Cell-free networks are considered as a promising distributed network architecture to satisfy the increasing number of users and high rate expectations in beyond-5G systems. However, to further enhance network capacity, an increasing number of high-cost base stations (BSs) is required. To address this problem and inspired by the cost-effective intelligent reflecting surface (IRS) technique, we propose a fully decentralized design framework for cooperative beamforming in IRS-aided cell-free networks. We first transform the centralized weighted sum-rate maximization problem into a tractable consensus optimization problem, and then an incremental alternating direction method of multipliers (ADMM) algorithm is proposed to locally update the beamformer. The complexity and convergence of the proposed method are analyzed, and these results show that the performance of the new scheme can asymptotically approach that of the centralized one as the number of iterations increases. Results also show that IRSs can significantly increase the system sum-rate of cell-free networks and the proposed method outperforms existing decentralized methods.Index Terms-Beamforming, cell-free networks, intelligent reflecting surface, decentralized optimization. I. INTRODUCTIONRecently, a user-centric network paradigm called cellfree networks has been considered as a promising technique to provide high network capacity and overcome the cellboundary effect of traditional network-centric networks (e.g., cellular networks) [1]-[4]. In cell-free networks, a large number of distributed service antennas, which are connected to central processing units (CPUs), coherently serve all users on the same time-frequency resource [2]. This distributed communication network can offer many degrees of freedom and high multiplexing gain. Recent results show that cellfree networks outperform traditional cellular and small-cell networks in several practical scenarios [2], [3]. To provide high directional gains, beamforming design is important in cell-free networks. To cooperatively design beamforming, a centralized zero-forcing (ZF) beamforming scheme is proposed in [5]. Since the CPU should collect all instantaneous channel state information (CSI) of all base stations (BSs), centralized approaches might be unsalable when the number of BSs and users (UEs) is large and the beamforming optimization at the CPU may be overwhelming due to the high dimensionality of aggregated beamformers. To avoid instantaneous CSI exchange among BSs via backhauling and reduce

For the capability of providing multi-giga BPS (bits per second) rates, millimeter wave (mmWave) communication is one of the key enabling technologies for the new and future generations of mobile communications, i.e., the fifth generation (5G) and beyond. Meanwhile, non-orthogonal multiple access (NOMA) can significantly increase the spectral efficiency by simultaneously serving multiple users in the same channel. Thus, mmWave NOMA networks have recently attracted considerable research attention. Meanwhile, a large number of confidential messages exchanged within highly interconnected systems has posed tremendous challenges on secure wireless communications, and thus in this article, we investigate the physical layer security of mmWave NOMA networks. Considering the limited scattering characteristics of mmWave channels and imperfect successive interference cancellation at receivers, we develop an analytic framework for the secrecy outage probability (SOP) for mmWave NOMA networks, in which legitimate users and eavesdroppers are randomly distributed. Based on the directional transmission property of mmWave signals, we propose a minimal angle-difference user pairing scheme to reduce the SOP of users. Considering the spatial correlation between the selected user pair and eavesdroppers, we develop two maximum ratio transmission (MRT) beamforming schemes to further enhance the secrecy performance of mmWave NOMA networks. Closed-form SOPs for the paired users with different eavesdropper detection capacities are derived. Numerical results show the effectiveness of our analysis and that there exists an optimal radius of network coverage ranges and transmit power to minimize the SOP of the user pair.

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Wireless content caching in cellular networks is an efficient way to reduce the service delay and alleviate backhaul pressure. For the benefits of sharing spectral and storage resources, clustering in cached networks has recently attracted significant research interests. Meanwhile, since the multimedia content (e. g.., video) of caching networks may require a huge transmission rates, millimeter wave (mmWave) communication is considered to be an efficient transmission scheme for cacheenabled networks. We investigate the ergodic rate and average service delay for typical user terminal (UT) in the clustered cacheenabled small cell networks (SCN) and ultra dense networks (UDN) with mmWave channels. In SCN, each cluster consists of cache-enabled UTs, and in the UDN a cluster is formed by cacheenabled UTs and small base stations (SBSs) with non-uniform caching capacity. The clusters are assumed to be discs and content sharing is only possible within clusters through mmWave device-to-device (D2D) tier and SBS tier communications. With stochastic geometry methods, the distributions of content sharing distance and signal-to-interference-noise-ratio (SINR) of typical UT in a cluster are derived for both SCN and UDN scenarios. To minimize the average service delay in high SINR region, we provide an algorithm to jointly optimize caching scheme for SBSs and UTs. By simulations, we validate our theoretical analysis and the performance of proposed caching scheme. The numerical results also show that there exists best radius in the design of cluster for UDNs.