# Device-To-Device Communication in 5G Environment: Issues, Solutions, and Challenges
## 6 Mode Selection
#### Recent Advances in Mode Selection  
[PERSON] et al., in [78], proposed an optimal content sharing mode selection scheme for socially aware D2D communications. The authors exploit social reciprocity in order to encourage mutual cooperation, and also refining the average link rate to achieve a successful content delivery. To ease the optimal matching between potential providers and demanders, the authors used the attributes of total unimodularity to develop a number of online implementations according to the existing linear programming algorithms. However, the authors did not take into consideration the sharing mode selection scheme under different popularity distributions, individual QoS constraints, etc. The authors  
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in [79] present a novel partial channel state information (CSI) model that has a lower overhead of feeding back the cross-link CSI. The authors proceed to propose a throughput-optimal, joint mode selection scheme, user scheduling, and the rate adaptation policy called CLISAA for D2D systems that exploit information about interference statistics from cross-links and integrate inter-cell interference. The authors produce the closed-form expressions to be used for feedback-conditioned goodput in the underlay mode that drives this optimal policy. However, the authors only consider single antennas at the BS and the user equipment in the simulation scenarios. [PERSON] et al., in [80], identify the challenges of selecting the suitable communication modes for all user equipment pairs in the D2D network. The authors analyzed the problem in the mode selection aspect for a multipair and multimode D2D network, in which the eNB is able to select one of the three modes in D2D communication (direct D2D mode, relay D2D mode, and local route mode) for each D2D pair. The authors proceeded to propose an optimal mode selection algorithm to achieve the maximum capacity. This paper aims to share the uplink channels of cellular users. The optimal mode selection issue can be replicated using an integer programming problem. The authors in [81] proposed an interlay mode for the nonorthogonal multiple access (NOMA) system, which allows power domain multiplexing of cellular users and D2D pairs using successive interference cancellation (SIC) decoding to eliminate interference between them. The authors proposed a resource allocation scheme and a joint D2D mode selection that makes up as a combinatorial optimization problem to increase the system sum rate and at the same time fulfill the SIC decoding constraint. [PERSON] et al., in [82], proposed an optimal mode selection scheme for full-duplex D2D cognitive networks that depends on its impact towards primary users. The authors study the impact towards D2D users in the proximity of active cellular users using the stochastic geometry tools. The authors then proceed with defining the crucial region where, if the receiver is inside of the guard zone, D2D links are able to operate in the half-duplex mode, whereas if the transmitter and receiver are both outside of the guard zone, D2D link can operate in the full-duplex mode.