Have someone experience or other knowledge about the "usrp_sounder.py"
CIR-channel sounder, FPGA-based tool ?
It should be a very valuable and usable Gnu-Radio communications tool if it
works !?!?
I appreciate any comments or ideas to my issues with it as described below.

/ Rickard


On Sat, May 28, 2011 at 2:09 AM, Rickard Radio <rickardra...@gmail.com>wrote:

> Dear all,
>
> I have been trying using the usrp_sounder.py script for measuring the
> channel impulse response (CIR), but without success.
>
> Q: - Have anyone succeeded? Does the FPGA-based script work as advertised?
>
> Here is what I got:
> Firstly, making a "loopback" test worked great
> > usrp_sounder.py   -g 50 -f 2.45e9 -d 12 -t -r -l -v  -F loopback.log
>
> followed by plotting the recorded CIR-values in Matlab:
> >> loopback = read_complex_binary('loopback.log',inf);
> >> plot(abs(loopback))
>
> This results in a nice looking "impulse-train", i.e., one strong impulse
> followed by near zero correlation values until the next PN-sequence frame
> (4095 values per 12-bit PN-period).
> So far, so good!
>
> However, when I try to send from one USRP with RFX2400 and receive with
> another (same setup) I do not get a representative output (I believe!).
> Distance between USRPs is about 10m, and placed in different rooms (with
> wooden walls).
> Rx> usrp_sounder.py -g 50 -f 2.45e9 -r -v  -F CIRtest.log
> Tx> usrp_sounder.py  -f 2.45e9 -t -v
>
> In this case I only see when plotting a continuous almost "random" sequence
> with relatively low but uniform max-amplitude, but no distinctive strong
> correlation peaks at all ?!?
> I do not understand why!?  Although the USRP to USRP distance is relatively
> short  (~10m) I would then expect a similar CIR-response as in the loopback
> test. That is, my expectation was not just one but a few relatively strong
> impulses, the rest of the long PN-period filled with much weaker background
> correlation noise. But I don't get any strong correlation-peaks at all !
> Not a single one that is well above the uniform bg correlation noise.
> Q: - Can anyone explain this result??
>
> It can not depend on a too weak received signal, since I tested to monitor
> it using the "ursp_fft.py" with the same UHDgain (= 50). In this case I can
> clearly see the broadband PN-signal in the frequency domain, well above the
> noise-floor (some 15-20 dB). Furthermore,  by turning on and off the
> PN-sequence transmitter during CIR-recording, I can also clearly notice a
> corresponding variation in the received signal strength when plotting in
> Matlab afterwards - but not a single distinctive peak at all !?!? Again just
> the uniform background correlation noise! Hmmm, why this then?!
>
> Q: - Have anyone had success with the "usrp_sounder.py" script?
> Q: - Does it work as advertised, or what can my problem be?
>
> I know it doesn't contain synchronization, so the CIR's should "roll"
> between PN-periods, but that is not the problem here, since I get no peaks?!
>
> Help me get nice strong peaks, please!
>
> Glad for any advice!
>
> / Rickard
>
>
> Below a summary of how I used the "ursp_sounder.py" script:
>
> Loopback:
> > usrp_sounder.py   -g 50 -f 2.45e9 -d 12 -t -r -l -v  -F loopback.log
> Using PN code degree of 12 length 4095
> Logging impulse records to file:  loopback.log
> Using smoothing alpha of 1.0
> Setting PN code degree to 12
> Enabling digital loopback.
> Enabling transmitter.
> gr_buffer::allocate_buffer: warning: tried to allocate
>    4 items of size 32760. Due to alignment requirements
>    512 were allocated.  If this isn't OK, consider padding
>    your structure to a power-of-two bytes.
>    On this platform, our allocation granularity is 4096 bytes.
> gr_buffer::allocate_buffer: warning: tried to allocate
>    4 items of size 32760. Due to alignment requirements
>    512 were allocated.  If this isn't OK, consider padding
>    your structure to a power-of-two bytes.
>    On this platform, our allocation granularity is 4096 bytes.
> Enter CTRL-C to stop.
>
> Q: -- Is it normal behaviour with the buffer-allocation warning above (I
> get it all the time when receiving, depending on the PN code length) ?
>
> Receiver:
> Rx > usrp_sounder.py -g 50 -f 2.45e9 -r -v  -F CIRtest.log
> Using PN code degree of 12 length 4095
> Sounding frequency range is 2.434G to 2.466G
> Logging impulse records to file:  CIRtest.log
> Using Flex 2400 Rx MIMO B for sounder receiver.
> Setting receiver gain to 50.0
> Using smoothing alpha of 1.0
> Setting receiver frequency to 2.45G
> Setting PN code degree to 12
> Disabling digital loopback.
> gr_buffer::allocate_buffer: warning: tried to allocate
>    4 items of size 32760. Due to alignment requirements
>    512 were allocated.  If this isn't OK, consider padding
>    your structure to a power-of-two bytes.
>    On this platform, our allocation granularity is 4096 bytes.
> gr_buffer::allocate_buffer: warning: tried to allocate
>    4 items of size 32760. Due to alignment requirements
>    512 were allocated.  If this isn't OK, consider padding
>    your structure to a power-of-two bytes.
>    On this platform, our allocation granularity is 4096 bytes.
> Enter CTRL-C to stop.
>
> Transmitter:
> Tx > usrp_sounder.py  -f 2.45e9 -t -v
> Using PN code degree of 12 length 4095
> Sounding frequency range is 2.434G to 2.466G
> Using Flex 2400 Tx MIMO B for sounder transmitter.
> Setting transmitter frequency to 2.45G
> Setting PN code degree to 12
> Disabling digital loopback.
> Enabling transmitter.
> Press return to exit.
>
> followed by plotting the recorded data in Matlab as shown above.
>
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