The performance of a concatenated coding scheme is studied over nonlinear channels when additive white Gaussian noise is considered in both the up and downlink. Bandwidth-efficient trellis codes using a 32-QAM signal set are used as inner codes and 3/4 rate convolutional codes as outer codes to achieve an overall bandwidth efficiency of 3 bit/s/Hz. Simulation results show that this coding system compared with an uncoded 8-PSK scheme, 3/4 convolutionally encoded 16-QAM systems and some TCM techniques, provides significant coding gains over linear and nonlinear channels.