Low-complexity switched soft demapper for rotated non-uniform constellation DVB-NGH systems

In this paper, we propose a low-complexity switched soft demapper for rotated non-uniform constellation (e.g., nonuniform QAM, non-uniform APSK) DVB-NGH systems. The constellation would be twisted seriously through an I/Q independent fading channel. The requirement for two-dimensional (2D) soft demapping leads to the exponentially increased complexity. The proposed demapper classifies the received symbols into the following two types based on a decision criterion: a) Those not seriously twisted symbols can be separated into two independent components after zero forcing (ZF) equalization for low-complexity one-dimensional (1D) soft demapping; b) Those seriously twisted ones require implementing 2D soft demapping to avoid performance degradation. Our simulations verify that the proposed scheme reduces the demapping complexity by 70% and 45% for a rotated non-uniform QAM (NU-QAM) and a nonuniform APSK (NU-APSK) respectively while the performance degradation is negligible as compared to the traditional full search algorithm. The demapper can be further simplified by combining the proposed scheme with other simplified algorithms for 2D soft demapping. The proposed demapper exploits the channel state information and can be applied not only to the rotated NU-QAM, but also to the NU-APSK constellation.