On the Degrees of Freedom of IRS-Assisted Non-Coherent MIMO Communications

Abstract

In this letter, we consider the problem of intelligent reflecting surfaces (IRS) assisted non-coherent MIMO communications. The transmitter employs non-coherent MIMO codes and we allow the IRS to send its own information by switching the phases of some/all of its reflecting elements. We prove that the phase-modulated information from the IRS is completely transparent to the non-coherent MIMO transmitter and receiver, with no need for any performance provisioning or even knowing that the IRS is transmitting data. Moreover, we characterize the achievable degrees of freedom of the proposed transmission scheme gained by allowing the IRS to send extra data. We show that although non-coherent communications do not require channel estimates at the receiver, IRSs can still be used to enhance performance as more energy can be geared towards the receiver, although no co-phasing is done in our case. Moreover, the IRS can increase the achievable degrees of freedom of the system without requiring any extra power at the IRS.