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Reconfigurable intelligent surfaces (RIS) is a new technology that aims to improve the spectral and energy efficiency for next-generation wireless technology (6G). It has recently emerged as a potential alternative for alleviating limitations caused by complex propagation environment, especially in urban areas and its ability to expand network coverage. The advantage of RIS is especially evident when line-of-sight transmission is not possible in aerial-terrestrial communication where unmanned aerial vehicles (UAVs) are used as access points (APs). In most literature, the RIS is shared among different access points (APs) and the phase shifters are designed jointly to optimize certain performance metric for all APs and receivers. This work takes a different approach by optimally determining which RIS elements should be assigned to each UAV-user pair and designing the phase shifters at the RIS to maximize the sum …