  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 (UEs), does it degrade the performance of UEs
served by operator Y? We answer this by analyzing the average and instantaneous
performances of the OOB operator considering both sub-6 GHz and mmWave bands.
Specifically, we derive the ergodic sum SE achieved by the operators under
round-robin scheduling. We also derive the outage probability and analyze the
change in the SNR caused by the IRS at an OOB UE 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
UE and providing diversity benefits. Finally, we show that the use of
opportunistic scheduling schemes can further enhance the benefit of the
uncontrolled IRS at OOB UEs. We numerically illustrate our findings and
conclude that an IRS is always beneficial to every operator, even when the IRS
is deployed & controlled by only one operator.
