In this paper, we present an analytical approach to evaluate the performance of dual-hop, two-way, and asymmetric D2D communications with and without network coding. In our approach, we first establish a relationship between link outage probability (LOP) and packet loss probability (PLP), where PLP is defined as a function of LOP. By distinguishing between two types of probabilities, we then investigate the system throughput and end-to-end packet loss probability (E2EPLP). Our evaluation results reveal that when PLPs of all links along one-way D2D communications are greater or smaller than those of their corresponding links along the other direction, network coding can achieve higher throughput (about 25%), as well as an lower E2EPLP (approximately 10%). We believe that the proposed analytical approach can provide a useful insight into the application of network coding in relay-based D2D networks.

Hybrid precoding architectures are widely studied for millimeter wave (mmWave) massive MIMO systems. A major challenge in designing hybrid precoders is the practical constraints on the number of RF chains, which can have a direct impact on the spectral and energy efficiencies of the communication systems. In this paper, we investigate tradeoff between the two performance metrics in both static and mobile communication scenarios via closed-form expressions, when the number of active RF chains can be selected. Based on these expressions, the computational complexity to configure the hybrid precoder is reduced, which can be used to adaptively activate required RF chains for the given MIMO system and channel condition. Numerical results indicate that a certain number of RF chains should be activated in order to maximize energy efficiency at high SNRs, which is generally different from the optimal configuration to maximize spectral efficiency. Further-more, for low SNRs, we have shown that a simple analog beamforming, which uses only a single RF chain, is optimal for both spectral and energy efficiencies. In addition, the proposed mobility-aware hybrid precoding is shown to be capable of effectively achieving beamforming gain between high-speed mobile devices.

While mode selection has been envisioned as the most cost-effective way to address the interference issue in Device-to-Device (D2D) communications, existing works have been largely conducted without consideration of the energy depletion of devices. In this paper we investigate simultaneous wireless information and power transfer (SWIPT) empowered mode selection based on stochastic geometry. As a mean of solving it, system energy efficiency is formulated by determining the closed-form ergodic energy-harvested and ergodic capacity of D2D and cellular users in reuse, dedicated, and cellular communication modes with the time switching and power splitting architectures of SWIPT. We then leverage the derived results, along with the energy efficiency to design an energy-efficient mode selection mechanism. Our simulation results show that the developed mechanism is able to select the best mode for D2D communication with better energy efficiency, especially in an ultra-dense cellular network as compared with a state-of-the-art mode selection approach.

As an essential part of vehicle networks, the Vehicle to Infrastructure (V2I) needs the support of millimeter wave and massive MIMO technologies to enable high data rate applications, such as automated driving, real-time high-quality multimedia services and so on. As the scale of the antenna array increases, the complexity of the beamforming and channel estimation algorithms under high mobility conditions also increases significantly. In particular, highly robust beamforming methods need to cope with fast changing transmission environments. In this paper, we adopt a biological inspired self-adaptive selection algorithm called attractor selection algorithm (ASA) to support uplink beamforming. The ASA requires only a little feedback information from the Road Side Infrastructure (RSI) to perform fast beam training, hence making the transmission link more stable. The simulation results indicate that the proposed ASA-assisted algorithm can significantly reduce the time required to achieve a timely beam training, which would be essential for V2I high communications under high mobility conditions.