Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
21
The VoWiFi network has to be designed in such a way that the adverse effect of these
reflections is minimized.
The MIMO feature used in the 802.11n standard util
izes more than one radio and one
antenna at the same time. This allows the AP and STA to use multiple streams of data which
are separated in the air by their phase because they have travelled different paths.
In a legacy WiFi network, receiving signals with different travel path and phas
e will cause
the signal to be corrupted and thus, not possible to be decoded by the receiver.
In the 802.11n standard the multipath signals can be decoded by the individual antennas/
radios, where each transm
itter and receiving antenna may be able to form a spatial stream.
If the antenna pairs are in line of radio sight to each other this will work just fine.
Contradictory to what most people are taught in classes that multipath is beneficial for
802.11n, even if the signals have been reflected in several ways on its route to the receiver,
too much multipath is bad for 802.11n. Each signal stream can be corrupted in the same
way as a single legacy stream, if the multipath propagation is too large.
The difference with the 802.11n standard is that to a cer
t
ain degree it can tolerate
multipath and it can use it to create multiple spatial streams. The establishment of multiple
spatial streams is up to the AP and the STA to negotiate. For a moving target like a voice
VoWiFi Handset this of course will be more difficult since the radio environment changes
constantly.