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 provide a new degree of freedom to mitigate interference in both angle and distance domains, which is not achievable using conventional far-field beam steering, allowing reliable communications for uses even residing at the same angular direction.

中文翻译：

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用于近场多用户 MIMO 通信的波束聚焦


大型天线阵列和高频段是未来无线通信系统的两个关键特征。大型天线与高传输频率的结合通常会导致通信设备在近场（菲涅耳）区域工作。在本文中，我们研究了在近场操作中可行的波束聚焦在促进高速多用户下行链路多输入多输出（MIMO）系统方面的潜力。由于实现波束聚焦的能力是由发射天线决定的，因此我们研究了考虑不同天线结构的近场信令，包括全数字架构、基于混合移相器的预编码器以及新兴的大规模动态超表面天线（DMA）架构MIMO 阵列。我们首先提供一个数学模型来表征近场无线信道以及所考虑的天线架构的传输模式。然后，我们制定波束聚焦问题，以最大化多用户网络中可实现的总速率。我们基于全数字、（基于移相器的）混合和 DMA 架构的总速率最大化任务提出有效的解决方案。仿真结果表明了所提出的波束聚焦方案对于单用户和多用户场景的可行性。特别是，所设计的聚焦光束提供了新的自由度来减轻角度和距离域中的干扰，这是使用传统的远场波束控制无法实现的，即使在相同的角度方向上也可以实现可靠的通信。