  Providing backhaul connectivity for macro and pico base stations (BSs)
constitutes a significant share of infrastructure costs in future heterogeneous
networks (HetNets). To address this issue, the emerging idea of flexible
backhaul is proposed. Under this architecture, not all the pico BSs are
connected to the backhaul, resulting in a significant reduction in the
infrastructure costs. In this regard, pico BSs without backhaul connectivity
need to communicate with their nearby BSs in order to have indirect
accessibility to the backhaul. This makes the radio resource management (RRM)
in such networks more complex and challenging. In this paper, we address the
problem of cross-layer RRM in HetNets with flexible backhaul. We formulate this
problem as a two-timescale non-convex stochastic optimization which jointly
optimizes flow control, routing, interference mitigation and link scheduling in
order to maximize a generic network utility. By exploiting a hidden convexity
of this non-convex problem, we propose an iterative algorithm which converges
to the global optimal solution. The proposed algorithm benefits from low
complexity and low signalling, which makes it scalable. Moreover, due to the
proposed two-timescale design, it is robust to the backhaul signalling latency
as well. Simulation results demonstrate the significant performance gain of the
proposed solution over various baselines.
