Cell-free massive multiple-input multiple-output (MIMO) is a promising technology for next-generation communication systems. This work proposes a novel partially coherent (PC) transmission framework to cope with the challenge of phase misalignment among the access points (APs), which is important for unlocking the full potential of cell-free massive MIMO technology. With the PC operation, the APs are only required to be phase-aligned within clusters. Each cluster transmits the same data stream towards each user equipment (UE), while different clusters send different data streams. We first propose a novel algorithm to group APs into clusters such that the distance between two APs is always smaller than a reference distance ensuring the phase alignment of these APs. Then, we propose new algorithms that optimize the combining at UEs and precoding at APs to maximize the downlink sum data rates. We also propose a novel algorithm for data stream allocation to further improve the sum data rate of the PC operation. Numerical results show that the PC operation using the proposed framework with a sufficiently small reference distance can offer a sum rate close to the sum rate of the ideal fully coherent (FC) operation that requires network-wide phase alignment. This demonstrates the potential of PC operation in practical deployments of cell-free massive MIMO networks.

翻译：无蜂窝大规模多输入多输出（MIMO）技术是下一代通信系统的关键候选技术。本文提出了一种新型部分相干（PC）传输框架，以应对接入点（AP）间相位偏差这一挑战，这对释放无蜂窝大规模MIMO技术的全部潜力至关重要。在PC操作下，仅要求AP在簇内实现相位对齐。每个簇向每个用户设备（UE）传输相同的数据流，而不同簇则发送不同数据流。我们首先提出了一种新颖的AP分簇算法，使得任意两个AP间的距离始终小于确保相位对齐的参考距离。随后，我们提出了新算法以优化UE侧的合并与AP侧的预编码，从而最大化下行链路总数据速率。此外，我们还提出了一种数据流分配新算法，以进一步提升PC操作的总数据速率。数值结果表明，采用所提框架且参考距离足够小时，PC操作获得的总速率接近于需要全网相位对齐的理想全相干（FC）操作的总速率。这充分展示了PC操作在无蜂窝大规模MIMO网络实际部署中的潜力。