Hybrid Frequency Transmission for Upload Latency Minimization of IoT Devices in HSR Scenario Aided by Intelligent Reflecting Surfaces

The explosively growing demand for Internet of Things (IoT) in high-speed railway (HSR) scenario has attracted a lot of attention amongst researchers. However, limited IoT device (IoTD) batteries and large information upload latency still remain critical impediments to practical service applications. In this paper, we consider a HSR wireless mobile communication system, where two intelligent reflecting surfaces (IRSs) are deployed to help solve the problems above. Considering the carrier aggregation method, the IRS needs to be optimized globally in hybrid frequency bands. Meanwhile, to ensure information security, the transmission to the mobile communication relay (MCR) on the train is covert to passengers in the carriage by IRS. This problem is challenging to handle since the variables are coupled with each other and some tricky constraints. We firstly transform the original sum-of-ratios problem into the more tractable parametric problem. Then, the block coordinate descent (BCD) algorithm is adopted to decouple the problem into two main sub-problems, and the downlink and uplink settings are alternatively optimized using low-complexity iterative algorithms. Finally, a heuristic algorithm to mitigate the Doppler spread is proposed to further improve the performance. Simulation results corroborate the performance improvement of the proposed algorithm.