Energy and Spectral Efficient Frequency Reuse of Ultra Dense Networks

Deploying ultra dense networks (UDNs) can boost the spectral efficiency (SE) and energy efficiency (EE) of cellular networks, where the density of base stations (BSs) may exceed the density of users. For UDNs, inter-cell interference is a limiting factor on improving both the SE and EE, and complicated interference coordination is highly undesirable. In this paper, we investigate energy and spectral efficient frequency reuse factors in randomly deployed cells, where BS sleeping is allowed for the cells without active users. Toward this goal, we find the frequency reuse factor that maximizes the SE or EE upper bound of the network with given ratio of BS density to user density, and quantify the SE and EE gains of universal frequency reuse over partial frequency reuse in UDNs. Our analytical results show that the universal frequency reuse is SE-optimal for the networks with arbitrary BS/user density ratios, but is EE-optimal only when the ratio exceeds a threshold, which highly depends on the entire bandwidth of the network and the number of antennas at each BS. Both the normalized SE and EE gains in UDNs increase with the BS/user density ratio. The simulation results are provided to validate our analysis.