The aim of this paper is to propose a method for reducing BER for signals with DOQPSK and CDOQPSK. In this method Viterbi algorithm is used to provide the reception of signals in general. It is shown that for BER 10[–5] Viterbi algorithm gives 2,1–3,4 dB energy gain (including peak-to-average power ratio) compared to π/4-DQPSK signals.