Specifications
22 Co-Channel Interference
Figure 4.
NOTE: The attenuation for the -a radio is, from a general point of view, higher than for -b/g.
Multipath Propagation 802.11n Radios
In relation to the two causes of signal corruption mentioned above, the main concern should be
the -a and -b/g radio difference of multipath (reflection, refraction, diffraction and scattering caus
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ing signal upfade) and delay spread of the RF signal path (causing signal downfade or even signal
corruption) between the handset and AP.
Multipath is that the receiver not only contains a direct line-of-sight radio wave, but also a larger
number of reflected radio waves. Because of multipath reflections, the channel impulse response
of a wireless channel looks like a series of pulses.
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 utilizes 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 phase will cause the sig-
nal 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 transmitter 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 peo
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ple are taught in classes that multipath is beneficial for 802.11n, even if the signals have been re-
flected 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 prop
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agation is too large.
The difference with the 802.11n standard is that to a certain 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 handset this of course will be
more difficult since the radio environment changes constantly.
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 width, only
one or two channels can be used in the WLAN system (depending on country regulations). Fur
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ther, interference with neighboring WLANs is more likely due to increased coverage.
b/g
Material Attenuation
Concrete 12 dB
Brick Wall 10 dB
Dry Wall 5 dB
Window 1 dB
Elevator Shaft 30 dB
Thin Door 2 dB
Book Shelf 2 dB
Plasterboard wall 3 dB