Alvarion BREEZENET BU-DS.11 Manuale Utente
Radio Signal Propagation
C-5
Link Parameters
Frequency: 2.4 GHz
P
P
out
= 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
Tx and Rx antenna gain (Gt and Gr) = 18 dBi
Distance between sites = 3 Km
Receiver sensitivity (Ps) = -84 dBm
Link Budget Calculation
EIRP = P
out
- Ct + Gt = 16 dBm
Pl = 32.4 + 20xLog(FMHz) + 20xLog(RKm) ≅ 110 dB
Si = EIRP - Pl + Gr - Cr = -82 dBm
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.
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
♦ 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.
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.
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.
(Fresnel zone) is clear of obstacles. A bad line of sight exists if the first Fresnel zone
is obscured.