Co-Channel Interference
Clear Channel Assessment
Co-Channel Interference
There are only three non-overlapping channels available in the 2.4 GHz band at 20 MHz that re-
sult in a high probability of channel reuse within a close proximity.
In b/g/n 40MHz channels should be avoided in the 2.4 GHz band. With 40 MHz channel width,
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 handsets can not use active scanning due
to DFS regulations.
NOTICE:
The handset can use the DFS channels, but the voice quality may be
distorted.
How closely these channels are reused is dependent on the geometrical prerequisites of the site
that shall be covered. If it is a one-floor hallway only, there is enough distance separation before
the reuse of the same channel is needed. For a multi-storey building with a large floor area, it is
impossible to have coverage at all places without having adjacent cells that use the same chan-
nel 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 share the RF channel as if they were present in
one cell.
2)
Error introduction. The STAs introduce transmission errors due to the “hidden node problem”
on page 19.
Clear Channel Assessment
802.11 specifies a distributed channel access function that basically can be summarized as “lis-
ten before talk”. The “listen” procedure is called clear channel assessment and reports if the me-
dia (air) is busy or idle. If an 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.
Clear Channel Assessment (CCA) is affected also by non-802.11 RF signals in the 2.4 GHz
band.
Even if APs that use the same channel are placed far away, there can be STAs present in the
cells that are closer and thereby causing transmission interruptions, see
cause problems even for far away STAs
on page 19.
A31003-M2000-P103-01-76A9, 29/04/2020
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System Planning, Planning Guide
