无小区大规模MIMO系统中基于叠加导频的上行频谱效率理论上限分析

Theoretical Upper Limit Analysis of Uplink Spectral Efficiency Based on Superimposed Pilots in Cell-Free Massive MIMO Systems

无小区大规模多输入多输出(Cell-Free Massive MIMO)突破传统蜂窝小区的概念,被认为是未来移动通信中十分富有潜力的技术。该技术通过部署大量的分布式接入点(Access Point, AP)以获得充分的宏分集增益,以提高用户服务质量。受信道相干时间的限制,系统中可供用户分配的正交导频资源非常有限,部分用户将不得不复用相同的导频,从而常规的复用导频(Multiplexed Pilot, MP)配置模式将引起严重的导频污染问题。因此,本文利用叠加导频(Superimposed Pilot, SP)配置模式,基于线性最小均方误差(Linear Minimum Mean Square Error, LMMSE)准则和最大比合并(Maximum Ratio Combining, MRC)技术,研究了无小区大规模MIMO系统的信道估计,并从理论上推导了系统上行频谱效率(Spectrum Efficiency, SE)的上限闭合表达式。仿真结果表明,采用SP配置模式可有效减轻导频污染问题带来的影响,从而明显地提升系统的SE。

Cell-Free massive MIMO breaks through the concept of traditional cells and is considered to be a promising technology in future mobile communication. The technology can obtain sufficient macro diversity gain by deploying a large number of distributed Access Points(AP) to improve the quality of user service. The orthogonal pilot resources available for user allocation are very limited due to the restriction of channel coherence time, some users will have to reuse the same pilot sequences in the conventional multiplexed pilot(MP) configuration mode, which may cause serious pilot contamination problem. Therefore, utilizing the superimposed pilot(SP) configuration mode and based on the linear minimum mean square error(LMMSE) criterion and the maximum ratio combining(MRC) technique, the channel estimation of Cell-Free massive MIMO system was investigated, and the upper bound closed-form expression of the uplink spectrum efficiency(SE) of the system was derived in this paper. The simulation results show that the SP configuration mode can effectively reduce the impact of pilot contamination, thereby significantly improve the SE of the system.