Page 432 - From GMS to LTE

418 From GSM to LTE-Advanced Pro and 5G

whether there are other 20 MHz networks active or not. The 802.11n standard also
applies to the 5 GHz band. Here, up to nine independent 40 MHz channels are available.
As this frequency range is still much less used at the time of publication, it is usually
possible to find an unused channel. In practice, many APs and client devices support
this band today so it has become a viable alternative. However, the downside of the
5 GHz band is the shorter transmission range as higher frequency signals have more
difficulties permeating walls and other obstacles as compared to a 2.4 GHz signal.
Whether an 802.11n device supports both frequency bands can usually only be noticed
when networks in the 5 GHz band are not detected. In particular, entry‐level smart-
phones and notebooks often only support 802.11n in the 2.4 GHz band.
To further increase transmission speeds and network range, the 802.11n standard
specifies a number of Multiple Input Multiple Output (MIMO) transmission schemes
for 20 MHz and 40 MHz channels. Most devices today offer MIMO spatial multiplex-
ing, which transmits several data streams over different transmission paths from the
transmitter to the receiver over the same channel. This requires several antennas at
both ends as each data stream originates from a separate antenna at the transmitter.
Figure 6.17 shows in a simplified manner how this is done. In practice, the two data
streams are usually not completely independent and hence a mathematical procedure is
required on the receiver side to remove the effect of the two data streams interfering
with each other on the way from the transmitter to the receiver.
The standard specifies up to four MIMO channels and APs must support at least two
independent transmissions chains. Devices can inform the AP of their capabilities dur-
ing the association procedure.
In practice, many devices use two or three MIMO channels, which can double or
triple the theoretical peak data transmission rate as compared to those of single stream
transmissions under ideal signal conditions.

First transmission path

No direct line
of sight Obstacle

Obstacle

Obstacle

Access point

Client
Second transmission Obstacle
path
Figure 6.17 2 × 2 MIMO.