초록 : We study the impact of the limited capacity of backhaul links on user scheduling in a heterogeneous network comprising macro base stations and small cells. Assuming a tree topology of the backhaul network, we formulate a global proportional fair (PF) scheduling problem and show that the backhaul capacity limitations have a fundamental impact on user scheduling. If the limitations are on the backhaul links between the macro base station and the small cells, the global PF user scheduling problem can be decomposed into a set of independent local PF user scheduling problems. Unlike the case with unlimited backhaul where the local PF scheduler would allocate equal time to each user, a local scheduler in this case can be of one of three types. We completely characterize these three types and also propose a simple heuristic for a quasi-optimal local PF scheduling.