Motivated by the ever-growing demand for \emph{green} wireless communications and the advantages of \emph{cell-free} (CF) massive multiple-input multiple-output (MIMO) systems, we focus on the design of their downlink for optimal \emph{energy efficiency} (EE). To address this fundamental topic, we assume that each access point (AP) is deployed with multiple antennas and serves multiple users on the same time-frequency resource while the APs are Poisson point process (PPP) distributed, which approaches realistically their opportunistic spatial randomness. Relied on tools from stochastic geometry, we derive a lower bound on the downlink average achievable spectral efficiency (SE). Next, we consider a realistic power consumption model for CF massive MIMO systems. These steps enable the formulation of a tractable optimization problem concerning the downlink EE per unit area, which results in the analytical determination of the optimal pilot reuse factor, the AP density, and the number of AP antennas and users that maximize the EE. Notably, the EE per unit area and not just the EE is the necessary metric to describe CF systems, where we meet multi-point transmission. Hence, we provide useful design guidelines for CF massive MIMO systems relating to fundamental system variables towards optimal EE. Among the results, we observe that a lower pilot reuse factor enables a decrease of the interference, and subsequently, higher EE up to a specific value. Overall, it is shown that the CF massive MIMO technology is a promising candidate for next-generation networks achieving simultaneously high SE and EE per unit area.
翻译:由于对无线通信的需求不断增加,而且对无线通信的需求不断增加,而且由于对无线通信的需求日益增长,以及无线通信(CF)大规模多投入多输出多输出(MIMO)系统的优势,我们注重设计其最佳度的下行链(EE)。为了解决这一基本议题,我们假设每个接入点(AP)都安装了多天线,并且在同一时间频资源上为多个用户服务,而AP是分布的Poisson点进程(PPPP),现实地接近了它们的机会性空间随机性。我们重新利用了来自随机性网络的工具,我们从下行链平均可实现的光谱效率(SEO)获得了较低的约束。接下来,我们考虑为CFEM大规模系统设计一个现实性电能消费模式,我们随后为EFE提供了一种最优化的系统,我们为EFE提供了一种必要的基本值。