  Small-cell architecture is widely adopted by cellular network operators to
increase network capacity. By reducing the size of cells, operators can pack
more (low-power) base stations in an area to better serve the growing demands,
without causing extra interference. However, this approach suffers from low
spectrum temporal efficiency. When a cell becomes smaller and covers fewer
users, its total traffic fluctuates significantly due to insufficient traffic
aggregation and exhibiting a large "peak-to-mean" ratio. As operators
customarily provision spectrum for peak traffic, large traffic temporal
fluctuation inevitably leads to low spectrum temporal efficiency. In this
paper, we advocate device-to-device (D2D) load-balancing as a useful mechanism
to address the fundamental drawback of small-cell architecture. The idea is to
shift traffic from a congested cell to its adjacent under-utilized cells by
leveraging inter-cell D2D communication, so that the traffic can be served
without using extra spectrum, effectively improving the spectrum temporal
efficiency. We provide theoretical modeling and analysis to characterize the
benefit of D2D load balancing, in terms of total spectrum requirements of all
individual cells. We also derive the corresponding cost, in terms of incurred
D2D traffic overhead. We carry out empirical evaluations based on real-world 4G
data traces to gauge the benefit and cost of D2D load balancing under practical
settings. The results show that D2D load balancing can reduce the spectrum
requirement by 25% as compared to the standard scenario without D2D load
balancing, at the expense of negligible 0.7% D2D traffic overhead.
