Wireless LAN – WLAN
116
3.8
Establishing outdoor wireless networks
BAT54-Rail/F..
Release
7.54
06/08
V Radio transmission through air is subject to power attenuation from the
so-called "free-space loss" x, which is logarhythmically related to the dis-
tance 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.
V A 10 dB safety margin is added to this attenuation so that the total loss for
this example can be taken as 127 dB.
V This 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.
V The receiving end also has amplifying and attenuating elements. If the
same antenna is used as at the transmitter, the antenna gain is 18 dB and
the loss from cable (again 4m), lightning protection and plug connectors
is 5 dB. The radio signal thus arrives at the receiver's radio module with
the following power:
- 97 dBm + 18 dBi - 5 dB = -84 dBm.
V From the table for reception sensitivity of the radio module, the attainable
data rate can be read off, in this case 24 Mbps:
Note: This values are the result of a calculation that includes a 'safety margin'
of 10dB. As every radio path is unique, these values can only serve as a
rough guide.
Reception sensitivity 802.11a [dBm]
Mbps 5.150 -5.725 GHz 5.725 -5.850 GHz
6
-90 -85
9
-89 -84
12
-88 -83
18
-87 -82
24
-85 -80
36
-81 -76
48
-76 -71
54
-73 -68
72 (Turbo)
-78 -73
96 (Turbo)
-73 -68
108 (Turbo)
-70 -65
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