On Thu, Jun 19, 2014 at 12:35 PM, Marcus Müller <marcus.muel...@ettus.com> wrote:
> Hi Marc, > > > Currently, I am trying to implement correlation receiver using FIR > filters. I am looking into polyphase filterbank channelizer to implement > correlation, but currently unable to do so because I currently do not > know how to assign taps for channelizer. > > I should have pointed you at Tom's article on PFB channelizers and > synthesizers [1]. > Thanks, nice article! I am going to test it soon. > > Oversampling at baseband ? I am currently sampling at 1Msps (usrp sink > > sampling rate) for a tone spacing of 125KHz. You mean after the USRP sink > > block I should upsample again and then do the correlation? Or you mean > > something else ? Please forgive me for asking too many question, I am > still > > learning. > don't worry; this list is meant for GNU Radio discussion, I think things > like exploring possibilities to do certain tasks in GNU Radio fit > nicely, especially because you are considering using something "cool and > new" like the PFBs in your approach. GNU Radio can always use more > people actually trying to implement stuff! > > So: Oversampling is what you do when getting more samples out of your > ADC than nyquist forces you to do. If you do 125k spacing 4FSK, then you > have a total bandwidth of 750kHz + X, so 1MS/s is "just enough"; anyway, > if you sample with let's say 3.125MS/s, than you'll have more samples. > Within that received bandwidth, you will have the same signal as before, > but now correlating against your known tone will give you a higher value > than with only 1MS/s, and because noise stays uncorrelated, this will > increase SNR. This is processing gain! > > ADC in N210 is sampling at 100Msps, so you are saying that I can get processing gain (better SNR) if I set the USRP sink sample rate higher than IMsps i.e usrp_sink.sample_rate(3.125e6) instead of usrp_sink.sample_rate(1e6) . Right ? Also I read [1] that if I oversample by 2 then I gain 3dB in SNR, if oversample by 4 I gain 6dB in SNR and so on, is that TRUE ? If this is true I can get a lot of SNR improvement because ADCs are operating at 100Msps. > >> ... encourage you to read a lot of books [1] and papers on this. > > > > Thanks for the link, nice stuff. > If you find something interesting, don't hesitate to add a new heading > like "FSK demodulator technology" and add the references there; I think > this list is something that could use broader usage :) > Sure! I 'll do that. > >> Assuming you don't want phase jumps between symbols, why not just use > >> the VCO block to generate frequencies out of your input values? > > Wow! modulation is easy using VCO. A question, why would someone want > phase > > jump between symbols? > In FSK systems, you'd usually want to avoid that, because phase jumps > lead to spectral components that are not at the frequency you shifted to. > With PSK, phase jumps is your only way to encode data, and with QAM > "half" of the information is encoded in phase. However, you usually see > a pulse shaper after these, because transmitting a real phase > discontinuity is somewhat physically impossible[2]. So these modulations > get the spectrum of a phase-jumping system, smoothed by the spectral > shape of the pulse shaping. > > Thanks for the detail. Also, I got FSK modulator working using the VCO :) Marc [1] http://www.dsprelated.com/showmessage/72731/1.php > Greetings, > Marcus > > [1] > > http://www.trondeau.com/examples/2014/1/23/pfb-channelizers-and-synthesizers.html > [2] Basically, imagine the baseband signal as a sequence of rectangles > in time; now, the fourier transform of a rectangle is a sin(x)/x, and > that has a support that doesn't fit into a limited bandwidth. >
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