Does an IRS Degrade Out-of-Band Performance?

Intelligent reflecting surfaces (IRSs) were introduced in the literature in order to enhance the performance of the wireless systems. However, from a cellular service provider's point of view, a concern with the use of an IRS is its effect on out-of-band (OOB) quality of service. Specifically, if there are two operators, say X and Y, providing services in a given geographical area using non-overlapping frequency bands, and if operator X uses an IRS to optimally enhance the throughput of it's users, does the IRS degrade the performance of operator Y? We study this scenario by analyzing the ergodic sum-rates achieved by both the operators under round-robin scheduling. We also derive the complementary cumulative distribution function of the change in the effective channel gain at an OOB user with and without the IRS, which provides deeper insights into the effect of the IRS on the overall channel quality. Surprisingly, we find that even though the IRS is randomly configured from operator Y's point of view, the OOB operator still benefits from the presence of the IRS, witnessing a performance enhancement for free. This happens because the IRS introduces additional paths between the transmitter and receiver, increasing the overall signal power arriving at the receiver and providing diversity benefits. We verify our findings via numerical simulations, and conclude that an IRS is always beneficial to every operator, even when the IRS is deployed to optimally serve only one operator in the system.