Installation guide

TD 92408EN
9 December 2011 / Ver. G
System Planning
Ascom VoWiFi System
19
the cells, see figure 3.
Figure 4.
Figure 3. Cell overlap between adjacent cells
The distance between the APs is often a tra
de-off between the amount of APs and
coverage.
To make up for fading effects in an indoor office environment it is recommende
d that the
radio signal strength at the cell coverage boundary does not drop below -70 dBm. The APs
should be placed to overlap their boundaries by approximately 6–10 dB.
This means that when the STA reaches a point where
the RSSI is -70 dBm, the STA is also
inside the adjacent cell and the RSSI from that AP is between -60 to -64 dBm. For
information on distance attenuation and attenuation in construction materials, see 5.2 RF
Signal Corruption in an VoWiFi System on
page 20.
The recommendations above
ensure a fading margin
of approximately 20dB which should
be appropriate for “normal” environments.
Note: The illustration in figu
re 3 is valid when all APs’ transmission power are configured to
100mW (20dBm). Since the Ascom VoWiFi
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 VoWiFi Handset and AP.
5.1 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.