Generalized PSK for improved iterative decoding and demodulation of coded DPSK systems

The transmission of coded DPSK signals over a time-varying channel is considered. Coded DPSK is similar to a serially concatenated coding scheme where the inner encoder is replaced by a differential modulator. At the receiver side, the a posteriori probability (APP) calculation algorithm is applied for differential demodulation, followed by an outer APP channel decoder. The likelihood values at the output of the channel decoder are fed back to the differential demodulator in an iterative decoding loop. Such a system shows large coding gain for transmission over a time-varying flat fading channel if the receiver has perfect channel state information. However, perfect channel knowledge is normally not available at the receiver side. Therefore, joint channel estimation and demodulation has to be applied, leading to a dramatic performance degradation for conventional DPSK. In order to maintain the large coding gain even without any channel knowledge, we propose a novel concept for DPSK by applying regular and generalized PSK symbols in an alternating manner. We evaluate the proposed system on the basis of extrinsic information transfer (EXIT) and bit error ratio (BER) charts.