  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.
