Re: [Discuss-gnuradio] FSK demodulation

[Marc Miller]

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
>