Distance-Aware Precoding for Near-Field Capacity Improvement in XL-MIMO

Extremely large-scale MIMO (XL-MIMO) communication is a promising technology to improve the capacity for future 6G networks. With a very large number of antennas, the near-field property of XL-MIMO systems becomes significant. Unlike the classical far-field line-of-sight (LoS) channel with only one available data stream, significantly increased degrees of freedom (DoFs) are available in the near-field LoS channel. However, limited by the small number of radio frequency (RF) chains, the existing hybrid precoding architecture widely used for 5G is not able to fully utilize the extra DoFs in the nearfield region. In this paper, to exploit the near-field effect as a new possibility for capacity improvement, the distance-aware precoding (DAP) architecture is developed, where each RF chain can be flexibly configured to active or inactive according to the distance-related DoFs. Moreover, based on the developed DAP architecture, a DAP algorithm is proposed to optimize the number of activated RF chains and precoding matrices to match the increased DoFs. Finally, simulation results verify that, the proposed DAP scheme can efficiently utilize the extra DoFs in the near-field region to improve the spectrum efficiency.