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This paper describes a coherent chirp spread spectrum (CSS) technique basedon the Long-Range (LoRa) physical layer (PHY) framework. LoRa PHY employs CSSon top of a variant of frequency shift keying (FSK), and non-coherent detectionis employed at the receiver for obtaining the transmitted data symbols. In thispaper, we propose a scheme that encodes information bits on both in-phase andquadrature components of the chirp signal, and rather employs a coherentdetector at the receiver. Hence, channel equalization is required forcompensating the channel induced phase rotation on the transmit signal.Moreover, a simple channel estimation technique exploits the LoRa referencesequences used for synchronization to obtain the complex channel coefficientused in the equalizer. Performance evaluation using numerical simulation showsthat the proposed scheme achieves approximately 1 dB gain in terms of energyefficiency, and it doubles the spectral efficiency when compared to theconventional LoRa PHY scheme. This is due to the fact that the coherentreceiver is able to exploit the orthogonality between in-phase and quadraturecomponents of the transmit signal.

In-phase and Quadrature Chirp Spread Spectrum for IoT Communications