A simultaneously transmitting and reflecting reconfigurable intelligent surface (STAR-RIS) assisted downlink (DL) active and uplink (UL) backscatter communication (BackCom) framework is proposed. More particularly, a full-duplex (FD) base station (BS) communicates with the DL users via the STAR-RIS's transmission link, while exciting and receiving the information from the UL BackCom devices with the aid of the STAR-RIS's reflection link. Non-orthogonal multiple access (NOMA) is exploited in both DL and UL communications for improving the spectrum efficiency. The system weighted sum rate maximization problem is formulated for jointly optimizing the FD BS active receive and transmit beamforming, the STAR- RIS passive beamforming, and the DL NOMA decoding orders, subject to the DL user's individual rate constraint. To tackle this challenging non-convex problem, we propose an alternating optimization (AO) based algorithm for the joint active and passive beamforming design with a given DL NOMA decoding order. To address the potential high computational complexity required for exhaustive searching all the NOMA decoding orders, an efficient NOMA user ordering scheme is further developed. Finally, numerical results demonstrate that: i) compared with the baseline schemes employing conventional RISs or space division multiple access, the proposed scheme achieves higher performance gains; and ii) higher UL rate gain is obtained at a cost of DL performance degradation, as a remedy, a more flexible performance tradeoff can be achieved by introducing the STAR-RIS.

翻译：提出了一种基于同时透射和反射可重构智能表面（STAR-RIS）辅助的下行主动通信与上行反向散射通信（BackCom）框架。具体而言，全双工基站（BS）通过STAR-RIS的透射链路与下行用户进行通信，同时借助STAR-RIS的反射链路激励并接收来自上行BackCom设备的信息。为提升频谱效率，下行和上行通信均采用非正交多址接入（NOMA）技术。本文以系统加权和速率最大化问题为目标，联合优化全双工基站主动接收与发射波束成形、STAR-RIS无源波束成形以及下行NOMA解码顺序，并受限于下行用户的个体速率约束。针对这一非凸优化难题，提出了一种基于交替优化（AO）的联合主动与无源波束成形设计算法，并预先设定下行NOMA解码顺序。为降低穷举所有NOMA解码顺序所需的高计算复杂度，进一步开发了一种高效的NMA用户排序方案。最后，数值结果表明：i）与采用传统RIS或空分多址接入的基线方案相比，所提方案实现了更高的性能增益；ii）上行速率增益的提升以下行性能损失为代价，而引入STAR-RIS可提供更灵活的性能权衡方案。