Receive Beamforming Schemes to Mitigate Inter-Operator Pilot Contamination in RIS-Aided MIMO Networks

When reconfigurable intelligent surfaces (RISs) are integrated into cellular networks, they can give rise to inter-operator pilot contamination, severely degrading network performance. While combating this effect is possible by orthogonalizing the RIS configurations, it requires inter-operator coordination and limits the degree of configuration freedom per RIS. Therefore, in this work, we explore the use of receive beamforming to mitigate inter-operator pilot contamination in RIS-aided multiple input multiple output systems, where two operators share infrastructure and deploy RISs to enhance network coverage. We focus on uplink channel estimation and data transmission and propose a method in which the base stations (BSs) apply a novel kind of receive beamforming to suppress pilot-contaminated interference. We compare our proposed method with a previous RIS orthogonalization approach and another approach that does not eliminate inter-operator pilot contamination by formulating two schemes. In Scheme 1, the BS beamforms towards its intended channel without nulling the interfering channel while orthogonal RIS configurations mitigate pilot contamination. In Scheme 2, the BS nulls the interfering channel, removing the need for orthogonalized RIS configurations and thereby halving the number of pilots. In the baseline scheme, the BS uses the same beamformer as in Scheme 1 and the same RIS configurations as in Scheme 2, hence does not eliminate pilot contamination. We assess the performance of both schemes under different channel conditions, in terms of channel estimation mean square error and capacity bounds with imperfect channel state information. Our numerical results indicate that Scheme 2 offers a superior rate at high signal-to-noise ratios (SNRs) due to fewer pilots and comparable channel estimation accuracy, while Scheme 1 performs better at very low SNRs due to capturing more energy. However, the reduced number of pilots in Scheme 2 makes it a favorable choice for practical systems, with minimal performance loss at low SNR. Overall, the proposed beamforming approach effectively mitigates inter-operator pilot contamination.