# A Compact Low-Power LoRa IoT Sensor Node with Extended Dynamic Range for Channel Measurements
## 2 Hardware Design
### RF-Section  
Among other things, the LoRa transceiver module is equipped with a LoRaWAN Protocol Stack, a 434 MHz and 868 MHz LoRa Technology Radio and a command processor providing the user with an easy-to-use ASCII-over-UART command interface. According to the transceiver's documentation, the module can theoretically achieve receiver sensitivities down to \(-146\) dBm by selecting the appropriate spreading factor (SF) and receiver bandwidth (RBW) of the LoRa modulation scheme [5]. Complemented by the integrated 14 dB power amplifier, very high link budgets can be obtained. In addition to LoRa modulation, the transceiver module also features Frequency Shift Keying (FSK) and Gaussian Frequency Shift Keying (GFSK) modulation [5]. With regard to channel characterization and link performance, the RN2483 transceiver can provide the user with the SNR of the last received packet. Unfortunately, no metrics on the measurement scheme used for this are given in the transceiver's documentation. Therefore a calibration procedure was carried out in an anechoic chamber, linking the measured SNR value to the received power, thus establishing a baseline for SNR measurements. The calibration method outlined in Figure 2 is performed in three phases. In the first phase, the LoRa transmitter (TX) is connected directly to a Rohde and Schwartz FSV40 spectrum analyser, to accurately measure the transmitted power. Second, the TX is placed in the anechoic chamber and is connected via low-leakage coaxial transmission lines and a high-precision stepped attenuator to the spectrum analyzer in the shielded control room. In this step, the losses induced by the coaxial cables are measured to be calibrated away. The setup with the TX in the anechoic chamber and the RX in the shielded control room is necessary to provide enough isolation for the third measurement phase, where the attenuation has to be increased to over 150 dB in order to reach the detection limit of the LoRa RX. In this third phase, using the stepped attenuator, the attenuation is adjusted from 50 dB to 162 dB, which is the maximum setting. For every data point in the calibration, ten  
Figure 2: Calibration setup. performed and averaged. This procedure was performed for both the 434 MHz and 868 MHz frequency bands. The results of the calibration measurements are shown in Figure 3.  
The calibration data show that using its default settings (SF = 12 and RBW = 125 kHz), the transceiver achieves a receiver sensitivity of \(-140\) dBm, which corresponds to an SNR of \(-20\) dB. This sensitivity is slightly better than the value of \(-136\) dBm specified by the manufacturer for the default parameters. Note for measured SNR values greater than 5, the detector is seen to saturate and behave in a highly non-linear fashion. In the linear region, first-order regression models are fitted to the calibration data, yielding the following formula to calculate the received power (P\({}_{\text{RX,dBm}}\)) based on the measured SNR (SNR\({}_{\text{meas,dB}}\)):  
\[\text{P}_{\text{RX,dBm}}=\frac{\text{SNR}_{\text{meas,dB}}-88.51}{0.7756} \tag{1}\]  
Because of the strong similarity of the models for both frequency bands (their difference is lower than the measurement resolution), it is concluded that this relation is valid for both bands. Furthermore, the coefficients found for these models are very similar to those of the model presented for 868 MHz in [20]. As can be seen in Figure 3, only a relatively small SNR detection range is actually available. To resolve this issue, the RF-section also features two stepped attenuators. These attenuators serve to increase the range of the signal level measurement by dynamically attenuating signals that are too strong. The measured received power is described by  
\[\text{P}_{\text{RX,dBm}}=\text{P}_{\text{ANT,dBm}}-\text{ATT}_{\text{dB}} \tag{2}\]  
with ATT\({}_{\text{dB}}\geq 0\) and P\({}_{\text{ANT,dBm}}\) equal to the received power at a well-matched antenna, neglecting the minimal losses of the short transmission lines.