Delay Constrained Optimal Power Allocation in High-speed Railway Scenarios

Widespread deployment of high-speed railways in recent years brings huge demands for high-mobility broadband wireless communications. To provide broadband wireless access for passengers in the train, a well acknowledged approach is to apply a two-hop architecture in which users communicate with base stations (BSs) via a mobile relay (MR). Considering quality of service (QoS) requirement of passengers, a crucial issue in this two-hop architecture is how to match user data arrival process and time-varying wireless channel service process with delay constraint at the MR. We deal with this problem from the perspective of power allocation. For constant-rate data arrival and large-scale-fading wireless channel in high-speed railway scenarios, we first present a necessary and sufficient condition for avoiding buffer overflow as well as infinite delay with constant transmit power, and when the transmit power can be adapted, we present the delay constrained optimal power allocation (DCOPA). We find that the proposed DCOPA bridges traditional channel inversion and water filling, which are only suitable for special cases where delay is not allowed and delay is quite loose respectively. Besides, with DCOPA, we present the inherent tradeoff between delay and minimal average transmit power in the system, which provides the optimal operating point for delay constrained traffic at the MR.