Centralized and decentralized multi-cell D2D resource allocation using flexible UL/DL TDD

Device to device (D2D) communication is expected to be an important aspect in future 5G communication systems. To this end, D2D has been identified as an important topic in the EU flagship project METIS, wherein commercial D2D use cases are also considered. In principle, direct D2D communication offers the double benefit of low latency and high data rate at the same time. Interference mitigation and performance optimization alongside cellular users are however some of the key challenges to D2D communication. In this paper, we present a flexible D2D concept which is based on integration of D2D communication in flexible UL/DL TDD scheduling. We then compare multi-cell decentralized and centralized approaches for joint resource allocation for direct D2D and cellular links. Simulation based results are shown in a challenging indoor scenario where direct D2D users are placed among a high density of indoor cellular small cells and users. We find that centralized multi-cell resource allocation can improve overall system performance by 24 % for supporting D2D communications. Our results also show that substantial system level gains above 30% are achieved by enabling resource reuse between direct D2D and cellular users in both uplink and downlink cellular time slots. Furthermore, we see that flexible TDD provides overall better system performance as compared to fixed TDD even with a decentralized scheduler.