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Active noise RADAR with frequency sweep

Experimental setup

The initial setup was running on a Dell Precision M6500 laptop with an Analog Devices Inc. PlutoSDR connected to the USB2 port and an Ettus Research B210 connected to the USB3 port. The random phase modulation on the carrier transmitted by the PlutoSDR (TX port) is collected with one B210 port (B - RX2) while the other port (A - RX2) is connected to the receiving antenna. The PlutoSDR is connected to a 10-dB coupler (Mini Circuits ZEDC-10-2B) whose direct output is connected to the transmitting antenna and coupled output is connected through a 20 dB attenuator to the B210 input.

Because installing the software on the laptop is hardly reproducible (GNU Radio 3.8, libiio + libad9361 + gr-iio supporting the PlutoSDR as described at https://wiki.analog.com/resources/tools-software/linux-software/gnuradio + need to modifiy gr-iio to increase sweep rate) and since the newer (April 2020) Raspberry Pi 4 (RPi4) is fitted with two USB 3 ports, the whole setup was ported to this platform.

In order to setup the system, Buildroot is downloaded at https://github.com/buildroot/buildroot since it supports the full 64-bit ARM processor of the RPi4. Additional packages needed are provided through a BR2_EXTERNAL mechanism using the additional files found at https://github.com/oscimp/oscimp_br2_external. After git cloning the latter repository, in a shell source sourceme.ggm will load the BR2_EXTERNAL additional packages. Then go to the buildroot directory, copy the provided raspberrypi4_64_defconfig into the Buildroot configs directory, and make raspberrypi4_64_defconfig followed by make. After a very long compilation, the output/images/sdcard.img is written (dd) to a micro-SD card for running on the RPi4.

RPi4 configuration:

Connect with a USB-TTL converter and set the IP address of the RPi4 to 192.168.0.165:

ifconfig eth0 192.168.0.165

The root password is root (needed for ssh to the board)

It is believed that at least the PlutoSDR must be powered from an external power supply (ie not from the USB cable going to the RPi4). It might be safe as well to power the B210 from an external supply, although it does not seem to be mandatory as the RPi4 seems to source enough current on its USB3 port.

Once the SD-card is written with sdcard.img, run it from the RPi4, and connect to the RPi4 after configuring the host computer Ethernet interface with a 192.168.0.X IP address (X != 165). The B210 firmware files must be copied from the host computer to the RPi4 (somehow uhd_images_downloader seems to be failing on the RPi4): on the RPi4,

mkdir -p /usr/share/uhd/images/

and then

scp 192.168.0.X:/usr/share/uhd/images/*b2* /usr/share/uhd/images

It is also possible to copy the files from the host computer to the output/target/usr/share/uhd/images directory of buildroot, then make to generate again sdcard.img to be dd to the SD-card.

Signal acqusisition

Copy from the host computer zeromq_demo_rev1.py to the root directory of the RPi4 (scp zeromq_demo_rev1.py 192.168.0.165:/root), connect to the RPi 4 with ssh 192.168.0.165 and from its shell, launch python3 zeromq_demo_rev1.py. This will start the GNU Radio flowchart and run a ZeroMQ Publish server as well as a TCP server.

From the host computer, run octave and launch zeromq_demo_rev1 (running hence the zeromq_demo_rev1.m script): the TCP server on the RPi4 must achnoledge the connection and data will start streaming from th RPi4 to the host computer. After the whole frequency sweep, spectra are concatenated and the high resolution correlation is displayed, hopefully with some echoes in the positive delay domain.

Signal processing

The 700_750_house_final4_with_protection.mat.gz dataset was collected with zeromq_demo_rev1.py controlling the B210 and the PlutoSDR, while the frequency sweep and data collection is controlled by zeromq_demo_rev1.m.

The dataset can be postprocessed using go.m running with GNU/Octave or Matlab.

A movie of the system running is found at http://jmfriedt.free.fr/active_radar_RPi4.mp4

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