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Re: [Discuss-gnuradio] Best Resample Method For Resampling LTE Data

From: Tom Rondeau
Subject: Re: [Discuss-gnuradio] Best Resample Method For Resampling LTE Data
Date: Mon, 1 Dec 2014 09:44:03 -0500

On Mon, Dec 1, 2014 at 3:01 AM, Tom Tsou <address@hidden> wrote:
Hi Paul,

On Sun, Nov 30, 2014 at 6:52 PM, Paul Creaser <address@hidden> wrote:
> I'm currently using the Rational Resampler in GUNURADIO to resample LTE
> data, for example from 30.72MHz to 15.36MHz and 50MHz to 15.36MHz. I`m able
> to decode the data, so every thing seems to be well.
> I chose the Rational Resampler, simply because it is easy to use. However I
> noticed, GNURADIO has a number of resample modules, Fractional Resampler,
> Polyphase Arbitrary Resampler etc... I'm guessing each resampler has it`s
> strengths and weaknesses. Perhaps one is better for audio, another is better
> for LTE data etc..

In your specific use case, rational/fractional and arbitrary
resamplers are mathematically equivalent since the 'arbitrary' portion
doesn't apply. For example, rather than rational 2-to-1 downsampling
in your example, an arbitrary rate conversion would handle something
like 2-to-Pi. A more realistic use case for arbitrary resampling is
symbol timing loops where the output sample rate is not fixed, but
continuously drifting over time.

That said, despite similarities in math, there may be implementation
differences between the blocks that you mention. So one or another
might be newer, better optimized, or just easier to use. Tom Rondeau
can more easily explain those differences if they exist.


I have very little to add to what Tom and Dan have already said. I tend to use the PFB arbitrary resampler myself because it's so easy to just plug in a number and go. A couple of things on the computational power, though:

1. The rational resampler should be the cheapest when you can find the right rational ratio for your rate change needs. This will still depend on the filter taps you use in it, however, which is the size of the prototype filter (the one you enter) divided by the interpolation rate.

2. The default taps for the PFB resampler will be a more expensive filter than the fractional resampler. The latter uses 8 taps at a time in the filter. You can tailor the size of the filter per arm (nfilts) to reduce the overhead of the filter itself.

3. The PFB resampler will produce nearly the same output as the rational resampler when the resampling rate of the PFB resampler is the same as the D/I ratio of the rational resampler. The PFB will still be slightly more expensive, even if you make the number of taps per arm (len(taps)/nfilts for the PFB and len(taps)/I for the RR). The PFB computes two filter outputs for every sample output, which is not needed when there is no arbitrary part required -- but it won't know that and so does the computation, anyways.

But as I said, like Dan, I just use the PFB resampler because it's so easy and I tend not to be that concerned with the computational issues. If I had to squeeze a bit more out of the overall flowgraph, I'd first tailor the resampler's filter to shorten it, or I'd work to see if I could make the RR work for me.


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