  In high speed railways (HSRs) communication system, when a train travels
along the railway with high velocity, the wireless channel between the train
and base station varies strenuously, which makes it essential to implement
appropriate power allocations to guarantee system performance. What's more, how
to evaluate the performance limits in this new scenario is also needed to
consider. To this end, this paper investigates the performance limits of
wireless communication in HSRs scenario. Since the hybrid information
transmitted between train and base station usually has diverse quality of
service (QoS) requirements, QoS-based achievable rate region is utilized to
characterize the transmission performance in this paper. It is proved that
traditional ergodic capacity and outage capacity with unique QoS requirement
can be regarded as two extreme cases of the achievable rate region proposed in
this paper. The corresponding optimal power allocation strategy is also given
to achieve the maximal boundary of achievable rate region. Compared with
conventional strategies, the advantages of the proposed strategy are validated
in terms of green communication, namely minimizing average transmit power.
Besides, the hybrid information transmission in a non-uniform generalized
motion scenario is analyzed to confirm the robust performance of proposed
strategy. The performance loss caused by non-uniform motion compared with that
in uniform motion is also indicated, where a deterministic worst case for
instantaneous speed realization is proposed to serve as the lower bound for
system performance.
