Design and Analysis of Chirp-Layered Superposition Coding for LoRa

TL;DR

This paper proposes a chirp-layered superposition coding scheme for LoRa that enhances spectral efficiency by superposing high-SF waveforms on low-SF signals with minimal impact on demodulation.

Contribution

It introduces a novel superposition coding method for LoRa that maintains demodulation integrity while increasing spectral efficiency.

Findings

High-SF superposed waveforms minimally affect LoRa demodulation.

Analytical error-rate expressions match Monte Carlo simulations.

Spectral efficiency of LoRa links is improved with simple transceiver modifications.

Abstract

This paper investigates the design of chirp-layered superposition coding for LoRa, where an additional waveform is linearly superposed on a standard LoRa transmission with minimal impact on the LoRa demodulation process. We first show that any non-zero superposed signal perturbs the output of the standard dechirp-and-DFT demodulator, and then characterize the class of superposed waveforms that minimize this degradation under a given power budget. In particular, we show that a high spreading factor (high-SF) LoRa waveform superposed on a low-SF signal (e.g., SF12 on SF7) can be designed so that its impact on the standard LoRa demodulation remains small. As a result, within each low-SF symbol interval, the high-SF segment can be treated as a quasi-narrowband carrier that conveys an additional BPSK stream. We derive analytical error-rate expressions for both the low-SF LoRa layer and the superposed high-SF layer, and validate them through Monte Carlo simulations. The proposed chirp-layered superposition coding scheme improves the spectral efficiency of LoRa-based links and uses a relatively simple transceiver architecture.