> Hi Marc, > > demod: > both can be implemented in GNU Radio. Correlation of course is more > computationally intensive; I haven't thought about 4-FSK so far, but I > guess for quadrature demodulation, you'd need multiple branches to > distinguish the possible symbols. There are other approaches (e.g. polar > discrimination by calculating the phase difference between two samples > etc. There's an exciting new polyphase filterbank channelizer > implementation, also, which you could abuse to do FSK, too). >
Hi Marcus! Thanks for your detailed reply. To what I have studied yet polar discriminator will be affected more by the amplitude variations during transmission. Currently I do not know how to cater these variations, I am reading literature on it. 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. > Generally, if you are synchronized, you might even just estimate the > frequency of a symbol. In the easiest case, this might break down to an > abs(FFT_4) (which, by the way, is mathematically *very* similar to > correlating with known frequencies). > > But since you asked about low-SNR situations: Employ a lot of > oversampling, if possible. Processing gain when correlating/averaging > should be proportional to oversampling factor. Oversampling, giving you > more bandwidth, would also be beneficial if you used a filterbank > approach, since it reduces the need for frequency synchronization in a > practical system, if your FSK shift is sufficiently larger than your > frequency offset. > > 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. > All in all, the question which demodulator architecture to use is an > academic/research one and not related to the implementation in GNU Radio > modulation. This is a highly interesting field and I can but encourage > you to read a lot of books [1] and papers on this. > > Thanks for the link, nice stuff. > modulation: > > 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? Marc > Greetings, > Marcus > > [1] a nice list: > http://gnuradio.org/redmine/projects/gnuradio/wiki/SuggestedReading > > On 18.06.2014 13:17, Marc Miller wrote: > > Hi, I am new to gnuradio, I want to develop 4FSK system. transmitter and > > receiver. Is there any existing 4FSK transceiver system in gnuradio ? > Also, > > which one of the following two is the best/optimum 4FSK demodulator for > low > > SNR? > > > > a. Quadrature demodulation > > b. Correlator demdulation (autocorrelate tones-->energy detect->pick > > largest) > > > > Which one of the above can be implemented in gnuradio ? > > > > > > Marc > > >
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