This paper optimizes the fronthaul bit allocation in massive multi-user multiple-input multiple-output (MU-MIMO) systems operating with limited-capacity fronthaul links. We consider an advanced antenna system (AAS) controlled by a centralized baseband unit (BBU). In the AAS, multiple antenna elements together with their radio units are integrated into a single unit. In this setup, a key challenge is allocating fronthaul bits between uplink channel state information (CSI) quantization and downlink precoding matrix quantization. We formulate the problem of maximizing the sum spectral efficiency (SE) for a given fronthaul capacity. We develop an SE expression for this scenario based on the hardening bound. We compute the expression in closed form for maximum ratio transmission, which reveals the relative impact of the two types of quantization distortion. We then formulate a bit split optimization problem and propose an algorithm that exactly solves it. Numerical results demonstrate how the relative importance of assigning bits to CSI and precoding varies depending on the signal-to-noise ratio.

翻译：本文研究了在前传链路容量受限条件下运行的大规模多用户多输入多输出（MU-MIMO）系统中的前传比特分配优化问题。我们考虑一种由集中式基带处理单元（BBU）控制的先进天线系统（AAS）。在AAS中，多个天线单元及其射频单元被集成在一个单一设备内。在此架构下，一个关键挑战是如何在前传链路上，为上行信道状态信息（CSI）量化和下行预编码矩阵量化分配比特。我们针对给定的前传容量，构建了最大化总频谱效率（SE）的优化问题。基于硬化界，我们推导了该场景下的SE闭式表达式。针对最大比传输方案，我们计算了该闭式表达式，揭示了两种量化失真类型的相对影响。随后，我们构建了比特分割优化问题，并提出了一种能够精确求解该问题的算法。数值结果表明，为CSI和预编码分配比特的相对重要性如何随信噪比的变化而变化。