Exploiting Device-to-Device Communications to Enhance Spatial Reuse for Popular Content Downloading in Directional mmWave Small Cells

With the explosive growth of mobile demand, small cells in millimeter-wave (mmWave) bands underlying macrocell networks have attracted intense interest from both academia and industry. MmWave communications in the 60-GHz band are able to utilize the huge unlicensed bandwidth to provide multiple Gb/s transmission rates. In this case, device-to-device (D2D) communications in mmWave bands should be fully exploited because there is no interference with the macrocell networks and higher achievable transmission rates. In addition, because there is less interference by directional transmission, multiple links including D2D links can be scheduled for concurrent transmissions (spatial reuse). With the popularity of content-based mobile applications, popular content downloading in the small cells needs to be optimized to improve network performance and enhance user experience. In this paper, we develop an efficient scheduling scheme for popular content downloading in mmWave small cells called popular content downloading scheduling (PCDS), where both D2D communications in close proximity and concurrent transmissions are exploited to improve transmission efficiency. In PCDS, a transmission path selection algorithm is designed to establish multihop transmission paths for users, aiming at better utilization of D2D communications and concurrent transmissions. After transmission path selection, a concurrent transmission scheduling algorithm is designed to maximize the spatial reuse gain. Through extensive simulations under various traffic patterns, we demonstrate that PCDS achieves near-optimal performance in terms of delay and throughput, as well as superior performance, compared with other existing protocols, particularly under heavy load. The impact of the maximum number of hops of transmission paths on its performance is also analyzed for a better understanding of the role of D2D communications.