A Local Oscillator Phase Compensation Technique for Ultra-Wideband Stepped-Frequency Continuous Wave Radar Based on a Low-Cost Software-Defined Radio.

Kazunori Takahashi,Takashi Miwa

doi:10.3390/s21030780

Abstract

The paper discusses a way to configure a stepped-frequency continuous wave (SFCW) radar using a low-cost software-defined radio (SDR). The most of high-end SDRs offer multiple transmitter (TX) and receiver (RX) channels, one of which can be used as the reference channel for compensating the initial phases of TX and RX local oscillator (LO) signals. It is same as how commercial vector network analyzers (VNAs) compensate for the LO initial phase. These SDRs can thus acquire phase-coherent in-phase and quadrature (I/Q) data without additional components and an SFCW radar can be easily configured. On the other hand, low-cost SDRs typically have only one transmitter and receiver. Therefore, the LO initial phase has to be compensated and the phases of the received I/Q signals have to be retrieved, preferably without employing an additional receiver and components to retain the system low-cost and simple. The present paper illustrates that the difference between the phases of TX and RX LO signals varies when the LO frequency is changed because of the timing of the commencement of the mixing. The paper then proposes a technique to compensate for the LO initial phases using the internal RF loopback of the transceiver chip and to reconstruct a pulse, which requires two streaming: one for the device under test (DUT) channel and the other for the internal RF loopback channel. The effect of the LO initial phase and the proposed method for the compensation are demonstrated by experiments at a single frequency and sweeping frequency, respectively. The results show that the proposed method can compensate for the LO initial phases and ultra-wideband (UWB) pulses can be reconstructed correctly from the data sampled by a low-cost SDR.

Highlights

Radar in the microwave frequency range is finding an increasing number of applications in many fields, such as imaging buried and concealed objects non-destructively, automotive, biomedical, and material characterization
The paper discussed the challenge of employing low-cost software-defined radio (SDR) for building Stepped-frequency continuous wave (SFCW) radar, and a solution to the challenge was presented
The phase is necessary for constructing SFCW radar in addition to the amplitude to reconstruct time-domain signals

Summary

Introduction

Radar in the microwave frequency range is finding an increasing number of applications in many fields, such as imaging buried and concealed objects non-destructively, automotive, biomedical, and material characterization. An SDR used for a random noise radar system in [14] has 400 MHz instantaneous bandwidth and achieved 37.5 cm range resolution These wideband SDRs are typically more expensive than low-cost ones in the market that this work is targeting. Nuand bladeRF is chosen in this work because it has I/Q input/output, full duplex capability, and a relatively wider carrier frequency bandwidth compared with other low-cost SDRs available in the market. These are necessary features to build a near-range UWB SFCW radar. Note that the paper extends the previous work [26] and provides more comprehensive explanations of the unknown initial phase of LO signals and experiment results

SDR-Based SFCW Radar

Working Principle

Implementation in SDR and Compensation of the Initial Phase of the LO Signal

At a Single Frequency

Frequency Sweeping

Discussion and Conclusions