基于LoRa的卫星物联网系统接收机同步与非相干软解调方案

Receiver Synchronization and Nonconherent Soft Demodulation Scheme for LoRa-Based Satellite IoT Systems

LoRa(Long Range)技术是一种改进的高阶啁啾扩频(Chirp Spread Spectrum,CSS)调制技术,如今已经成为了地面物联网(Internet of Things,IoT)的主流调制技术之一.不同于地面物联网,卫星IoT将会实现全球范围内的无缝覆盖,但其通信距离遥远和高速移动使得接收机通常工作在极低信噪比下并面临较大时延和多普勒频移.鉴于此,本文将LoRa技术应用到卫星IoT系统中并研究相应的接收机同步与非相干软解调方案.首先给出了卫星IoT上行链路的系统模型和数据帧结构.接着根据LoRa信号的特点,设计了一种联合时延与多普勒频移估计方案.为了进一步消除相偏和其他残留项的影响以及考虑采用基于软判决译码的编码方案,又设计了一种低复杂度的非相干软解调算法.仿真结果表明基于所提同步方案和软解调算法的turbo-LoRa系统可以在极低信噪比和较小导频开销下获得接近于理想情况的误码性能,且在BER=10-5下优于Hamming-LoRa系统3 dB.

LoRa(Long Range)technique is an improved high-order chirp spread spectrum(CSS)modulation tech-nique,and becomes one of the mainstream modulation techniques for terrestrial internet of things(IoT).Different from the terrestrial IoT,satellite IoT can achieve seamless global coverage,however,the corresponding receiver often works under the extremely-low signal-to-noise ratio(SNR)and also faces large delay and Doppler shift due to long distance and high-speed movement.To this end,this paper applies the LoRa technique to satellite IoT systems and studies the relevant receiv-er synchronization as well as noncoherent soft demodulation scheme.First,considered system model and data frame struc-ture of the uplink satellite IoT are given.Then,according to the feature of LoRa signals,a joint delay and Doppler shift esti-mation scheme is proposed.In order to further eliminate the impact of the phase offset as well as other residual terms and consider using soft-decision decoding based coding schemes,a low-complexity noncoherent soft demodulation algorithm is also designed.Simulation results show that the turbo-LoRa system based on the proposed synchronization scheme and soft demodulation algorithm can achieve near ideal error performance under extremely low SNR and small pilot overhead,and can provide 3 dB gains over the Hamming-LoRa system at a bit error rate(BER)of 10-5.