RIScatter: Unifying Backscatter Communication and Reconfigurable Intelligent Surface

Backscatter Communication (BackCom) nodes harvest energy from and modulate information over an external electromagnetic wave. Reconfigurable Intelligent Surface (RIS) adapts its phase shift response to enhance or attenuate channel strength in specific directions. In this paper, we show how those two seemingly different technologies (and their derivatives) can be unified into a single architecture called RIScatter. RIScatter consists of multiple dispersed or co-located scatter nodes, whose reflection states are adapted to partially modulate their own information and partially engineer the wireless channel. The key principle is to render the probability distribution of reflection states as a joint function of the information source, Channel State Information (CSI), and Quality of Service (QoS) of coexisting active primary and passive backscatter links. This enables RIScatter to softly bridge BackCom and RIS; boil down to either under specific input distribution; or evolve in a mixed form for heterogeneous traffic control and universal hardware design. To reap the benefits of RIScatter, we also propose a co-located Successive Interference Cancellation (SIC)-free receiver that semi-coherently decodes the backscatter information, recovers the reflection pattern, and coherently decodes the primary link. For a single-user multi-node RIScatter network, we characterize the achievable primary-(total-)backscatter rate region by designing the input distribution at scatter nodes, the active beamforming at the Access Point (AP), and the energy decision regions at the user. Simulation results demonstrate RIScatter nodes can exploit the scattered paths to smoothly shift between backscatter modulation and passive beamforming.