Cell-free (CF) massive multiple-input multiple-output (MIMO) is considered as a promising technology for achieving the ultimate performance limit. However, due to its distributed architecture and low-cost access points (APs), the signals received at user equipments (UEs) are most likely asynchronous. In this paper, we investigate the performance of CF massive MIMO systems with asynchronous reception, including both effects of delay and oscillator phases. Taking into account the imperfect channel state information caused by phase asynchronization and pilot contamination, we obtain novel and closed-form downlink spectral efficiency (SE) expressions with coherent and non-coherent data transmission schemes, respectively. Simulation results show that asynchronous reception destroys the orthogonality of pilots and coherent transmission of data, and thus results in poor system performance. In addition, getting a highly accurate delay phase is substantial for CF massive MIMO systems to achieve coherent transmission gain. Moreover, the oscillator phase of UEs has a larger effect on SE than that of the APs, because the latter can be significantly reduced by increasing the number of antennas.
翻译:无细胞(CF)大规模多输出多重输出(MIMO)被认为是实现最终性能极限的有希望的技术,然而,由于其分布式结构和低成本接入点,用户设备收到的信号极有可能是零星的。在本文件中,我们调查了无同步接收的大型CF大型MIMO系统的性能,包括延迟和振动阶段的影响。考虑到阶段同步化和试点污染造成的频道状态信息不完善,我们分别获得了具有连贯和非一致数据传输计划的新型和封闭式下链节光谱效率表达式(SE),模拟结果表明,无同步接收摧毁了试点和数据连贯传输的或多倍数性,从而导致系统性能不佳。此外,对大型CFCMIMO系统来说,一个非常准确的延迟阶段对于实现一致传输增益非常重要。此外,UE的振荡阶段对SE的影响大于APs的反光谱传输计划,因为后者的天线可以大幅减少。