Introduction

FMCW Positioning Radar project is a work of team made up of three people. We were trying to get the qualification to attend ESDC through this project. Sadly, we were out of the game in the stage of school contest.

The radar works in the 2.7 GHz - 3.7 GHz band. It has 4 transmitting antenna and 12 receive antenna. With a transmission power of less than 10 dBm, it localizes a person within a median of 5 cm in the y dimension. Within 5 meters, it has an accuracy of about 20 cm. Without further tests, the maximum distance is unknown. However, it can indicate a person behind a 25 cm concrete wall.

Code and document here. The final presentation with hardware and data processing flow introduction see here or below.

FMCW

Frequency-modulated continuous-wave radar (FM-CW) – also called continuous-wave frequency-modulated (CWFM) radar – is a short-range measuring radar set capable of determining distance (Continuous-wave radar wiki). If you have no idea about it, the tutorial below is a good starting point.

• Frequency-Modulated Continuous-Wave Radar (FMCW Radar): a brief introduction of the principle of FMCW
• Build a Small Radar System Capable of Sensing Range, Doppler, and Synthetic Aperture Radar Imaging: an MIT course to build an FMCW radar yourself
• GUEST POST: TRY RADAR FOR YOUR NEXT PROJECT: an article written by the instructor of the course above
• Intro to mmWave Sensing: FMCW Radars - Module 1: Range Estimation: a series of 5 short videos providing a concise yet in-depth introduction to sensing using FMCW radars

SDR

Software-defined radio (SDR) is a radio communication system where components that have been traditionally implemented in hardware (e.g. mixers, filters, amplifiers, modulators/demodulators, detectors, etc.) are instead implemented by means of software on a personal computer or embedded system.

The project uses a USRP N210 with an LFRX daughter board. SDR is not an important part of this project cause the RF front end are made with RF modules and the FMCW generator is build separately. The USRP here acts as a role of ADC and is accessed via Simulink. See how to use a USRP with Simulink below:

• USRP® Support Package from Communications System Toolbox: Matlab toolbox for USRP
• Digital Communication Systems Engineering Using Software Defined Radio: a tutorial of SDR using Simulink

Software

Software part includes ADF4159 evaluation board configuration, mbed code, and Simulink model.

ADF4159

The ADF4159 evaluation board is configured using ADF4158 and ADF4159 Evaluation Board Software via USB. The configuration file here can be loaded into the software. The board is configured as follows:

• Ramp mode: Continuous sawtooth
• RF frequency: 2.7 GHz - 3.7 GHz
• Ramp frequency: 2000 Hz
• Charge pump: 1.25 mA
• Muxout: Digital lock Detect. It is used as a sync signal to trigger the switching of antennas.

The 4 Tx and 12 Rx makes up 4*12 antenna pairs. With ramp frequency of 2000 Hz, every antenna pair is switched to for 41.7 times per second.

mbed code

The mbed board switches antenna pairs after every trigger edge from the ADF4159 Muxout pin. And it also overlay the antenna pair number to the sync signal. Considering that MCU has some unstable reaction delay between the trigger edge of the sync signal and the first down edge of the output, the MCU first pulls down the sync signal then output bits of the number. In this way, the down edge of the overlaid signal has no jitter. Code of mbed board is here.

Simulink model

All the models are in the simulink folder. RadarImagingAndPositioning.slx block implements the basic signal processing logic and output the 2D heat map and target position. The data processing flow is shown in the presentation above. usrp_4t12r_heatmap.slx connects the basic blocks to show the imaging result and targets. For more information about using USRP with Simulink, see the Introduction section above.