This paper is concerned with the analytical performance analysis and evaluation of 49-ary Quadrature Partial Response Signaling when transmitted through nonlinear satellite channel in the presence of Additive White Gaussian Noise and Co-channel Interference ; in both uplink and downlink channels . The main source of the nonlinearities is the Traveling Wave Tube Amplifier on-board of the satellite. The transponder nonlinearities considered in this paper are due to : input amplitude -to-output amplitude conversion and input amplitude-to-output phase conversion . The cochannel interference results from interferers in the passband of the coherent ,receiver . The results in terms of the dependence of the average symbol error probability upon the uplink and downlink , signal-to-noise ratio's and Signal -to cochannel interference signal ratio's at different values of Back-Off from saturation operation of the Traveling Wave Tube Amplifier on-board of the satellite are illustrated . The results showed that the Back-Off value is the dominant factor in determining the system performance .The appropriate values of the threshold levels ; d"s and compensation phases ; θ's at the receiver are highly correlated and can only be arrived at by minimizing the average symbol error probability .