无源物联网：背景、概念、挑战及研究进展

近年来，随着物联网(IoT)部署范围不断扩大，数以千亿计的智能设备将接入物联网，对网络接入能力、能量供应、成本等提出了极大挑战，无源物联网呼之欲出。该文梳理了无源物联网相关概念并给出其定义，首次系统研究了无源物联网面临的能量密度低、转化效率低、后向散射通信距离有限、能量信息同时传输兼顾难等4大挑战，详细分析了问题原因并对研究进展进行综述：针对能量密度低挑战，从波束成形、能量收集天线设计、智能反射表面3个方面综述；针对能量转换效率低挑战，从接收机架构优化、波形设计、阻抗匹配优化、整流器优化4个方面综述；针对后向散射通信距离有限挑战，从新的调制方式、频移型后向散射新机制、MIMO增强、新的信道编码机制、新的信号检测方法、智能反射表面增强以及半有源模式7个方面综述；针对能量信息同时传输兼顾难问题，从能量信息同时传输架构优化、能量信息兼容信道编码2个方面综述。针对每个优化方向，对比分析了各类方法的优劣并指出了未来研究方向。

Passive Internet of Things: Background,Concept, Challenges and Progress

In the past decades, the scope of Internet of Things (IoT) is expanded continuously. With hundreds of billions of smart devices connect to IoT, huge challenges are arisen from several aspects such as device cost, connectivity capability, and power supplies. Fortunately, the new paradigm - passive IoT is coming which is one of the effective solutions for these challenges. Some related concepts are analyzed and the definition of passive IoT is proposed. For the first time, the four challenges faced by passive IoT, such as low energy density, low conversion efficiency, limited distance of backscatter communication, and difficulty in transmission of power and information simultaneous, are studied. The problems are analyzed in deeply and the research progress are surveyed. For the challenge of low energy density, the research progress is reviewed from three aspects: beamforming, antenna design for energy harvesting, and intelligent reflecting surface. For the challenge of low energy conversion efficiency, from receiver architecture optimization, waveform design, impedance matching, rectifier optimization. For the challenge of limited distance of backscatter communication, the research progress is reviewed from seven aspects: new modulation scheme, new frequency-shifted backscattering scheme, MIMO, new channel coding scheme, new signal detection method, intelligent reflecting surface enhancing, and semi-active mode. Considering the difficulty in transmission of power and information simultaneous, the research progress is reviewed from two aspects: optimization of the receiver architecture and the energy information compatible signal coding scheme. For each aspect, the advantages and disadvantages of various methods are analyzed and the future research directions are pointed out.