  With the explosive growth of mobile data, the demand of high-speed railway
(HSR) passengers for broadband wireless access services urgently needs the
support of ultra-highspeed scenario broadband wireless communication.
Millimeterwave (mmWave) can achieve high data transmission rates, but it is
accompanied by high propagation loss and vulnerability to blockage. To address
this issue, developments of directional antennas and unmanned aerial vehicles
(UAVs) enhance the robustness of the mmWave train-ground communication system.
In this paper, we propose a UAV and MRs relay assistance (UMRA) algorithm to
effectively overcome link blockage, which can maximize the number of
transmission flows on the premise of meeting QoS requirements and channel
qualities. First, we formulate a mixed integer nonlinear programming (MINLP)
problem for UAV trajectory design and transmission scheduling in the
full-duplex (FD) mode. Then, in UMRA, the relay decision algorithm and
transmission scheduling algorithm based on graph theory are proposed, which
make a good tradeoff between computation complexity and system performance.
Extensive simulation results show that a suitable UAV position will greatly
improve the performance of the UMRA algorithm and make it close to the optimal
solution. Compared with the other two existing benchmark schemes, with the high
channel quality requirements and large-area blockage, UMRA can greatly improve
the number of completed flows and system throughput.
