Optimized Rotations for LDPC-Coded MPSK Constellations with Signal Space Diversity

For multi-level modulation methods, rotation of the signal constellation together with in-phase and quadrature phase channel interleaving (signal space diversity) are known to provide good performance gains over fading channels. This paper studies the extension of such schemes with a low density parity check (LDPC) code. It is shown that for both coded and uncoded Gray-mapped MPSK modulation formats with signal space diversity on a Rayleigh fading channel, a well-considered choice of the rotation angle may lead to a significant gain over the conventional unrotated constellation. However, the optimum rotation angle for the coded scheme may be different from the corresponding optimization angle of the uncoded scheme.