Link budget refresher:
 Received Carrier Power:
P_{r} = P_{t }+ G_{t} – L_{p} + G_{r }(dBW)
 Received Noise Power:
P_{n }= 10 log (k T B) (dBW)
 Carrier to Noise Ratio:
C/N = P_{t} – P_{n}
= P_{t} + G_{t} – L_{p} + G_{r} – 10 log (T)  10
log (B) – 10 log (k) (dB)
Carrier to Interference refresher:
 Received Wanted Carrier Power:
P_{w} = P_{t} + G_{t} (θ) – L_{p} + G_{r}
(φ) (dBW)
 Received Interfering Carrier Power
Pi = P’_{t} + G’_{t}(θ’) – L’_{p} + G_{r}(φ’)
(dBW)
 So carrier to interference ratio:
C/I = P_{w} – P_{i}
= [P_{t} + G_{t} (θ) – L_{p} + G_{r} (φ)]
– [P’_{t} + G’_{t}(θ’) – L’_{p} + G_{r}(φ’)]
Normally simplify by assuming that L_{p} = L’_{p}.
Note that the G_{r} terms both refer to the same antenna (the receiving system
gain).
Define receive antenna discrimination, D_{r} = G_{r }(φ)  G_{r}
(φ’), typically –3 or –4 dB for a satellite receive antenna, generally much
higher, say +20 to +40 dB for an earth station receive antenna.
So simplify terms:
C/I = P_{t} + G_{t} (θ) + D_{r} – P’_{t} – G’_{t}(θ’)
(dB)
This is a very simple equation; satellite to satellite coordination is not technically
difficult to analyse!
Some additional complications to bear in mind:
 P_{t} and P’_{t} can be power spectral densities (dBW/Hz) or powers
(dBW).
 If P_{t} and P’_{t} are densities then strictly you have calculated
C_{o}/I_{o} not C/I (Co/Io ~ C/I for digital carriers).
 If P_{t} and P’_{t} are powers then they may have different
bandwidths  normalise C/I inside the bandwidth of the wanted carrier.
