Enhanced LTE Device-to-Device Proximity Services

Designated for public safety communications, device-to-device (D2D) proximity services (ProSe) have emerged as one of the most innovative technologies in 3GPP Release 12 and 13. When some or all infrastructures are unavailable, Release 12 D2D ProSe supports direct data exchanges between user equipments (UEs) without signal relays via eNBs. As a further enhancement, Release 13 D2D ProSe subsequently supports two-hop UE relays and priority control for mission-critical multimedia applications. With these particular features, D2D ProSe has been regarded as the key foundation toward the next generation social-aware communication and networking paradigm. To fully comprehend this innovation, in this article we provide comprehensive knowledge about D2D ProSe, with essential physical channels and mechanisms in Release 12, and UE-to-network relay as well as priority control in Release 13. By investigating the system-level performance of the Release 13 D2D designs, open issues are further revealed. The provided insights facilitate the practice and incorporation of D2D ProSe into the state-ofthe-art and future cellular network evolutions.