In this letter, we analyze the performance of mixed coherent and non-coherent transmissions approach, which can improve the performance of cell-free multiple-input multiple-output orthogonal frequency division multiplexing (CF mMIMO-OFDM) systems under asynchronous reception. To this end, we first obtain the achievable downlink sum-rate for the mixed coherent and non-coherent transmissions, and then provide a closed-form expression for the case with the maximum ratio precoding. Subsequently, an efficient clustering algorithm is proposed to group access points into different clusters with the same quantized phase shift in each cluster. Numerical results demonstrate that the mixed coherent and non-coherent transmissions can effectively improve the sum-rate of CF mMIMO-OFDM systems under asynchronous reception.

翻译：在本通信中，我们分析了混合相干与非相干传输方法的性能，该方法可提升异步接收条件下无蜂窝多输入多输出正交频分复用系统的性能。为此，我们首先推导了混合相干与非相干传输的可达下行链路和速率，并针对最大比预编码给出了闭式表达式。随后，提出了一种高效聚类算法，将接入点分组为不同簇，使每个簇内具有相同的量化相移。数值结果表明，在异步接收条件下，混合相干与非相干传输能有效提升无蜂窝大规模MIMO-OFDM系统的和速率。