Low complexity two-stage soft demapper for rotated constellation in DVB-T2

The rotated constellation with cyclic Q-delay provides the diversity gain for the signal space over the fading channel when properly demapped at the receiver. However, the implementation of optimal soft demapper is impractical due to the exponentially increasing complexity with modulation order. In this paper, we propose a two-stage soft demapper with low complexity applicable to the DVB-T2 receiver design. It is motivated by the observation that 2D LLR contours have special patterns depending on faded I and Q channel information. Simulation results show that the proposed demapper substantially reduces the computational complexity, 87.5% from the optimal solution for rotated 64 QAM under Rayleigh channel with 5% erasures, while maintaining the optimal BER performance for a given channel condition. The superiority over previous low complexity soft-demapper is also discussed.