Re: [Discuss-gnuradio] rx_ofdm Does Not Work Past Synchronizer

[Jonathan Fox]

Thanks for the responses.

When I use the USRPs the UHD sink is placed after the OFDM Cyclic Prefixer, right after the Mutliply Const (50m). I haven't changed any settings because I remember reading in a few previous emails that the most common question people ask is "Did you change anything?"

Sadly, the delay wasn't the case, at least for the loopback mode.

I reduced the amplitude of the signal by an additional 0.02, so the multiply constant is at 0.03 as opposed to 0.05. It brought the clipping well within +1, -1. Before the signal clipped past -1 but never rose above +1. No effect but I am not ruling out that it wouldn't help.

The video is interesting enough that I am going to continue watching after the first half, maybe I can figure out more from there.

Jon

On Wed, Mar 19, 2014 at 7:50 AM, Martin Braun <address@hidden> wrote:

On 03/19/2014 12:25 AM, Jonathan Fox wrote:
> So I have the stock python scripts (tx_ofdm and rx_ofdm) located in the
> gr-digital examples and put USRP sources and sinks in the appropriate
> spots. I left sampling alone (100K for TX, 3.2M for RX) and had the
> TX/RX frequency at 500 MHz. The USRPs used are N210s with WBX
> daughterboards. I tested the USRP used for TX out by outputting a signal
> to another and looking at it on the GNU Radio spectrum analyzer. Also,
> this is through SMA cables and a 30 dB attenuator, no over the air
> transmission. I placed file sinks after each block in the process; each
> file is a binary file. After multiple runs I felt like something wasn't
> working after looking at the binary files.

Maybe you're probably screwing up your tx signal by clipping, although I
can't say for sure from your flow graphs (where exactly do you put the
UHD sink?) See
http://video.fosdem.org/2014/AW1125/Sunday/Tutorial_OFDM_Packet_Transceivers.webm,
starting around ~27 mins for an explanation, or search the mailing list
archive.

Martin

>
> When I execute, only the file sinks placed after the Schmidl & Cox OFDM
> Synchronizer freq_offset and detect have data in them, at least their
> files have some size to them, implying something has been written. The
> file sinks after FFT (or the header/payload demux) have no data, 0 KB.
>
> When I discovered that the receive didn't work, I took the grc file
> another user made back in October with both TX mod and RX demod in the
> same flowgraph to see if that would work (files attached). Again same
> issue, all the binary files after the FFT have no data so something is
> getting screwed up in my demod.
>
> Has any user on this list run into this issue before or can offer some
> insight to why the receiver is failing?
>
> Thank you,
>
> Jon
>
> Attached is the python script, grc file, and screen shot of the TXRX
> combo in GRC.
>
>

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