Device-to-Device Relaying: Optimization, Performance Perspectives, and Open Challenges Towards 6G Networks

A relaying can significantly improve performance of contemporary mobile networks in terms of capacity and/or energy consumption. Nevertheless, an incorporation of conventional relay stations into the mobile networks is usually expensive in terms of both capital and operational expenditures for the mobile operators. With an evolution of Device-to-device (D2D) communication, the relaying can be also facilitated via a user equipment (UE) by means of D2D relaying. In the D2D relaying, the users relay data for others, thus significantly cutting the mobile network operators' expenditures. The D2D relaying also offers a plenty of opportunities for the relaying as, in theory, each UE can play a role of the relay. To fully unlock all benefits of the D2D relaying, however, there are still many challenges to be addressed, such as an efficient radio resource and interference management, mobility management, security and trust issues, or finding proper incentives to motivate users to help others. In this survey, we introduce a detailed taxonomy of the D2D relaying concept encompassing an overview of various relaying scenarios, types of relays, and radio resource management techniques to be optimized. Then, we provide a comprehensive overview of research works targeting the D2D relaying addressing the above-mentioned challenges, compare these works from various perspectives, identify their potential drawbacks and limitations, and also draw some interesting lessons for the readers. Last, based on the gaps in the current literature, we identify key open research challenges deserving further attention of the researches to make the D2D relaying feasible and attractive option for mobile network operators as well as end-users in emerging 6G networks.