Inverted BER Trends for Energy-Detected GRSM-MQAM Massive MIMO Downlink

This work investigates Generalized Receiver Spatial Modulation (GRSM) in a massive MIMO downlink scenario over millimetre wave channels. While GRSM enhances spectral efficiency (SE) and reduces power consumption, indexing additional bits using the spatial dimension increases the vulnerability to detection errors. These errors primarily stem from thresholddependent spatial detection. We propose a novel predefined threshold computation method minimizing spatial detection errors, rigorously validated through the Maximum A Posteriori (MAP) criterion. Furthermore, we derive an analytical Average Bit Error Probability (ABEP) expression tailored for energy detection, exploiting inherent constellation energy distributions. The analytical derivation was validated via link-level simulations under two scenarios: (i) perfect spatial detection, and (ii) practical spatial detection. The results show spatial errors dominating overall performance, shifted from theoretical values by practical spatial detection using multiple thresholds for different energy levels in 16QAM. Crucially, an inverted error trend revealed between GRSM-4QAM and GRSM-16QAM, highlighting a tradeoff between error resilience, complexity, and energy efficiency.