Due to the heavy reliance of millimeter-wave (mmWave) wireless systems on directional links, beamforming (BF) with high-dimensional arrays is essential for cellular systems in these frequencies. Thus, performing the array processing in a power-efficient manner is a fundamental challenge. Analog and hybrid BF require few analog-to-digital and digital-to-analog converters (ADCs and DACs), but can only communicate in a small number of directions at a time, limiting directional search, spatial multiplexing, and control signaling. Digital BF enables flexible spatial processing but must be operated at a low quantization resolution to stay within reasonable power levels. This decrease in quantizer resolution distorts the received as well as the transmitted signal. To assess the effect of coarse quantization at the receiver, this paper presents a system level analytic framework based on a simple additive quantization noise model (AQNM). The analysis verified through extensive simulations reveals that at moderate resolutions (3–4 bits per ADC), there is negligible loss in downlink cellular capacity from quantization. In essence, the low resolution ADCs limit the high SNR, where cellular systems typically do not operate. For the transmitter, it is shown that DACs with 4 or more bits of resolution can support high order modulations, and do not violate the adjacent carrier leakage limit set by 3rd Generation Partnership Project (3GPP) New Radio (NR) standards for cellular operations. In fact, our findings suggest that low resolution digital BF architectures can be a power-efficient alternative to analog or hybrid BF for both transmitters and receivers at millimeter-wave.

中文翻译：

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毫米波数字波束成形案例

由于毫米波 (mmWave) 无线系统严重依赖定向链路，因此具有高维阵列的波束成形 (BF) 对于这些频率的蜂窝系统至关重要。因此，以高效节能的方式执行阵列处理是一项基本挑战。模拟和混合 BF 需要很少的模数和数模转换器（ADC 和 DAC），但一次只能在少数方向上进行通信，限制了方向搜索、空间复用和控制信令。数字 BF​​ 支持灵活的空间处理，但必须在低量化分辨率下运行以保持在合理的功率水平内。量化器分辨率的这种降低会使接收和发送的信号失真。为了评估接收端粗量化的影响，本文提出了一个基于简单加性量化噪声模型 (AQNM) 的系统级分析框架。通过大量模拟验证的分析表明，在中等分辨率下（每个 ADC 3-4 位），量化造成的下行链路蜂窝容量损失可以忽略不计。本质上，低分辨率 ADC 限制了高 SNR，而蜂窝系统通常无法在高 SNR 中运行。对于发射机，显示具有 4 位或更多位分辨率的 DAC 可以支持高阶调制，并且不会违反第三代合作伙伴计划 (3GPP) 新无线电 (NR) 标准为蜂窝操作设定的相邻载波泄漏限制。事实上，我们的研究结果表明，对于毫米波的发射器和接收器，低分辨率数字 BF​​ 架构可以成为模拟或混合 BF 的节能替代方案。