Tagged: wifi

Using an RTL-SDR and Broadcast FM Radio RDS Signals to improve WiFi Networking

The performance of WiFi networks can depend heavily on how crowded the WiFi channels are in your area. For example when your neighbours start streaming a movie over their own separate WiFi network, it can cause your own WiFi connection to slow down. This happens because generally separate WiFi networks do not collaborate with one another, and when two packets are sent on the same channel at the same time, they collide causing no packets to get through.

There are several methods that attempt to stop collisions, but none are very efficient because WiFi nodes are not synchronized to one another. If each WiFi node could be synchronized to a common reference time, then avoiding collisions is made easier.

Marcel Flores, Uri Klarman, and Aleksandar Kuzmanovic from Northwestern University have been working on this idea and have come up with a system they have termed Wi-FM which is based on FM RDS signals. Many FM radio stations transmit a digital Radio Data System (RDS) subcarrier on their broadcast frequency. This RDS signal is often used to simply display information on the radio such as the station name and current song playing.

Since each nearby WiFi node should be able to receive the same RDS signal at the exact same time, it can be used as a common synchronization signal. Then once synchronized each WiFi node can listen to the other nodes and work out what their transmit scheduling is like and then optimize their own transmit schedule.

In their prototyping they used an RTL-SDR dongle connected to a PC running GNU Radio. The GNU Radio program decodes the RDS signal and the resulting information is sent to the Linux kernel which handles the WiFi transmit schedule processing.

This story was also covered on Hackaday.

WiFM radio processing path.
WiFM radio processing path.

Seeing through walls with WiFi signals and USRP software defined radios

Researchers at the University College of London have found a way to use WiFi signals to see through walls, using a USRP software defined radio and software written in LabView. The researchers have shown that they are able to utilize local WiFi signals to detect and monitor moving objects such as people behind a wall in a similar fashion to how radar systems work. The advantage over traditional radar is that their system is completely passive, requiring no transmitter, other than the already ubiquitous WiFi signal.

In a demonstration the researchers showed how they were able to not only detect the presence of a person behind a wall, but also detect small hand gestures that were made.

Detecting body gestures from WiFi signals in LabView.
Detecting body gestures from WiFi signals in LabView.

It appears the researchers are patenting their work and are looking to market their technology towards military and security surveillance operators as well as towards other applications such as traffic monitoring and the monitoring of children and the elderly.

We aren’t sure what type of radio accuracy is required for a system such as this, but it may be possible that SDR’s that cost less than the USRP may also work, assuming the software technology can ever be replicated/licensed.

wifi_hostage
A proposed application of the technology: Allowing police to see through walls in a hostage situation.