Radio Signal Propagation

C-5

Link Parameters
Frequency: 2.4 GHz
Pout = 4 dBm (2.5 mW)
Tx and Rx cable length (Ct and Cr) = 10 m. cable type RG214 (0.6 dB/meter)
Tx and Rx antenna gain (Gt and Gr) = 18 dBi
Distance between sites = 3 Km
Receiver sensitivity (Ps) = -84 dBm
Link Budget Calculation
EIRP = Pout - Ct + Gt = 16 dBm
Pl = 32.4 + 20xLog(FMHz) + 20xLog(RKm) ≅ 110 dB
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.
Figure C-6: Multipath Reception
♦ Bad Line of Sight
An optical line of sight exists if an imaginary straight line can connect the anten-
nas 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.