Efficient Indexing Schemes for Quadrature Reflection Modulation in RIS-Based BackCom

Besides steering the incoming signals toward a receiver, a reconfigurable intelligent surface (RIS) can simultaneously serve as a backscatter node to communicate additional data by modulating the same signal. Quadrature reflection modulation (QRM) is a promising contender among index modulation (IM)-based techniques for implementing RIS-based backscatter communications (RIS-BackCom). In this paper, we demonstrate that the error performance of RIS-QRM heavily relies on the employed indexing scheme, which configures each RIS element into one of the two orthogonally-reflecting modes of QRM to encode the transmitted data onto the reflection pattern of RIS elements. In this context, we present an efficient guideline for designing indexing schemes that ensure the resulting channel-dependent QRM symbols are evenly spaced with maximum separation. Following this guideline, we propose two specific indexing techniques: the incremental and uniform indexing schemes, both of which substantially enhance the error performance of RIS-QRM without increasing detection complexity. We derive a closed-form expression for the bit error probability (BEP) of RIS-QRM with the incremental indexing scheme. Furthermore, we propose a low-complexity detector for RIS-BackCom systems employing RIS-QRM. Extensive computer simulations illustrate the effectiveness of our proposed schemes in improving the error performance of RIS-QRM-based BackCom systems.