Specifications

Basic Cell Planning 21
The recommendations above ensure a fading margin of approximately 20dB which should be ap-
propriate for “normal” environments.
NOTE: The illustration in <Blue>Figure 3 is valid when all APs’ transmission power are configured
to 100mW (20dBm). Since the handset transmission power is pre-configured to approximately 100
mW, this ensures a symmetric wireless link.
Note that the illustration also is valid for other transmission power settings, but the same power
setting must be set in both the handset and AP.
Range vs. Transmission Rate
In order to maintain high capacity in each cell, the radio signal strength must be sufficient at all
places in the cell where STAs are expected.
802.11 STAs have the possibility to choose transmission (Tx) rate on a per packet basis. The rates
spans from 1Mbit/s to 54Mbit/s (a/b/g) 65Mbit/s (n) and only affects the payload portion of each
packet. The different Tx rates are obtained by the use of different modulation schemes. A higher
transmission rate uses a more complex modulation scheme than a lower transmission rate.
• 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.
RF Signal Corruption in an VoWiFi System
There are several causes of signal corruption in a VoWiFi system, and the primary causes are sig-
nal attenuation due to distance, penetration losses through walls and floors and multipath propa-
gation.
Free Space Loss
Free space loss (FSL) means that there is a weakening of the RF signal due to a broadening of
the wave front (signal dispersion). The RF signals grow weaker as the cell grows larger or the dis
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tance becomes greater.
Distance Attenuation
The distance attenuation is highly dependent on the construction of the building, floor plan layout
and wall construction material. Some rough figures of attenuation for different materials are pre
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sented in the tables below.