Link Budget Calculation
EIRP = Pout - Ct + Gt = 16 dBm
Pl = 32.4 + 20xLog(FMHz) + 20xLog(RKm) ? ?110 dB
Si = EIRP - Pl + Gr - Cr =
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.
Figure 30: Multipath Reception
??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.
62
