RIScatter: Unifying Backscatter Communication and Reconfigurable Intelligent Surface

—Backscatter Communication (BackCom) nodes har- vest 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 to leverage their beneﬁts simultaneously into a single architecture called RIScatter. RIScatter consists of multiple dispersed or co-located scatter nodes, whose reﬂection states can be adapted to partially engineer the wireless channel of the existing link and partially modulate their own information onto the scattered wave. This contrasts with BackCom (resp. RIS) where the reﬂection pattern is exclusively a function of the information symbol (resp. Channel State Information (CSI)). The key principle in RIScatter is to render the probability distribution of reﬂection states (i.e., backscatter channel input) as a joint function of the information source, CSI, and Quality of Service (QoS) of the coexisting active primary and passive backscatter links. This enables RIScatter to softly bridge, generalize, and outper- form 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. For a single-user multi-node RIScatter network, we characterize the achievable primary- (total-)backscatter rate region by optimizing the input distribution at the nodes, the active beamforming at the Access Point (AP), and the backscatter detection regions at the user. Simulation results demonstrate RIScatter nodes can exploit the additional propagation paths to smoothly transition between backscatter modulation and passive beamforming, and the proposed practical receiver accommodates the double fading, double modulation, and symbol period difference of active-passive coexisting net- works without Successive Interference Cancellation (SIC).