Optimal Power Allocation With Delay Constraint for Signal Transmission From a Moving Train to Base Stations in High-Speed Railway Scenarios

Widespread deployment of high-speed railways in recent years has created huge demand for high-mobility broadband wireless communications. To provide broadband wireless access for passengers on the train, a well-acknowledged approach is to apply a two-hop architecture, under which passengers communicate with base stations (BSs) via an access point (AP) installed in the train. We consider the uplink transmission from the AP to the BSs on the ground along the railway. The key problem we discuss is how to match user-data arrival process and time-varying channel service process with a delay constraint at the AP. We first assume the constant-rate data arrival and the wireless channel affected only by large-scale fading in high-speed railway scenarios. We present the optimal power-allocation policy given the delay constraint. Based on the policy, we show that there exists a power-delay tradeoff in our system model, with the delay within a limited range. Besides, we find that the average transmit power and the train velocity have a tradeoff, which is similar to the power-delay tradeoff. Besides, given average transmit power constraint, the delay constraint is inversely proportional to the train velocity. Finally, when small-scale fading is considered, we modify the optimal power-allocation policy and provide a convenient approach to counteract Nakagami-m fading in high signal-to-noise ratio (SNR) regimes. The modified power-allocation policy can be well adapted to different fading statistics in different terrestrial environments experienced by the highspeed train.