We consider network-assisted full-duplex (NAFD) cell-free massive multiple-input multiple-output (CF-mMIMO) systems, where full-duplex (FD) transmission is virtually realized via half-duplex (HD) hardware devices. The HD access points (APs) operating in uplink (UL) mode and those operating in downlink (DL) mode simultaneously serve DL and UL user equipments (UEs) in the same frequency bands. We comprehensively analyze the performance of NAFD CF-mMIMO from both a spectral efficiency (SE) and energy efficiency (EE) perspectives. Specifically, we propose a joint optimization approach that designs the AP mode assignment, power control, and large-scale fading (LSFD) weights to improve the sum SE and EE of NAFD CF-mMIMO systems. We formulate two mixed-integer nonconvex optimization problems of maximizing the sum SE and EE, under realistic power consumption models, and the constraints on minimum individual SE requirements, maximum transmit power at each DL AP and UL UE. The challenging formulated problems are transformed into tractable forms and two novel algorithms are proposed to solve them using successive convex approximation techniques. More importantly, our approach can be applied to jointly optimize power control and LSFD weights for maximizing the sum SE and EE of HD and FD CF-mMIMO systems, which, to date, has not been studied. Numerical results show that: (a) our joint optimization approach significantly outperforms the heuristic approaches in terms of both sum SE and EE; (b) in CF-mMIMO systems, the NAFD scheme can provide approximately 30\% SE gains, while achieving a remarkable EE gain of up to 200\% compared with the HD and FD schemes.
翻译:本文研究网络辅助全双工(NAFD)无细胞 massive MIMO系统,其中全双工(FD)传输通过半双工(HD)硬件设备实现。在 Uplink(UL)模式下运行的 HD 接入点(AP)和同时服务于 DL和UL用户设备 (UE)的 DL模式下运行的 AP,在相同的频段中实现 DL 和 UL 传输。我们从光谱效率(SE)和能量效率(EE)两个方面全面分析 NAFD CF-mMIMO 的性能。具体而言,我们提出一个联合优化方法,设计 AP 模式分配、功率控制和大尺度衰落(LSFD)权重,以提高 NAFD CF-mMIMO 系统的性能。我们提出了两个混合整数非凸优化问题:在现实的功耗模型和最低个别 SE 要求的约束下,分别最大化总 SE 和 EE。这些具有挑战性的问题被转化为可处理的形式,并提出了两个使用连续凸近似技术来解决它们的新算法。更重要的是,我们的方法可用于联合优化功率控制和 LSFD 权重,以最大化 HD 和 FD CF-mMIMO 系统的总 SE 和 EE,但此前尚未研究。数值结果表明:(a)我们的联合优化方法在总 SE 和 EE 方面比启发式方法显著优越;(b)在 CF-mMIMO 系统中,NAFD 方案可以提供约 30% 的 SE 增益,同时相对于 HD 和 FD 方案,实现了可观的 EE 收益,高达 200%。