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We consider providing services for passengers in a high-speed train and localusers (quasi-static users) in a single OFDMA system. For the train, we apply atwo-hop architecture, under which, passengers communicate with base stations(BSs) via a mobile relay (MR) installed in the train cabin. With thisarchitecture, all passengers in the train can be represented by the MR. Sincethe channels of the MR and local users vary differently, we consider allocatingsystem resources (power and subcarriers) over two time-scales for them. Weformulate the problem as a capacity optimization problem for the MR subject tothe sum capacity constraint of local users. We treat the inter-carrierinterference (ICI) at the MR as additive Gaussian noise and derive an explicitexpression for the ICI using the two-path Doppler spread model. Then we discussthe optimization problem and propose an optimal power and subcarrier allocation(OPSA) policy. The capacity obtained using OPSA is compared with that ofconstant power and subcarrier allocation (CPSA) policies. Simulation resultsjustify the optimality of the OPSA. Besides, by comparing the capacity boundsachieved by OPSA with and without ICI, we find that only in specific regions,where the gap between the capacity bounds is large, do practical ICIcancellation methods provide meaningful rate gain.

Downlink Resource Allocation for the High-speed Train and Local Users in OFDMA Systems