A modified hybrid RQD demapper based on novel hard to soft assigning algorithm

Rotated constellation with Q-Delay (RQD) is an innovative scheme that appeared to give better performance compared to the traditional unrotated modulation. The bit error rate (BER) decreases due to the RQD scheme signal space diversity. The authors presented four novel computationally efficient demodulation schemes with hybrid soft-hard outputs in [1] that took advantage of the unique projections the rotated cells have in the RQD system. The Coded Modulation (CM) system can utilize the hybrid computationally efficient demodulation schemes for obtaining better results in terms of BER and computation complexity. One of the hybrid schemes was tested with Low Density Parity Check (LDPC) coder/decoder through two proposed Hard to Soft Assigning algorithms (HSA). These algorithms assign estimated Log-Likelihood Ratio (LLR) values to the hard mapped bits, so that the LDPC decoder receives an all-soft stream. Numerical results show that HSA algorithms give better performance than the original all-soft stream at a moderate SNR range.