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Re: [Discuss-gnuradio] OFDM Benchmark Tx Issue?

From: Michael Dickens
Subject: Re: [Discuss-gnuradio] OFDM Benchmark Tx Issue?
Date: Tue, 21 Feb 2012 09:14:02 -0500

Hi Florian - Interesting observation about better reception when using larger 
bandwidths.  I tried it out, and yes indeed they do seem better at 1 MS/s 
compared with 250 or 500 kS/s -- meaning that more packets are received 
correctly at the higher rate than the lower rates.  I didn't try > 1 MS/s yet, 
but I will later.  I need to review my OFDM to get a better clue as to why this 
might be the case.

Though this is interesting and useful, my problem wasn't the packet error rate.

My issue was that the Tx center frequency was 1 MHz high with a center 
frequency of 5 GHz [1], no matter which USRP1 I used for Tx, when using GNU 
Radio -> UHD.  When I wrote my own C++ program to use UHD for doing Tx of a 
real sinusoid -- "real" so as to see both the + and - spikes, so as to be able 
to determine where the center frequency is -- everything centered correctly 
when I set the UHD frequency to 5 GHz [2].

I don't have time right now to investigate further.  Mostly throwing this out 
to see if anyone else had encountered it. - MLD

[1] 1 MHz off at 5 GHz is 200 ppm (yes?).  IIRC a reasonable spec on these 
boards is 5-10 ppm.   So, > 10x off the spec?  I doubt it.  My guess is it's 
something in GNU Radio that wasn't fixed when the benchmarks were transitioned 
from gr-usrp* to UHD.

[2] The offset was ~1 kHz, or ~0.2 ppm, which is well within hardware spec.

On Feb 20, 2012, at 12:23 PM, Florian Schlembach wrote:
> we encountered a very similar issue and spent already a lot of time on that. 
> Configuration is the same except we are using an USRP2 though.
> We also did not receive anything at the RX side although the spectrum looked 
> quite good as we have checked that via usrp_fft.py.
> Ultimately, we found out that it was about the sampling rate. We were able to 
> receive something with a sampling rate (respective bandwidth ) from 1.5 
> MSamples/s, better 2 MS/s.
> We are assuming that this might be a decimation rate issue. We facing the 
> same problems with both a XCVR2450 and an RFX daugherboards. We checked 
> whether lower sampling rates are working with both daughterboards (with 
> another SDR implementation called Iris, also uses UHD) and they do! Thus, 
> this might eventually be a problem of GNU Radio.
> Could you check whether you are receiving something with higher sampling 
> rates? Just invoke your benchmark scripts with a higher bandwidth, e.g. -W 2M

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