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Link Budget Calculation
EIRP = P
- Ct + Gt = 16 dBm
out
Pl = 32.4 + 20xLog(FMHz) + 20xLog(RKm)
Si = EIRP - Pl + Gr - Cr = -82 dBm
In conclusion, the received signal power is above the sensitivity threshold, so the
link should work. The problem is that there is only a 2 dB difference between
received signal power and sensitivity. Normally, a higher margin is desirable due to
fluctuation in received power as a result of signal fading.
Signal Fading
Fading of the RF signal is caused by several factors:
Multipath
The transmitted signal arrives at the receiver from different directions, with dif-
ferent path lengths, attenuation and delays. The summed signal at the receiver may
result in an attenuated signal.
Bad Line of Sight
An optical line of sight exists if an imaginary straight line can connect the antennas
on either side of the link.
Radio wave clear line of sight exists if a certain area around the optical line of sight
(Fresnel zone) is clear of obstacles. A bad line of sight exists if the first Fresnel zone
is obscured.
Distance
Weather conditions (rain, wind, etc.)
At high rain intensity (150 mm/hr), the fading of an RF signal at 2.4 GHz may reach
a maximum of 0.02 dB/Km. Wind may cause fading due to antenna motion.
Figure 30: Multipath Reception
62
110 dB
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