Backscatter communication (BackCom), one of the core technologies to realize zero-power communication, is expected to be a pivotal paradigm for the next generation of the Internet of Things (IoT). However, the "strong" direct link (DL) interference (DLI) is traditionally assumed to be harmful, and generally drowns out the "weak" backscattered signals accordingly, thus deteriorating the performance of BackCom. In contrast to the previous efforts to eliminate the DLI, in this paper, we exploit the constructive interference (CI), in which the DLI contributes to the backscattered signal. To be specific, our objective is to maximize the received signal power by jointly optimizing the receive beamforming vectors and tag selection factors, which is, however, non-convex and difficult to solve due to constraints on the Kullback-Leibler (KL) divergence. In order to solve this problem, we first decompose the original problem, and then propose two algorithms to solve the sub-problem with beamforming design via a change of variables and semi-definite programming (SDP) and a greedy algorithm to solve the sub-problem with tag selection. In order to gain insight into the CI, we consider a special case with the single-antenna reader to reveal the channel angle between the backscattering link (BL) and the DL, in which the DLI will become constructive. Simulation results show that significant performance gain can always be achieved in the proposed algorithms compared with the traditional algorithms without the DL in terms of the strength of the received signal. The derived constructive channel angle for the BackCom system with the single-antenna reader is also confirmed by simulation results.

翻译：反向散射通信(BackCom)作为实现零功耗通信的核心技术之一，有望成为下一代物联网(IoT)的关键范式。然而，"强"直射链路(DL)干扰(DLI)传统上被认为是有害的，通常会淹没"弱"反向散射信号，从而降低BackCom性能。与以往消除DLI的努力不同，本文利用相长干扰(CI)，使DLI对反向散射信号产生贡献。具体而言，我们的目标是通过联合优化接收波束成形向量和标签选择因子来最大化接收信号功率，但由于Kullback-Leibler(KL)散度的约束，该问题非凸且难以求解。为解决此问题，我们首先对原问题进行分解，然后提出两种算法：通过变量变换和半定规划(SDP)求解波束成形设计的子问题，以及通过贪心算法求解标签选择的子问题。为深入理解CI，我们考虑单天线阅读器的特殊情形，揭示反向散射链路(BL)与DL之间的信道角度关系，在此条件下DLI将变为相长性。仿真结果表明，与不考虑DL的传统算法相比，所提算法在接收信号强度方面总能获得显著的性能增益。针对单天线阅读器BackCom系统推导的相长信道角度也通过仿真结果得到验证。