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Re: [Discuss-gnuradio] Channel Sounding @ 5 GHz


From: Jordan J Riggs
Subject: Re: [Discuss-gnuradio] Channel Sounding @ 5 GHz
Date: Mon, 13 Jul 2009 09:48:40 -0600

I'm not an expert on the matter, but I suspect that the XCVR2450 board requires some sort of command to select the 5GHz band, which would need to be implemented in the sounder's FPGA code.
At a chip rate of 32MHz, you have a resolution of 3e8/32e6 meters. The length of the PN sequence determines the maximum unambiguous distance at which multipaths can be detected. So with a degree-12 sequence, you have (2^12 - 1)*3e8 meters to work with. With long sequences, however, you are limited by the capabilities of the USRP. Depending on your DBoard, you have ~20mW of output. Your antenna also makes a big difference. Especially if you want to do any work outside, I would use something like a log-periodic antenna. If you can get your hands on an amplifier, you can significantly boost your range.
In my experience (with a sounder of my own design), averaging multiple IR's appreciably reduces the noise.

While I'm at it, can anyone explain why a complex correlation vector is coming out of the sounder FPGA? Doesn't a mathematical correlation return real values?

Jordan

On Fri, Jul 10, 2009 at 5:49 AM, Martin Braun <address@hidden> wrote:
Hi,

I was wondering if there's a way to use the channel sounder (gr-sounder)
with the XCVR d'boards at 5 GHz. I gave it a try, but the channel
impulse responses only popped up +-1s and zeros. Well, I wasn't
expecting it to work since the XCVR is not mentioned in the README, but
can it be tweaked?

Plus, does anyone have any experience with the sounder they'd share?
If so, there's a couple of things I'd be interested to know:

- What's a sensible distance for getting useful results without any
 additional hardware?
- Can I tune the amp/gain settings to increase my SNR without distorting
 the result?
- Assume the channel's stationary. In that case, every IR vector
 should contain the same data (up to some phase shift, probably). I'm
 figuring I could further increase the SNR by averaging
 (phase-correctly) over several IRs, right?

If anyone has any interesting stories on channel sounding with GR/USRP,
I'd be grateful for any info.

Cheers,
MB

--
Dipl.-Ing. Martin Braun           Phone: +49-(0)721-608 3790
Institut fuer Nachrichtentechnik  Fax:   +49-(0)721-608 6071
Universitaet Karlsruhe (TH)       http://www.int.uni-karlsruhe.de/

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