3GPP device-to-device communications for beyond 4G cellular networks

Since 2012, 3GPP Release 12 (the beyond fourth generation (B4G) network) has been adopted as the next generation public safety network to be deployed in 2015. In addition to conventinal high data rate transmissions in LTE-A, a public safety network is required to further support communications for users out of base station (BS) coverage. Avoiding any signal relay via BSs, direct data exchanges among users, known as deviceto- device (D2D) communications, have consequently emerged as the most crucial innovation and the mandatory technology for a public safety network. However, due to the tight schedule for standardization, a special form of "broadcasting" communications has been adopted by D2D communications in Release 12. To fully comprehend such an unprecedented technology in cellular networks, this article provides a comprehensive overview of D2D operations in Release 12. From the system architecture to the radio interface, the provided insights boost the knowledge to practice D2D communications over cellular networks.