# Toward High Bit Rate LoRa Transmission via Joint Frequency-Amplitude Modulation
## 1 Introduction
For instance, BIC-LoRa [22] employs non-linear chirp shapes to encode extra bits, while Golden Modulation (GM) [23] leverages Zadoff-Chu sequences to enable massive multiuser access. Inspired by this paradigm shift and seeking further gains, we propose QR\(-\)LoRa, a novel PHY-layer modulation scheme that leverages joint frequency-amplitude modulation to improve the bit rate of LoRa transmissions. To this end, we propose the QR\(-\)LoRa, a novel PHY-layer modulation scheme that leverages the joint frequency-amplitude modulation to improve the bit rate of LoRa transmissions. In addition to the standard frequency modulation of a LoRa chirp, QR\(-\)LoRa can modulate information data on the amplitude domain. Specifically, we replace the constant envelope of the LoRa chirp with a cyclic right-shifted ramp signal that can encode information in the amplitude dimension. Then, we generate two signals, one is a ramp signal whose amplitude changes from the minimum to the maximum, and the other is a chirp signal whose frequency varies linearly with time. We perform a cyclic right-shifted operation on the ramp signal to encode the information data. Finally, the cyclic right-shifted ramp signal multiplies with the chirp signal to generate the QR\(-\)LoRa chirp. During the demodulation process, we propose a differential algorithm to decode the amplitude-modulated data bits in the QR\(-\)LoRa symbol and utilize the traditional dechirp method to decode the frequency-modulated data bits. Through theoretical analyses, the data rate of QR\(-\)LoRa can be up to \(R_{b}^{QR}\geq\frac{SF}{2^{SF}}BW\). To put this high-level idea into practice, the challenges in design and implementation must be addressed, as summarized below. (i) Due to the large-scale fading and multipath effect of the real channel, the envelope of the QR\(-\)LoRa chirp produces nonlinear distortion. In particular, the fast fading causes the envelope to fluctuate, which interferes with the amplitude demodulation result. Aiming at solving the above problem, we propose a _denoising algorithm_ to smooth and reshape the envelope. (ii) When the channel noise appears at the envelope discontinuous point, it results in multiple peaks around the envelope wrapping point, which affects the accuracy of QR\(-\)LoRa amplitude demodulation; thus, we introduce the _Minimum Aided Decoding Algorithm_(_MADA_) as an assistant to eliminate the ambiguity of amplitude peaks. (iii) The sensitivity of amplitude modulation is lower than that of frequency modulation, which decreases the communication range of QR\(-\)LoRa. In the end, we design QR\(-\)LoRa to work in two modes, joint or single modulation, according to the current SNR. Our main contributions are as follows:  
* We design a novel PHY-layer scheme named QR\(-\)LoRa, which leverages the joint frequency-amplitude modulation to improve the bit rate of LoRa transmissions. * We propose a set of simple but effective algorithms to solve the practical challenges during the modulation and demodulation process of the QR\(-\)LoRa. * We verify and evaluate the performance of QR\(-\)LoRa through theoretical analysis and real-world environments. The results show that the bit rate gain of QR\(-\)LoRa can grow to 2\(\times\) compared with the existing LoRa device. **Roadmap. ** The rest of this paper is structured as follows: Section 2 presents the preliminary, and Section 3 describes the QR\(-\)LoRa design. We analyze QR\(-\)LoRa performance in Section 4. Section 5 presents the implementation and evaluation results. Finally, we conclude this paper in Section 6.