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Reconfigurable intelligent surface (RIS) has been attracted lots of attention due to the fact that it can reconfigure propagation environment with low cost for resource-constrained systems. In this article, RIS is considered to support wireless communication system by taking both reflection path and direct path into consideration and investigate the system performance under general Nakagami-$m$ fading. A novel analytical closed-form expression is first derived for outage probability. To get more insights, we provide simplified approximations for the high SNR regime and obtain the diversity order and coding gain in closed form. We demonstrate that $\sum _{l = 1}^{N + 1} {m_{l}^*}$ diversity order which can be obtained by RIS-assisted system, where $N$ is the number of reflect elements in the RIS and ${m_{l}^*}$ refers to the channel fading parameters. Furthermore, we investigate the performance of ergodic achievable rate and provide tight bounds for it in closed form. Based on these results, high SNR slope is discussed. Notably, our analytical results are suitable for any integer fading parameters and arbitrary topologies, transmit power. Numerical results validate the accuracy of our analytical results and show the great performance improvement provided by the RIS-assisted system.

This paper investigates the device-to-device (D2D) communication underlaying multi-user multi-input multi-output (MU-MIMO) cellular networks. It is assumed that D2D users reuse the downlink time-frequency resources of cellular links, and the base station (BS) is assumed to be equipped with multiple antennas. We investigate the ergodic achievable sum rate of the system when the interference cancellation (IC) pre-coding strategy is employed at the BS. The distributions of the received signal-to-interference-plus-noise ratio (SINR) for each link are firstly analyzed, and an exact ergodic achievable sum rate of the whole system with closed-form expressions is then derived. Furthermore, we present novel upper and lower bounds with simpler expressions, which are later verified to be fairly close to the Monte-Carlo simulations. All the expressions we presented are suitable for arbitrary network topology and arbitrary number of antennas at BS. Based on the derived bounds, the influence of the antennas at BS on system performance is then analyzed. We reveal that the system performance increases along with the number of antennas at BS in a logarithmic way. The accuracy of our analytical results is validated via comparisons with Monte-Carlo simulations.

In this paper, we investigate the energy efficiency and spectrum efficiency, including one-hop device-to-device (D2D) communications mode and two-way amplify-and-forward (AF) relaying D2D communications mode in underlay D2D communications enabled cellular networks. An analysis of average energy efficiency and spectrum efficiency are developed and closed-form expressions are obtained for two types of D2D communications modes under the effect of Rayleigh fading channel, path loss, and co-channel interference. Analytical results are validated through numerical simulations. Based on the simulation, the effects of the interference, the distance between D2D pair and the position of relay node on the energy efficiency and spectrum efficiency of D2D communications are investigated. The optimal D2D transmission powers of these two modes to maximize the energy efficiency are also investigated.