Beam Focusing for Near-Field Multi-User MIMO Communications

Abstract

Large antenna arrays and high-frequency bands are two key features of future wireless communication systems. The combination of large-scale antennas with high transmission frequencies often results in the communicating devices operating in the near-field (Fresnel) region. In this paper, we study the potential of beam focusing, feasible in near-field operation, in facilitating high-rate multi-user downlink multiple-input multiple-output (MIMO) systems. As the ability to achieve beam focusing is dictated by the transmit antenna, we study near-field signalling considering different antenna structures, including fully-digital architectures, hybrid phase shifter-based precoders, and the emerging dynamic metasurface antenna (DMA) architecture for massive MIMO arrays. We first provide a mathematical model to characterize near-field wireless channels as well as the transmission pattern for the considered antenna architectures. Then, we formulate the beam focusing problem for the goal of maximizing the achievable sum-rate in multi-user networks. We propose efficient solutions based on the sum-rate maximization task for fully-digital, (phase shifters based-) hybrid and DMA architectures. Simulation results show the feasibility of the proposed beam focusing scheme for both single-and multi-user scenarios. In particular, the designed focused beams are such that users residing at the same angular direction can communicate reliably without interfering with each other, which is not achievable using conventional far-field beam steering.Index terms-Beam focusing, dynamic metasurface antennas, near-field multi-user communication.

Near-Field Wireless Power Transfer with Dynamic Metasurface Antennas

“…For multi-user wireless communications operating in radiating near-field region, beam focusing has been exploited to mitigate cochannel interference and hence maximize the sum-rate in our previous work [14]. Despite the similarity between multi-user near-field WPT (considered here) and communications (considered in [14]), there are several fundamental differences in both the design objectives and proposed algorithms. Specifically, for wireless communications, focused beams are designed to reduce co-channel interference which is harmful to data transmission rate.…”

“…According to (15), we have q = 1 2 b + e j π 2 1 , where 1 denotes a N × 1 all ones vector. Hence, we transform (14) into…”

“…The search space in ( 16) is the product of N complex circles, which is a Riemannian submanifold of C N . Thus, ( 16) can be tackled using the Riemannian conjugate gradient (RCG) algorithm [14], [23].…”

“…Unlike the far-field case, where the EM field is a plane wave, in the radiating near-field region, the EM field is a spherical wavefront. In such settings, transmitters can generate focused beams [13], which were shown to mitigate interference in multi-user communications [14], and it was recently envisioned that this capability can facilitate efficient WPT with minimal energy pollution [15]. This motivates the exploration of the ability to achieve energy focusing using emerging antenna architectures, such as DMAs.…”

Near-field Wireless Power Transfer for 6G Internet-of-Everything Mobile Networks: Opportunities and Challenges

“…This illustration demonstrates that in the mmWave regime, EM-based WPT from a medium-sized antenna often results in the receivers being located in the radiating near field. the formulation of highly focused beams (as opposed to conventional directional beams achievable in the far field), which can be harnessed to mitigate interference in multi-user communications [7]. The fact that IoE devices are likely to receive signals in the near-field regime combined with the potential beam focusing capabilities arising in such domains motivate the exploration of radiating near-field WPT for 6G IoE networks.…”