Re: [Discuss-gnuradio] DVB-S2 Physical Layer Framer: Why outputs a zero after each sample?

[Francisco Albani]

Thanks!

Why not moving the zero stuffing downstream and use a polyphase arbitrary resampler with RRC taps? I'm not familiar with performance comparison between PFAR and FFT FIR Filter.

2016-06-15 19:41 GMT-03:00 Ron Economos <address@hidden>:

The primary design decision was to use the FFT filter instead of the interpolating FIR filter for performance. The FFT filter is considerably faster, especially at the 100 taps used in the flow graph. However, (as far as I know) the FFT filter does not have an interpolation option, just decimation. So the 2X interpolation is done in the Physical Layer Framer block instead.

You can remove the zero stuffing from the Physical Layer Framer and use the Interpolating FIR Filter block instead of the FFT filter block for RRC filtering. If you try that, be sure to remove the factor of two in lines 535, 548 and 680 in the PL framer block.

Here's a link that explains why zero stuffing is the correct way to do interpolation.

http://www.dspguru.com/dsp/faqs/multirate/interpolation

Ron

On 06/15/2016 02:37 PM, Francisco Albani wrote:

Hi to all! Specially to Ron Economos! (and thanks for sharing your work)

I was trying to understand the example dvbs2_tx.grc and, after putting graphical sinks after many blocks, I noticed that the one called "Physical Frame Layer" outputs one complex zero after each constellation point (see attached screenshot). Moreover, the FFT Filter that gives the RRC shape is not instructed to interpolate to fit any samples_per_symbol requirement. Moremoreover, the sample rate is hardcoded to symbol_rate*2.

This makes me think this transmitter can only work for 2 samples per symbol. (I suppose one can resample to an arbitrary sps value if needed.)

The point of this message is to confirm my suspicion and to ask why this was a design decision. I hope to learn some insight.

Here is the code: https://github.com/gnuradio/gnuradio/blob/master/gr-dtv/lib/dvbs2/dvbs2_physical_cc_impl.cc#L684

Relevant lines: 80, 684, 704 and 727.

Bye and thanks!

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