D2D mode selection and resource allocation with flexible UL/DL TDD for 5G deployments

Network-facilitated device to device (D2D) communication is expected to play a central role in 5th generation cellular systems, as this allows to substantially reduce the end-to-end latency in mission-critical communication use cases, but also to obtain network and spectral efficiency improvements through the exploitation of proximity gains, hop gains and reuse gains in mobile broadband scenarios. The likely complement of evolved legacy standards with non-backward compatible radio interfaces in the 5G context will allow to design radio technology that natively and efficiently supports D2D from the very beginning. However, there are various fundamental design questions to be answered, focusing in this work on the question of how mode selection between D2D and device-infrastructure-device communication should ideally be conducted, as this will have a major impact on protocol stack design. We compare a fast, instantaneous SINR based mode selection (likely implemented on MAC level) against a typically assumed path-loss based slow mode selection (PDCP or RRC level). Through extensive simulation studies, it is shown that fast mode selection does allow to improve D2D performance without impairing cellular performance, but likely requires highly coordinated RRM, and it is thus needed to carefully consider whether the depicted gains would justify the likely more complicated and overhead-prone implementation of mode selection on MAC level.