Revisiting Scheduling in Heterogeneous Networks When the Backhaul Is Limited

We study the impact of the limited capacity of back-haul links on downlink user scheduling in a heterogeneous network comprising macro base stations and small cells. Assuming a tree topology of the backhaul network, we formulate a backhaul-aware global a-fair time-domain user scheduling problem and study it under three different scenarios of backhaul limitations. For the scenario where the backhaul links are not the bottleneck, we derive closed-form scheduling solutions to the scheduling problem under certain assumptions. For the scenario where the backhaul links between the macro base station and the small cells are the bottleneck, we show that the global a-fair user scheduling problem can be decomposed into a set of independent local a-fair user scheduling problems. However, unlike the previous case, a local scheduler in this case is not of a unique type but can be of one of three types, depending on the available backhaul capacity. We completely characterize these three types and also propose a simple heuristic for optimal a-fair scheduling. When the link between the macro base station and the core network is a potential bottleneck, we show how each base station can still perform a local scheduling as in the previous case as long as there is amaster problem that allocates feasible virtual backhaul capacities to each BS. However, computing the optimal virtual capacities is complex and expensive in terms of the amount and frequency of information exchanges. For this scenario, we propose realization-agnostic heuristic schemes that are simple to implement and perform quite well.