Road ahead for D2D communications

Device-to-device (D2D) is a key enabling technology for the fifth-generation Internet-of-Things, where the global number of D2D connections specifically is expected to reach 14.7 billion devices by 2023. In D2D communication, the spectrum can be shared between D2D and cellular users via two methods, namely, in-band (licensed) and out-band (unlicensed) communication. The integration of D2D in the network requires a substantial modification in both core and access networks to support the smart services in future innovative infrastructures. User equipment communicates through radio network links and core network in conventional cellular communication. Efficient discovery of the nodes in proximity is fundamental to the D2D communications. Vehicle-to-vehicle communications have attracted great interest due to the potential of improving traffic safety, reducing energy consumption, and enabling new services related to intelligent transportation systems. Visible light communication-based D2D can potentially be a venue for more structured research and development efforts in near future.