Distance-aware beamspace modulation for near-field spectral efficiency improvement

In the near-field region of extremely large-scale multiple-input multiple-output (XL-MIMO), the spatial degrees of freedom (DoFs) increase sharply. Beamspace modulation (BM) is an effective scheme for leveraging the massive DoFs of XLMIMO with a small number of radio frequency (RF) chains. This letter proposes a distance-aware BM (DABM) strategy to achieve greater spectral efficiency (SE) improvement. By introducing selection switches, the number of activated RF chains is dynamically adjusted based on distance-related DoFs. Then, we derive a concise upper bound for the SE of BM. Based on this, we derive the optimal number of RF chains to reach the maximum SE improvement at different spatial DoFs. Finally, simulation results demonstrate that DABM effectively adapts to varying DoFs at different distances, thereby enhancing SE.