Re: [Discuss-gnuradio] Measure the Distance to another 802.11 device

[Marcus Müller]

Hi Florian,

that's an interesting approach!


On 06/07/2017 02:58 PM, Florian Adamsky wrote:
> Hi all,
>
> in one of our projects we need to measure the distance between two
> 802.11 devices as accurately as possible. Our idea is to use the
> round-trip time (RTT). To avoid any delay from the operation system and
> from the network stack, our idea is to measure the arrival time of the
> acknowledgment control frame. Means, we take a timestamp when device A
> sent a small data frame to device B; when B has received the frame, it
> replies with an acknowledgment control frame and when A has received it
> we will take another timestamp. Of course we would repeat that n-times
> to avoid outliers.
Your observation, that Wifi chipsets typically delegate most of what is
necessary to be a working Wifi device to the operating system. Highly
timing-sensitive things, however, are typically handled by the chip
firmware itself.
If I remember correctly, there's some degree of adjustability in that
firmware, at least in Atheros chipsets; [1] might be an interesting talk
for you.
>
> We bought a HackRF and tried to get the examples from gr-ieee802-11
> running. After some minor problems (dc offset) we were able to receive
> 802.11 frames. However, we are not able to send any 802.11 packets,
> because the hackrf driver does not support burst transmission with
> tagged streams. One reader of this mailing list suggested to give
> soapysdr a try. We did that as well, but again without success. Here we
> didn't see any "UUUUU" in the debug console but we were still not able
> to see any packets with another wireless card in monitor mode.
Another takeaway from [1] that in usual operation, the hardware doesn't
even hand over packets that don't make the checksum test – maybe it'd be
interesting to disable that filtering.

Best regards,
Marcus

[1]
https://www.irongeek.com/i.php?page=videos/defcon-wireless-village-2014/20-inside-the-atheros-wifi-chipset-adrian-chadd