Installation guide

TD 92408EN

9 December 2011 / Ver. G

System Planning

Ascom VoWiFi System

6 Co-Channel Interference

There are only three non-overlapping channels available in the 2.4 GHz band at 20 MHz

which results in a high probability of channel re-use within a close proximity.

In b/g/n 40MHz channels should be avoided in the 2.4 GHz band. With 40 MHz channel

wid

th, only one or two channels can be used in the WLAN system (depending on country

regulations). Further, interference with neighboring WLANs is more likely due to increased

coverage.

There are 19 channels available in total in Europe and 24 in

the USA (FCC channels), whereof

there are four non-DFS in Europe and nine non-DFS in the USA. Data traffic only can use DFS

channels, but it is not recommended for voice, since VoWiFi Handsets can not use active

scanning due to DFS regulations.

Note: The VoWiFi Handset ca

n use the DFS cha

nnels, but the Voice quality may be distorted.

How closely these channels are reused is depende

nt on the geometrical prerequisites of the

site that shall be covered. If it is a one-floor hallway only, there will be enough distance

separation before re-use of the same channel is needed. For a multi-story building with a

large floor area, it will be impossible to have coverage at all places without having adjacent

cells that use the same channel to some extent.

Installing two adjacent cells working on the

same channel introduces the following

problems:

1 Capacity reduction. All STAs in the two cells will share t

he RF channel as if they were

present in one cell.

2 Error introduction. The STAs will

introduce transmission errors due to the “hidden

node problem” described in 6.2 Hidden Node Problem on pa

ge 23.

6.1 Clear Channel Assessment, CCA

a/b/g

802.11 specifies a distributed channel access function that basically can be summariz

ed as

“listen before talk”. The “listen” procedure is called clear channel assessment and reports if

the media (air) is busy or idle. If a STA wants to transmit a packet, it must first determine if

the media is idle, then it can transmit the packet. If the media is busy, the STA has to wait

for the media to be idle. The same channel access rules apply for an AP.

CCA is affected also by non-802.11 RF signals in the 2.4 GHz band.