  Load balancing is an effective approach to address the spatial-temporal
fluctuation problem of mobile data traffic for cellular networks. The existing
schemes that focus on channel borrowing from neighboring cells cannot be
directly applied to future 5G wireless networks, because the neighboring cells
will reuse the same spectrum band in 5G systems. In this paper, we consider an
orthogonal frequency division multiple access~(OFDMA) ultra-dense small cell
network, where Device-to-Device~(D2D) communication is advocated to facilitate
load balancing without extra spectrum. Specifically, the data traffic can be
effectively offloaded from a congested small cell to other underutilized small
cells by D2D communications. The problem is naturally formulated as a joint
resource allocation and D2D routing problem that maximizes the system sum-rate.
To efficiently solve the problem, we decouple the problem into a resource
allocation subproblem and a D2D routing subproblem. The two subproblems are
solved iteratively as a monotonic optimization problem and a complementary
geometric programming problem, respectively. Simulation results show that the
data sum-rate in the neighboring small cells increases 20\% on average by
offloading the data traffic in the congested small cell to the neighboring
small cell base stations~(SBSs).
