# Toward High Bit Rate LoRa Transmission via Joint Frequency-Amplitude Modulation
## 5 Evaluations
### Compatibility with Existing LoRa System  
To enable the proposed scheme to be commercially feasible, we explore the compatibility with the commercial LoRaWAN device, such as the commercial off-the-shelf LoRa node equipped with a Semtech SX1276 RFIJS. Inspired by previous work [5], we can transform the existing LoRa node into a QR\(-\)LoRa node by plugging in an external expansion board, as shown in Figure 19.  
The expansion board consists of an MCU, a switch, a passive surface acoustic wave filter (SAW Filter) [30], and an envelope detector [31]. The SAW filter is a narrowband bandpass filter with a center frequency of 433.92 MHz and a passband bandwidth of 500 kHz. The red dotted line in Figure 19 denotes the generation route of the QR\(-\)LoRa chirp, the blue dotted line represents the frequency demodulation route of the QR\(-\)LoRa signal and the green dotted line shows its amplitude demodulation route.  
For the transmitter, as in the red dotted line shown in Figure 19, the MCU sends instructions to the LoRa node to generate a standard LoRa chirp. Then, the MCU controls the switch to the Passive SAW Filter branch, where the standard LoRa chirp is transformed into a QR\(-\)LoRa chirp through the Passive SAW Filter. For the receiver, as in the blue and green dotted lines shown in Figure 19, the QR\(-\)LoRa signal received by the antenna is divided into two parts, one is directly fed into the LoRa node through the RF switch controlled by the MCU for standard LoRa frequency demodulation. On the other hand, the QR\(-\)LoRa amplitude demodulation process is as follows: (1) The envelope detector extracts the envelope of the received QR\(-\)LoRa signal and sends it to the MCU. (2) The MCU samples the envelope and performs amplitude demodulation through differential algorithm.  
Figure 19: QR\(-\)LoRa node.  
Figure 18: BER measured results under LOS and NLOS conditions.  
It should be noted that the function of the Passive SAW Filter is to realize the mapping from frequency modulation to amplitude modulation. Reference [5] utilizes the steep transition band of the Passive SAW Filter so that the amplitude of the LoRa chirp passing through the critical band varies with the LoRa chirp frequency. This characteristic is exactly in line with the QR\(-\)LoRa amplitude modulation requirements. For the detailed working principle of the Passive SAW Filter, please refer to [5].  
In summary, the QR\(-\)LoRa node is compatible with a LoRa node through an external expansion board. The Passive SAW Filter, envelope detector and RF switch in the expansion board are passive devices and the MCU can use the ultra-low power Apollo2 (Reference [5] evaluates that its power consumption is only 19.6 uW). Therefore, the power consumption of the expansion board will not increase the power consumption cost too much. In addition, the above-mentioned passive devices are small in size and low in price, which makes QR\(-\)LoRa a commercial prospect.  
The detailed design and implementation compatible with LoRa commercial devices is a very interesting and meaningful task, which we view as part of our future work. The pulse-encoded transition (PET) technique and stacked-FET switches discussed in [32] offer low-loss, high-power handling capabilities suitable for the commercialization of QR\(-\)LoRa. In our future work, we aim to implement these techniques to optimize ACLR and bandwidth for better LoRa compatibility. Additionally, we plan to explore the impact of different pulse shapes, such as raised cosine, as part of the ongoing improvements in our research.