 40 MHz devices
belonging to two different 80 MHz networks want to transmit data simultaneously, they
would each use the primary channel of their respective network, which does not overlap
with the primary channel of the other network. Each of the devices senses that the
channel is free and no collision will occur. If a transmission on a non‐primary channel
is already ongoing, a device can either wait for another transmission opportunity or
transmit only on the primary channel. Such dynamic bandwidth usage can significantly
improve the overall spectrum usage in networks with devices that are unable to use the