This paper proposes a new method to decode π/4 shift DQPSK signal named as Output Shifted Coding Modulation (OSCM). OSCM is a combination of channel coding and modulation, also known as coded modulation. Traditionally, receive signal from π/4-shift DQPSK demodulator is converted to binary signal for hard decision decoding or quantized signal for soft decision decoding. The decoding methods that usually used are Viterbi decoder (VD) and Turbo Code. Although turbo code are powerful coding, the implementation are complicated compared to VD. For this new method, received signal from demodulator is directly decoded by OSCM decoder using modified trellis diagram. OSCM introduce two types of decision methods for error correction calculation which are multilevel decision and soft decision. The result from simulation and calculation shows that BER for OSCM are improving compared to high constraint length, k hard decision Viterbi decoder.

Channel coding, Convolutional codes, Viterbi decoding, Trellis codes