Design of LDPC code strategy for orthogonal modulation

M-ary bi-orthogonal keying (MBOK) is a kind of orthogonal modulation that uses the spectrum efficiency to obtain a power gain in the power limited system. In this paper, coding strategy combined with MBOK is investigated to get an optimal combination performance. Specifically, advantages of orthogonal modulation is analyzed and compared with spread spectrum method. Then the error pattern of MBOK modulated signals through AWGN channel is studied to illustrate problems of the combination of LDPC and MBOK. Meanwhile, we investigate the matched combination strategy when modulated by MBOK. And BICM is employed to preserve the soft iterative decoding information when unmatched. At last, we test the performances of these three combination strategies in AWGN channel. According to simulation results, matched GF(q) LDPC strategy for MBOK modulation has at least 0.5db advantage over the unmatched combination when the BER is 1E-5. The gap to Shannon limit could be less than 0.5db. We also discover the unmatched combination could be improved by using BICM, but the performance is still a little worse than the matched combination.