Diversity combining methods for convolutional coded and soft-decision Viterbi decoded channels in mobile satellite communication systems are evaluated. Computer simulations confirm that the pre-Viterbi-decoding maximal-ratio combining method has better performance than other methods in Rician fading channels. Bit error probability performances derived from the analysis model using the probability density function of Rician fading and the bit error probability performance of Viterbi decoding in additive white Gaussian noise (AWGN) channels are shown to agree with the simulation results. The diversity method is applied to trellis-coded 8PSK modulation and coherent detection/differential detection, and their performances are compared with conventional QPSK modulation and coherent detection with high-coding-rate and high-coding-gain Viterbi decoding. QPSK with high-coding-rate Viterbi decoding can be an attractive candidate for mobile satellite systems as well as trellis-coded 8PSK.