  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.
