8-ary Unbalanced PSK Turbo Coded Modulation with Low Complexity Soft-Demapping

The 8-ary phase shift keying (8-PSK) constellation remains today widely used in turbo coded modulation schemes transmitting 3 coded bits per signal waveform. The present paper investigates a binary turbo coded modulation scheme using unbalanced 8-PSK (8-UPSK) constellations. A 8-UPSK with a structure similar to that of quadrature amplitude modulation (QAM) is proposed, yielding a low complexity calculation of the soft-decision associated with each bit. We suggest an architecture for computing the exact logarithm of the maximum likelihood ratio with a complexity reduced by more than 50% compared to that required for 8-PSK demapping. In addition, Monte-Carlo simulations show that some UPSK turbo coded modulations outperform the regular PSK modulation, hence achieving improved error performance with a reduced computational complexity.