Re: [Discuss-gnuradio] IEEE 802.11 a/g/p transceiver - Data not being received at any frequency other than 5.89G

[Qurat-Ul-Ann Akbar]

Hi,

I changed the sample rate to 20 MHz and changed the beacon frames to be 11g and now the receiver receives some WiFi packets. But it shows a parse error which says : 

new mac frame  (length 10)
=========================================
frame too short to parse (<20)
WIRESHARK: received new message
message length 10
WIRESHARK: d_msg_offset: 0   to_copy: 43   d_msg_len 43
WIRESHARK: output size: 32768   produced items: 43

And there are some "Ds" printed on the console as well. I read through the previous threads about the frame not being able to parse but didn't find anything useful. I checked Wireshark and the packets make sense over there and I can see the different beacon frames and RTS/CTS transmissions as well. I also saw your wifi receiver video on Youtube and my constellation plot looks nothing like yours. Its very random. Can you kindly tell me what you think could be going on ?

Best,

Annie

On Tue, Jan 24, 2017 at 1:25 AM, Bastian Bloessl <address@hidden> wrote:

Hi,

On 01/24/2017 12:17 AM, Qurat-Ul-Ann Akbar wrote:

I did the following things that you suggested:

1) The gain is normalized and in the example file wifi_rx and tx it was
set to 0.75. I changed the gain to a bunch of values but nothing is
really changing. Also the scope plot in the receiver just shows noise
and even when I get some decoded message on the console I don't see
anything changing in the scope plot and the constellation plot is random
as well during the transmission. At 5.89 GHz (which is default in the
script when I downloaded it), it shows some packets on the console with
random sequence numbers but doesn't work at all at any other frequency
which I set by the frequency in the GUI.

I don't understand what exactly you are doing. The module is creating/decoding a baseband signal. It just instructs the hardware to tune to a certain frequency. So if your device works only on one particular frequency, it might be an issue of the device, the interference on the other channels, the antenna, etc ...

However, I think that it is much more likely that you changed something in the flow graph and forgot to put the "freq" parameter properly in the HW source/sink. Maybe one device is not retuning when you click the GUI.

2) I followed the instructions for testing wifi_rx with a wifi card on
your page. The beacon frames are not being detected by wifi_rx and
nothing gets printed on the console. I dont know whats wrong exactly. I
have tested my setup with a simple script sending a single and double
tone message at a frequency of 2.5 GHz and its successful so I dont
think its a hardware problem.

Make sure that these are 11a or 11g beacons (OFDM). Not 11b or some compatibility mode. You will have to set the sample rate to 20MHz.

Can you kindly tell me what could be going on ?

Sorry, but I have no idea. Make sure that there are no overruns/underruns ('O's and 'U's printed on the console).

Best,
Bastian

Thanks a lot,
Annie

On Wed, Jan 18, 2017 at 12:24 AM, Bastian Bloessl <address@hidden

<mailto:address@hidden>> wrote:

    Hi,

    On 01/17/2017 11:57 PM, Qurat-Ul-Ann Akbar wrote:

        1) The sequence numbers are not in accordance with the data.
        Sometimes
        the first packet I get has sequence number 0 but at other times
        I get
        sequence numbers like 5 or 41. The mac frame length is 524. An
        example
        sequence numbers pattern was 5,36,452,753,961. I do not
        understand why
        are the sequence numbers random? The antennas are very close to each
        other so shouldn't the packet loss rate be very low? And are packets
        corrupted and GNU Radio can not decode them properly?


    The unreliability can have multiple reasons, like
    - too high gain
    - too low gain
    - overruns/underrruns
    - DC offset (try different LO offsets)

    The constellation plot is usually a good indicator how good it works.
    Placing the USRPs side by side doesn't always make things better.


        2) My transmission does not work for any frequency other than
        5.89G. I
        can't understand what could be the reason for this. Not even
        5.88G. My
        daughter board supports 1.2G to 6GHz.



    Can you describe a bit more verbose what you did and what happened?
    Actually, that should work by just changing the frequency in the GUI.

    To debug, it might also be helpful to test your setup with a WiFi
    card. This is helpful to understand if the problem is in the sender
    or the receiver.

    Best,
    Bastian

--
Dipl.-Inform. Bastian Bloessl
Distributed Embedded Systems Group
University of Paderborn, Germany
http://www.ccs-labs.org/~bloessl/