In this paper, a reduced-state trellis for M-ary CPFSK is presented under the condition that the MSED is not reduced therefore the complexity of Viterbi decoding is reduced.

Viterbi algorithm is a kind of maximum likelihood decoding method, which can obtain high decoding speed and cost fixed decoding time, so it is widely used in deep space communication.

So this paper focuses on Convolutional code and Viterbi decoding algorithm.