[论文解读] Modulation in the Air: Backscatter Communication over Ambient OFDM Carrier

Gang Yang, Ying‐Chang Liang+2 more|arXiv (Cornell University)|Apr 7, 2017

Energy Harvesting in Wireless Networks参考文献 26被引用 6

一句话总结

本文提出了一种新型的漫反射背向散射通信（AmBC）收发器设计，利用扩频原理和循环前缀结构，在漫反射OFDM载波上抑制直射链路干扰。该文提出一种最优最大似然检测器，其检测阈值具有闭式表达式，适用于单天线系统；并提出一种多天线接收机的线性组合检验统计量，与传统能量检测方法相比，在误比特率（BER）性能、数据速率和通信范围方面均取得显著增益。

ABSTRACT

Ambient backscatter communication (AmBC) enables radio-frequency (RF) powered backscatter devices (BDs) (e.g., sensors, tags) to modulate their information bits over ambient RF carriers in an over-the-air manner. This technology also called "modulation in the air", thus has emerged as a promising solution to achieve green communications for future Internet-of-Things. This paper studies an AmBC system by leveraging the ambient orthogonal frequency division multiplexing (OFDM) modulated signals in the air. We first model such AmBC system from a spread-spectrum communication perspective, upon which a novel joint design for BD waveform and receiver detector is proposed. The BD symbol period is designed to be in general an integer multiplication of the OFDM symbol period, and the waveform for BD bit `0' maintains the same state within a BD symbol period, while the waveform for BD bit `1' has a state transition in the middle of each OFDM symbol period within a BD symbol period. In the receiver detector design, we construct the test statistic that cancels out the direct-link interference by exploiting the repeating structure of the ambient OFDM signals due to the use of cyclic prefix. For the system with a single-antenna receiver, the maximum-likelihood detector is proposed to recover the BD bits, for which the optimal threshold is obtained in closed-form expression. For the system with a multi-antenna receiver, we propose a new test statistic, and derive the optimal detector. Moreover, practical timing synchronization algorithms are proposed, and we also analyze the effect of various system parameters on the system performance. Finally, extensive numerical results are provided to verify that the proposed transceiver design can improve the system bit-error-rate (BER) performance and the operating range significantly, and achieve much higher data rate, as compared to the conventional design.

研究动机与目标

• 为解决使用环境OFDM信号的漫反射背向散射通信（AmBC）系统中强直射链路干扰的关键挑战。
• 设计一种鲁棒的收发器，以在低功耗、无电池的物联网环境中实现可靠、高速通信。
• 通过仅需估计背向散射信道强度而非完整信道状态信息，降低接收机复杂度。
• 开发适用于实际AmBC部署场景的实用定时同步算法。
• 评估系统参数对传输速率和检测性能的影响。

提出的方法

• 从扩频通信的角度建模AmBC系统，利用具有循环前缀的OFDM信号的重复结构。
• 设计BD波形，使得比特'0'在BD符号周期内保持恒定状态，而比特'1'包含符号中间的跳变。
• 构建一种检验统计量，通过利用OFDM信号中循环前缀的周期性来抵消直射链路干扰。
• 推导出单天线接收机的最大似然检测器，并以闭式表达式获得最优检测阈值。
• 为多天线接收机提出每根天线检验统计量的线性组合，实现在最小信道知识条件下的最优检测。
• 提出针对所设计波形和信号结构的高效定时同步算法。

实验结果

研究问题

• RQ1 如何在使用环境OFDM信号的AmBC系统中有效消除直射链路干扰？
• RQ2 何种波形设计可在利用OFDM信号固有结构的同时实现可靠检测？
• RQ3 是否仅通过背向散射信道强度估计即可实现最优检测，而非依赖完整CSI？
• RQ4 与传统能量检测方法相比，所提系统在误比特率（BER）、数据速率和工作范围方面的性能如何？
• RQ5 关键系统参数对检测性能和传输速率有何影响？

主要发现

• 所提系统相较于基于传统能量检测的设计，显著降低了误比特率（BER）性能。
• 由于有效的干扰抑制和检测灵敏度提升，通信范围得到显著扩展。
• 系统支持远高于传统AmBC系统的数据速率，实现了高速率背向散射通信。
• 单天线系统的最优检测阈值以闭式表达式推导得出，支持低复杂度实现。
• 多天线部署提升了BER性能，证明了所提接收机设计的可扩展性。
• 所提定时同步算法在实际部署场景中被证明具有实用性和高效性。