A Low-Complexity APSK Iterative Receiver With Convergence- and Oscillation-Aware Early Termination Strategies

In satellite-oriented and resource-constrained communication systems, low-complexity receiver processing is essential due to the limited power budget and computational capability. In this study, we propose a low-complexity soft demapping and iterative processing framework applicable to high-order APSK-coded modulation systems. A constellation-aware soft-decision demapper is developed for 16-APSK modulation. It exploits the geometric characteristics of APSK constellations to obtain channel log-likelihood ratios (LLRs) without resorting to exponential or logarithmic operations. The framework is further extended to 32-APSK modulation. We employ the Max-Log approximation method to construct an efficient bit-interleaved LDPC-coded modulation with iterative decoding (BILCM-ID) architecture. A unified factor-graph formulation is established to characterize the probabilistic relationships in the receiver structure. Furthermore, we introduce an early termination method in iterative message-passing algorithms (iMPAs) to adaptively terminate decoding iterations as well as external demapping-decoding iterations by monitoring the global convergence behavior. The proposed framework provides a unified low-complexity solution for high-order APSK iterative reception, while maintaining the decoding gain and scalability. Simulation results over AWGN and Rayleigh fading channels demonstrate that the proposed framework provides an effective low-complexity solution for APSK BILCM-ID receivers. The simplified demapper achieves substantial complexity reduction with marginal BER degradation compared to the conventional schemes. In addition, the proposed convergence- and oscillation-aware early termination strategy further reduces computational complexity while incurring only negligible performance loss.