An innovative approach to hybrid analog-digital precoding for the downlink of wideband massive MIMO systems is developed. The proposed solution, termed Rank-Constrained Coordinate Ascent (RCCA), starts seeking the full-digital precoder that maximizes the achievable sum-rate over all the frequency subcarriers while constraining the rank of the overall transmit covariance matrix. The frequency-flat constraint on the analog part of the hybrid precoder and the non-convex nature of the rank constraint are circumvented by transforming the original problem into a more suitable one, where a convenient structure for the transmit covariance matrix is imposed. Such structure makes the resulting full-digital precoder particularly adequate for its posterior analog-digital factorization. An additional problem formulation to determine an appropriate power allocation policy according to the rank constraint is also provided. The numerical results show that the proposed method outperforms baseline solutions even for practical scenarios with high spatial diversity.

翻译：摘要：本文提出了一种面向宽带大规模MIMO系统下行链路的创新混合模拟-数字预编码方法。所提出的解决方案称为秩约束坐标上升（RCCA），首先通过约束整体发射协方差矩阵的秩，在全频域子载波上最大化可达和速率，从而寻求全数字预编码器。通过将原始问题转化为更合适的模型，并对发射协方差矩阵施加便利结构，规避了混合预编码器中模拟分量的频率平坦约束以及秩约束的非凸性。该结构使得最终的全数字预编码器特别适合后续的模拟-数字分解。此外，还提出了根据秩约束确定适当功率分配策略的补充问题公式。数值结果表明，即使在高空间分集的实际场景中，所提方法仍优于基线方案。