Effective Electromagnetic Degrees of Freedom in Backscatter MIMO Systems

TL;DR

This paper defines and analyzes the electromagnetic degrees of freedom in backscatter MIMO systems, revealing their dependence on mutual coupling and illumination, and demonstrating how to optimize them in various environments.

Contribution

It introduces a novel definition of BS-EEMDOFs based on the Jacobian, deriving a closed-form expression and exploring their distribution and optimization in backscatter MIMO systems.

Findings

BS-EEMDOFs depend on mutual coupling and illumination.

The number of BS-EEMDOFs can be optimized using coherent illumination.

Experimental results validate the theoretical analysis.

Abstract

While the definition of the effective electromagnetic degrees of freedom (EEMDOFs) of a static linear multiple-input multiple-output (MIMO) system is well established, the counterpart for a backscatter MIMO (BS-MIMO) system is so far missing. A BS-MIMO system encodes the input information into the loads of backscatter elements. Due to mutual coupling, the mapping from load configuration to observed fields is fundamentally non-linear, which complicates the analysis of BS-EEMDOFs. We introduce a definition of BS-EEMDOFs based on the Jacobian of the observed fields with respect to the load configuration. We derive a closed-form expression from multiport network theory which demonstrates that the number of BS-EEMDOFs is fundamentally a distributed variable, whose distribution depends on the mutual coupling between the backscatter elements and the coherent illumination. The modes associated with BS-EEMDOFs lie in the column space of the end-to-end channel matrix from backscatter array ports to receiver ports, but the number of BS-EEMDOFs is generally different from the number of benchmark EEMDOFs associated with the same array being coherently fed rather than tunably terminated. The dependence on the coherent illumination yields optimized coherent illumination as a control knob for the number of BS-EEMDOFs. We present numerical and experimental results for the evaluation and optimization of the number of BS-EEMDOFs in different radio environments with reconfigurable intelligent surfaces.