SOVA decoding of concatenated convolutional encoders and CPM scheme over AWGN and fading channels

Continuous phase modulation (CPM) is investigated. We employ the decomposed CPM model where the model is decomposed into a continuous phase encoder (CPE) and a memoryless modulator (MM). This allows us to concentrate on the encoding aspects without interference from the modulator. By designing a convolutional channel encoder (CE), to operate over the same algebra as the CPE, the CE and the CPE can be combined and treated in a unified manner. This combination is called an extended CE, as the CPE is effectively an extension of the CE and is a convolutional encoder. In this research, a 1/2 rate CE over the ring of integers modulo 4 is combined with the CPE of a 4-ary CPM scheme whose modulation index is h=1/4. Using this trellis coded modulation design the paper explains the design of a serially concatenated channel coded system, where the inner encoder is the quaternary extended CE and the outer encoder a quaternary convolutional encoder. SOVA decoding is employed. Simulations show that a BER of 2.6×10^-4 at 0.3 dB SNR is obtainable in an AWGN channel