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.

翻译：反向散射通信（BackCom）节点从外部电磁波中获取能量并调制信息。可重构智能表面（RIS）通过调整其相位响应来增强或抑制特定方向的信道强度。本文展示了这两种看似不同的技术（及其衍生技术）如何被统一到单一架构RIScatter中。RIScatter由多个分散或共位的散射节点组成，其反射状态被自适应调整以部分调制自身信息、部分调控无线信道。核心原理是将反射状态的概率分布表示为信息源、信道状态信息（CSI）以及共存主动主链路与被动反向散射链路服务质量（QoS）的联合函数。这使得RIScatter能够平滑桥接BackCom与RIS；在特定输入分布下退化为任一技术；或以混合形式演进以支持异构流量控制与通用硬件设计。为充分发挥RIScatter的优势，我们还提出了一种共位免串行干扰消除（SIC-free）接收机，其采用半相干方式解码反向散射信息、恢复反射模式，并相干解码主链路。针对单用户多节点RIScatter网络，我们通过设计散射节点的输入分布、接入点（AP）的主动波束赋形以及用户的能量判决区域，刻画了主链路与（总）反向散射链路的可达速率区域。仿真结果表明，RIScatter节点可利用散射路径在反向散射调制与无源波束赋形之间平滑切换。