Near-field orthogonality and cosine beams for near-field space division multiple access in 6G communications and beyond

TL;DR

This paper introduces near-field space division multiple access (NF-SDMA) using cosine beams, enabling orthogonal communication in the near-field for 6G and beyond, extending classical SDMA concepts to near-field scenarios.

Contribution

The paper proposes the concept of NF-SDMA, designs cosine beams that maintain orthogonality in the near-field, and extends beam correlation analysis from ULAs to UPAs.

Findings

Orthogonality of cosine beams is preserved from near-field to far-field.

Designed beams are numerically verified to be orthogonal at the receiver in near-field conditions.

Proposed codebooks are effective for receivers with many or single antennas.

Abstract

Spatial division multiple access (SDMA), a powerful method routinely applied in multi-user multiple-input multiple-output (MIMO) communications, relies on the angular orthogonality of beams in the far field, to distinguish multiple users at different angles. Yet, with the gradual shift of wireless connectivity to the near-field of large radiating apertures, the applicability of classical SDMA becomes questionable. Therefore, to enable near-field multiple access, it is necessary to design beams that have the desired orthogonality in the near-field. In this work, we propose the concept of near-field space division multiple access (NF-SDMA), to enable SDMA in the near-field. We demonstrate analytically that the orthogonality of beams is preserved at any location of the receiver, from the near-field to the far-field of the transmitter. By judicious design, we select the family of cosine beams and we prove that they satisfy the orthogonality condition, offering a multitude of communication modes in the near-field. We demonstrate how the correlation of beams generated with uniform linear arrays (ULAs) is extended to uniform planar arrays (UPAs) in a straightforward and insightful manner. To test our analytical findings, we propagate the designed beams numerically, and we measure their orthogonality both at the transmitter and the receiver. We verify that the orthogonality of the proposed beams is successfully retrieved at a receiver that resides in the near-field of the transmitter, and is also robust to displacements of the receiver. Based on our findings, we propose codebook designs for NF-SDMA that are applicable for receivers with many elements and even with single antennas.