Low complexity LLR for PSK modulated STBC system

The optimum detection of the transmitted symbol at the receiver requires complicated arithmetic operations which involve comparison of distances between all constellation points and received symbols. In this paper, low complexity LLR (Log Likelihood Ratio) operation for the phase modulated scheme is applied to the STBC(Space-Time Block Code) decoder. The LLR operation utilizes the fact that the distance between the origin to each constellation point is equal for all cases. Also the decision boundary is adjusted to align with the in-phase or quad-phase axis in the signal space. The approximated LLR value can be obtained by calculating the distances between updated bit decision boundaries and received symbols. Simulation results show that the performances of the applied scheme are comparable to the conventional maximum likelihood detection method.