User's Manual

February 2003
Rev. A
Page 32 of 47
5. Appendix B: Calculation RF Link Budget
Proper RF link planning ensures that the AU/SU receives sufficient signal power to maintain the
desired Bit Error Rate (BER). The following section gives a brief description of the basic RF terms
and describes the calculation of the maximum safe distance versus the antenna gain.
A typical radio system is given hereunder:
AU
(Transmitter)
SU
(Receiver)
Gt
Gr
Pt Pr
Ct
Cr
Figure 5-1: Radio Link – General description
The following variables are used to calculate the link budget:
P
t
- Transmitted Power in dBm.
C
t
– Transmitter Cable Attenuation in dB.
G
t
- Transmitting antenna Gain in dBi.
EIRP – Effective Isotropic Radiated Power in dBm. This is the power radiating from the antenna,
taking into account the output power from the transmitter, connector losses, cable losses
and antenna gain.
PL - Path Loss in dB. This is the signal loss as it travels through the air.
G
r
- Receiving antenna Gain in dBi.
C
r
– Receiver Cable Attenuation in dB.
P
r
– Receiving Power Level at Receiver in dBm.
S
r
– Receiver Sensitivity in dBm (The minimum RF signal power level required at the input of the
receiver for certain performance IE-5 BER)
GM – Gain (Fade) Margin in dB. The fade margin is the amount by which the system gain plus
total gain exceeds the path loss or in other words this is the number of dB that the received
signal strength exceeds the minimum receiver sensitivity. Any wireless system requires some
level of fade margin to compensate for RF path fading due to weather conditions or multipath
interference. (The transmitted signal arrives at the receiver from different directions, with
different path length, attenuation and delays. The summed signal at the receiver may result
an attenuated signal). The GigAccess™ recommended fade margin at 2.4 GHz is 10 dB
minimum.