On the Impact of an IRS on the Out-of-Band Performance in Sub-6 GHz & mmWave Frequencies

Intelligent reflecting surfaces (IRSs) were introduced to enhance the performance of wireless communication systems. However, from a service provider's viewpoint, a concern with the use of an IRS is its effect on out-of-band (OOB) quality of service. Specifically, if two operators, say X and Y, provide services in a given geographical area using non-overlapping frequency bands, and if operator X uses an IRS to enhance the spectral efficiency (SE) of its users, does it degrade the performance of users served by operator Y? We answer this question by analyzing the average and instantaneous performances of the OOB operator considering both sub-6 GHz and mmWave bands, accounting for their corresponding channel characteristics. Specifically, we derive the ergodic sum-spectral efficiency achieved by the operators under round-robin scheduling. We also derive the outage probability and analyze the change in the SNR witnessed by an OOB user in the presence of the IRS using stochastic dominance theory. Surprisingly, 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, in both sub-6 GHz and mmWave bands. This is 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 numerically illustrate our findings and conclude that an IRS is always beneficial to every operator, even when the IRS is deployed and controlled by only one operator to serve its own users.