RSSI measurement / questions

Hello

I would like to be able to measure the received signal power by the
USRP2’s antenna at any compatible frequency. My idea is store the value
of the received signal power in a variable, which I’ll use to determine
if the channel is busy or not (based on a configurable threshold).

I’ve taken a look at USRP2_fft example and I’ve thought this:

[USRP2 source] --> [Complex to Mag ^2]–> [FFT sink]

When I run this graph (using grc) in a computer, I obtain the USRP2’s
measurement modulus. On another computer I run a program which generates
802.11 a/g/p frames. The result is: there’s a big difference between
received signals (when I’m sending 802.11 frames and when I’m not
sending).

My questions are:

  • Is it a good way to test the ‘RSSI’ by the USRP2’s antenna? Is it
    accurate?

  • How could I store some resultant samples in a file (instead of
    graphical sink)? If I could do this, I’ll be able to estimate whether
    the channel is busy or not.

  • Does it exist another method to do the RSSI measurement “easily”?

  • I’ve been reading and googling before posting, and I found this post
    http://lists.gnu.org/archive/html/discuss-gnuradio/2007-08/msg00088.html
    but it’s from 08/2007. Is it possible to check the value of “rssi
    registers” directly? Are they deprecated because they’re related to
    USRP1 & inband signaling?

Regards,

J. Gutierrez

Hi,

  • How could I store some resultant samples in a file (instead of
    graphical sink)? If I could do this, I’ll be able to estimate whether
    the channel is busy or not.

  • Does it exist another method to do the RSSI measurement “easily”?

I assume you are using the XCVR2450 when dealing with 802.11.
If so,you can access the rssi output from the maxim chip directly with
the aux
adc (ad7922) on the USRP2. You can add the lsadc_read_rx() command in
the
firmware to get the adc output and then pass the 12 bits through the
metadata.word0 to the host. The rest of the adc output to dBm estimation
can
be done on the host side. That works for us with version 3.2.2 as an
indication of the received signal strength but I am not sure if it will
affect
other functionalities. We are still testing this way. Any comments are
welcome!

BR Christoph

Christoph T. wrote in post #908064:

I assume you are using the XCVR2450 when dealing with 802.11.
If so,you can access the rssi output from the maxim chip directly with
the aux
adc (ad7922) on the USRP2. You can add the lsadc_read_rx() command in
the
firmware to get the adc output and then pass the 12 bits through the
metadata.word0 to the host. The rest of the adc output to dBm estimation
can
be done on the host side.

can you explain the details of reading the rssi with USRP2?how to use
the code in firmware ? Is the value in the usrp or PC?