Coverage and Spectral Efficiency of Device-to-Device Relay-Assisted Cellular Networks

Two-hop relay transmissions can be exploited to improve the coverage and spectral efficiency of cellular networks. Using stochastic geometry, we develop analytical models for the uplink coverage probability and spectral efficiency of two-hop device-to-device (D2D) relay-assisted cellular networks, in which user equipments (UEs) with poor direct links to the base station (BS) can complete their transmissions with the help of a relay UE. We assume overlay inband D2D operation such that D2D link transmissions (from the source UE to the relay UE) use uplink spectrum, but do not interfere with uplink transmissions in the same cell. We model the base stations (BSs) and candidate relay UEs as Poisson point processes (PPPs) and derive the two-hop coverage probability capturing the correlation of the two links in two-hop transmission. We then model the uplink spectral efficiency, which takes into account the D2D link resource usage and depends on the mode selection strategy. Numerical results show that the analytical models provide reasonable approximations of the cellular uplink performance with a Rayleigh fading assumption. They also reveal insights into the coverage and spectral efficiency gains achievable when leveraging D2D relays on the uplink of a cellular network.