  In the scenario where small cells are densely deployed, the millimeter wave
(mmWave) wireless backhaul network has been widely used. However, mmWave is
easily blocked by obstacles, and how to forward the data of the blocked flows
is still a significant challenge. To ensure backhauling capacity, the quality
of service (QoS) requirements of flows should be satisfied. In this paper, we
investigate the problem of optimal scheduling to maximize the number of flows
satisfying their QoS requirements with relays exploited to overcome blockage.
To achieve a practical solution, we propose a relay-assisted and QoS aware
scheduling scheme for the backhaul networks, called RAQS. It consists of a
relay selection algorithm and a transmission scheduling algorithm. The relay
selection algorithm selects non-repeating relays with high link rates for the
blocked flows, which helps to achieve the QoS requirements of flows as soon as
possible. Then, according to the results of relay selection, the transmission
scheduling algorithm exploits concurrent transmissions to satisfy the QoS
requirements of flows as much as possible. Extensive simulations show RAQS can
effectively overcome the blockage problem, and increase the number of completed
flows and network throughput compared with other schemes. In particular, the
impact of relay selection parameter is also investigated to further guide the
relay selection.
