In this paper, we investigate the spatial-wideband effects in cell-free massive MIMO (CF-mMIMO) systems in mmWave bands. The utilization of mmWave frequencies brings challenges such as signal attenuation and the need for denser networks like ultra-dense networks (UDN) to maintain communication performance. CF-mMIMO is introduced as a solution, where distributed access points (APs) transmit signals to a central processing unit (CPU) for joint processing. CF-mMIMO offers advantages in reducing non-line-of-sight (NLOS) conditions and overcoming signal blockage. We investigate the synchronization problem in CF-mMIMO due to time delays between APs. It proposes a minimum cyclic prefix length to mitigate inter-symbol interference (ISI) in OFDM systems. Furthermore, the spatial correlations of channel responses are analyzed in the frequency-phase domain. The impact of these correlations on system performance is examined. The findings contribute to improving the performance of CF-mMIMO systems and enhancing the effective utilization of mmWave communication.

翻译：本文研究了毫米波频段无小区大规模MIMO（CF-mMIMO）系统中的空间宽带效应。毫米波频段的应用带来了信号衰减等挑战，并需要超密集网络（UDN）等更密集的网络架构以维持通信性能。CF-mMIMO作为一种解决方案被引入，其中分布式接入点（AP）将信号传输至中央处理单元（CPU）进行联合处理。CF-mMIMO在减少非视距（NLOS）条件和克服信号遮挡方面具有优势。我们探究了因AP间时延导致的CF-mMIMO同步问题，并提出了最小循环前缀长度以缓解OFDM系统中的符号间干扰（ISI）。此外，在频域-相位域分析了信道响应的空间相关性，并考察了这些相关性对系统性能的影响。研究成果有助于提升CF-mMIMO系统性能，并促进毫米波通信的有效利用。