Weak-Coverage User Compensation for RIS-Assisted Plateau LoRa Multi-User Downlink Coverage Enhancement

Abstract

This paper studies coverage imbalance in reconfigurable intelligent surface (RIS)-assisted plateau LoRa multi-user downlink transmission. Terrain blockage, propagation distance, and scattering diversity cause heterogeneous channel qualities, which may leave weak users with high bit error rate (BER) and outage risk even when the average link quality is improved. To address this issue, a weak-coverage user compensation method is proposed. Users are first classified according to initial channel quality, and inverse-quality weights are constructed to increase the influence of weak users. A weighted multi-user contribution metric is then used to select dominant RIS elements, and a local candidate phase codebook is generated around a weighted reference phase. Finally, a coverage-fairness-aware score selects the discrete RIS phase vector. Simulations show that the proposed method reduces average BER, outage probability, weak-user BER, and inter-user performance variation while reducing the candidate search space from QN to ${L^{{K_d}}}$.