In-phase and Quadrature Chirp Spread Spectrum for IoT Communications

TL;DR

This paper introduces a coherent CSS scheme for IoT communications that encodes data on both in-phase and quadrature components, leading to improved energy and spectral efficiency over traditional LoRa PHY.

Contribution

It proposes a novel coherent detection method for CSS in LoRa, utilizing channel estimation from reference sequences to enhance performance.

Findings

Achieves approximately 1 dB gain in energy efficiency.

Doubles spectral efficiency compared to conventional LoRa.

Uses channel equalization based on reference sequences.

Abstract

This paper describes a coherent chirp spread spectrum (CSS) technique based on the Long-Range (LoRa) physical layer (PHY) framework. LoRa PHY employs CSS on top of a variant of frequency shift keying (FSK), and non-coherent detection is employed at the receiver for obtaining the transmitted data symbols. In this paper, we propose a scheme that encodes information bits on both in-phase and quadrature components of the chirp signal, and rather employs a coherent detector at the receiver. Hence, channel equalization is required for compensating the channel induced phase rotation on the transmit signal. Moreover, a simple channel estimation technique exploits the LoRa reference sequences used for synchronization to obtain the complex channel coefficient used in the equalizer. Performance evaluation using numerical simulation shows that the proposed scheme achieves approximately 1 dB gain in terms of energy efficiency, and it doubles the spectral efficiency when compared to the conventional LoRa PHY scheme. This is due to the fact that the coherent receiver is able to exploit the orthogonality between in-phase and quadrature components of the transmit signal.