LANCOM Reference Manual LCOS 3.50 Chapter 11: Wireless LAN – WLAN
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Wireless LAN – WLAN
The data transmission rate is set according to the reception power. A
WLAN module has an input sensitivity equivalent to a power level of,
for example, -80dBm. If the received power falls below this level, th en
a lower data rate can be switched in that corresponds with an
improved sensitivity with a lower level of power.
햳Outdoor wireless connections are usually realised with external antennae
and extension cables together with lightning protection for safety. The
power loss from the cable is approx. 1 dB per metre. A cable 4 m long thus
reduces power by 4 dB, the lightning protection and the various plug
connections also lead to the loss of a further 1 dB. Thus the power of the
external antenna is:
17 dBm - 4 dB - 1 db = 12 dBm.
햴The power received by the antenna is then amplified. An AirLancer
Extender O-18a (with an emitting angle of 18°) supplies an antenna gain
of 18 dBm. The total power output from the antenna is thus:
12 dBm + 18 dBm = 30 dBm.
This power emission must be within the legal limits of the country
where the antenna is in operation!
햵Radio transmission through air is subject to power attenuation from the
so-called "free-space loss" x, which is logarhythmic ally related to the
distance d (in km) between transmitter and receiver.
x = 100 + 20 * log (d) [dB] in the 2.4-GHz band
x = 105 + 20 * log (d) [dB] in the 5-GHz band
A 802.11a transmission over a distance of 4 km results in a free-space loss
x of:
x = 105 dB + 20 * log (4) dB = 105 dB + 12 dB = 117 dB.
햶A 10 dB safety margin is added to this attenuation so that the total loss
for this example can be taken as 127 dB.
햷This loss between the transmitting and receiving antenna is subtracted
from the output power of the transmitting antenna:
30 dBm - 127 dBm = - 97 dBm.
This determines the reception power at the receiving antenna.