Decoding real world LR-FHSS signals: design, implementation, and approaching the theoretical limit.

Abstract

Long Range-Frequency Hopping Spread Spectrum (LR-FHSS) is a new physical layer option added to the LoRa family, promising higher network capacity than the previous versions of LoRa. Since the announcement, LR-FHSS has gathered growing interest. Various studies have attempted to evaluate and enhance its communication range and network capacity, while others have compared its performance with previous version of LoRa. However, the actual network capacity of LR-FHSS and the effectiveness of proposed methods remain unknown, as most existing studies rely on mathematical analysis or simulations with certain simplifying assumptions. Our goal is to reveal the actual capacity of LR-FHSS and develop methods to enhance its performance while evaluating these methods in a setting as close to real-world conditions as possible.

In this dissertation, we design and implement a software LR-FHSS receiver from scratch to convert the baseband waveform into bits and pass the Cyclic Redundancy Check (CRC). To the best of our knowledge, this work is the first of its kind that processes signals transmitted by an actual LR-FHSS device while accounting for real-world issues such as frequency estimation errors. Also, we design customized methods to enhance receiver's capacity, including error correction decoding and Successive Interference Cancellation (SIC), which were not mentioned in earlier studies but effectively handle collisions. Furthermore, we develop an analytical bound for the theoretical capacity of LR-FHSS networks.

The evaluation of our receiver was based on real-world packet traces collected using an LR-FHSS device and demonstrated through real-world experiments on the POWDER wireless platform, in addition to trace-driven simulations for large networks. Our result shows that LR-FHSS outperforms the previous version of LoRa, meets expectations in communication range, achieves significantly higher network capacity than those reported earlier, and confirms that the capacity of our software receiver is approaching the upper bound of LR-FHSS networks. We overcame a number of challenges such as lack of documentation of LR-FHSS and open-source resources, header acquisition, match reconstructed waveform with the received waveform under heavy collisions, and find a good approximation of the residual power of packets that have been decoded and canceled.