Air-ground interference mitigation is a challenging issue for integrating unmanned aerial vehicles (UAVs) into cellular networks, since UAVs can cause/suffer severe interference to/from a large number of co-channel base stations (BSs) in their uplink/downlink communications due to line-of-sight (LoS) UAV-ground channels. In this letter, we propose a new and efficient interference mitigation scheme to tackle this challenge by exploiting the intelligent reflecting surfaces (IRSs) deployed near the BSs. Specifically, in the downlink, the IRS near each co-channel BS tunes its passive beamforming to suppress the BS's interference to the UAV. While in the uplink, its passive beamforming is tuned to suppress/enhance the interference from the UAV for facilitating the linear/nonlinear interference cancellation (IC) at each co-channel BS. To achieve the optimal UAV-ground performance trade-off, we further propose a hybrid IC scheme to maximize the weighted sum rate of the UAV and ground users in the uplink, by jointly optimizing the set of BSs applying the linear/nonlinear IC and the UAV's transmit power. Simulation results demonstrate the performance gain of the proposed schemes over benchmark schemes and draw important design insights for IRS-aided UAV communication in cellular networks.
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