# Performance Analysis of DF Relay-Assisted D2D Communication in a 5G mmWave Network
## 1 Introduction
### Related Works  
As discussed above regarding the advantages of implementing DF relaying to enhance network coverage and provide the seamless operation of D2D communication in mmWave networks while satisfying the QoS constraints, DF relaying has widespread applications that enhance the SINR and thereby increase the data rate of D2D users. Some of the techniques related to the performance enhancement of relay-aided D2D communication are discussed as follows. In [5], the authors addressed the power control problem between the D2D source and the relay node at a new algebfor EE in DF relay-aided D2D communication. The EE of the relay node to the D2D link was maximized while satisfying the minimum data rate of the cellular link. The maximization was performed by obtaining the optimal power at both the D2D transmitter and the relay node. In [6], a stochastic geometry was used to obtain the coverage probability for a relay-aided mmWave cellular network in the downlink channel. Blockages and beamforming gains were also considered while obtaining the coverage probability and SE for line-of-sight, as well as non-line-of-sight paths. In [7], three communication transmission modes were proposed to investigate the achievable sum rates of D2D communication assisted by DF relay nodes. The corresponding expressions for the ergodic achievable sum rates of each transmission mode were obtained to compare the performance metrics of direct communication to those of DF relay-assisted D2D communication. It was found that the DF relay-aided D2D communication exhibited better performance than direct communication. Again, the authors in [8] derived an ergodic rate for a D2D communication system aided by a two-way DF relay node. They formulated two scenarios, the weak interference case and the high-SNR case, giving better insight into the problem and deriving the closed-form power allocation strategy to showcase the improvement in system performance. The authors derived closed-form expressions of the coverage probability and transmission capacity for a full-duplex amplify-and-forward (FDAF) relay-aided D2D communication network in [9]. The parameters such as the D2D user density, relay node density, and distance between D2D pairs were also considered to simulate the environment. The authors in [10] considered two interference cancellation models to evaluate the transmission capacity of relay-assisted D2D communication. Furthermore, they investigated the effect of these parameters on the overall network performance. The authors in [11] put forward a power control problem in which two optimization objectives were considered, i.e., the maximization of the minimum SINR of the D2D users and the maximization of the sum rate of the D2D users for a two-way AF relay to assist the underlay D2D communication. In [12], an interference-free dual-hop D2D-aided cooperative relaying strategy (CRS) based on spatial modulation was proposed. Two scenarios were assumed, each operating in two different time slots. Interference-free information was received at the end user node without allocating any fixed transmit power. The bit error rate and SE were also derived to show the performance of the proposed work. A multi-objective combinatorial optimization problem was modeled in [13], which helped balance the trade-off between the total transmit power and the system data rate. The authors also proposed a centralized relay selection and power allocation algorithm, which achieved the Pareto-optimal solution in polynomial time. This method reduced the total transmit power and improved the system throughput. The authors in [14] proposed a scheme with manifold objectives in the form of the ability to improve the SE by using underlay spectrum sharing mode, alleviate co-channel interference, and enhance comprehensive performance while satisfying multiple QoS metrics.