The spatial degrees of freedom (DoFs) greatly increase in the near-field region of millimeter wave or terahertz multiple-input multiple-output communications with extremely large antenna arrays (XL-MIMO). To employ the increased spatial DoFs, a beamspace modulation (BM) strategy is introduced to the near field of XL-MIMO. BM can work with a fixed small number of RF chains. It exploits the increased spatial DoFs as modulation resources for capacity improvements. The achievable spectral efficiency and its asymptotic capacity are analyzed. Both theoretical and simulation results show that the proposed BM strategy considerably outperforms the existing benchmark that only selects the best beamspace for data transmission in terms of spectral efficiency.

翻译：毫米波或太赫兹超大阵列多输入多输出通信的近场区域中，空间自由度显著增加。为利用这些增大的空间自由度，本文提出一种适用于XL-MIMO近场的波束空间调制策略。该策略可在固定少量射频链路的条件下工作，将增大的空间自由度作为调制资源以提升容量。我们分析了可实现频谱效率及其渐近容量。理论与仿真结果表明，相较于仅选择最优波束空间进行数据传输的现有基准方法，所提波束空间调制策略在频谱效率方面具有显著优势。