Installation guide
TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
20
• The lower the transmission rate, the more energy per bit is available at the receiver's
detector. Thereby the transmission range is increased by lowering the transmission rate
and thus the transmission will take longer.
As an 802.11 STA moves away from an AP, the Tx rate is lowered in
order to increase the
range. This has effects on the capacity in the cell. Since all STAs in a cell shares the capacity
(air time), a reduction in Tx rate for one STA reduces the overall available capacity for all
STAs in that cell.
5.2 RF Signal Corruption in an VoWiFi System
There are several causes of signal corruption in a VoWiFi system, and the primary causes are
signal attenuation due to distance, penetration losses through walls and floors and
multipath propagation.
5.2.1 Free Space Loss
Free space loss (FSL) means that there is a weakening of the RF signal d
ue t
o a broadening
of the wave front (signal dispersion). The RF signals grow weaker as the cell grows larger or
the distance becomes greater.
5.2.2 Distance Attenuation
The distance attenuation is highly dependent on the
construction of the building, floor plan
layout and wall cons
truction material. Some rough figures of attenuation for different
materials are presented in the tables below.
Figure 5.
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
Table 1 - Estimation of attenuation for differe
nt construction ma
terials for -b/g radio.
Note: The attenuation for the -a radio is, from a general
point of view, higher than for -b/g.
5.2.3 Multipath Propagation 802.11n Radios
In relation to the two causes of sign
al corruption mentioned above, the main concern should
be the -a and -b/g radio difference of multipath (reflection, refraction, diffraction and
scattering causing signal upfade) and delay spread of the RF signal path (causing signal
downfade or even signal corruption) between the VoWiFi Handset and AP.
Multipath is that the receiver not only contains a di
rect 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.