IQ Impedance Modulator Front-End for Low-Power LoRa Backscattering Devices

This article describes the design of an IQ impedance modulator radio front-end which is used to generate long range (LoRa) symbols with backscattering techniques, allowing building low-power devices that may be compatible with the current LoRa networks. It is shown that a linear frequency modulated (LFM) chirp can be generated by properly varying the phase of a reflected wave instead of directly varying its instantaneous frequency. The proposed device consists of two radio frequency (RF) transistors that generate a set of impedances by changing their gate bias, allowing reflecting their incident wave with predefined phase values. By joining these two reflected waves in quadrature, a new set of impedances is obtained whose phases vary 360°, on a constant Voltage Standing Wave Ratio (VSWR) circle. In order to validate the proposed design, several LoRa symbols were generated and successfully transmitted and decoded. Measurements of the bit error rate (BER) versus signal-to-noise ratio (SNR) were conducted and shown to be in accordance with other related work. Moreover, the impact of intentional perturbations added to the control bias voltages was analyzed and tests over-the-air (OTA) were performed for different indoor propagation scenarios.