Re: Σχετ: Re: QAM constellation script

[Marcus Müller]

Hi George,

that flow graph doesn't frequency-hop :) It does something different, 
rather cool. Please correct me if I got anything wrong about your 
self-written blocks below:

What you do is:

"VCO generator":

Produces always the same 10240-long vector with pseudo-random values,


"Repeat":

Repeat that vector you get 100 times (but that makes no difference, you 
could have just taken 100 vectors from "VCO generator", they are all the 
same)
This does **not** repeat the elements within the vector 100 times


"VCO (complex)":

Takes every input values and uses it (scaled with a factor) as phase 
increment relative to the last value it has generated.
With the uncorrelated random [0,1] source at the input, and the scaling 
being such that the phase increment range is always [0;2pi], this just 
generates a pseudo-random point on the unit circle each output item


"Complex Conj":

Well, takes the complex conjugate of each item individually – still just 
points of uncorrelated random phase!


"Multiply":

You're multiplying the samples from your reception with a different 
sample each from a repeating, white pseudorandom noise:
This is spectral whitening :) But not frequency hopping.


Best regards,
Marcus

On 4/28/23 14:03, George Katsimaglis wrote:
> Hi,
>
> Thanks for the answers.
> I attach the Rx flowchart and grc of the frequency hopping. I have 
> successfully used it on QO100 satellite.
>
> George SV1BDS
>
>
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>     Στις Πέμ, 27 Απρ, 2023 στις 14:57, ο χρήστηςMarcus Müller
>     <mmueller@gnuradio.org> έγραψε:
>     Hi George,
>
>     > Also I have implemented 1.000.000 hops/sec frequency hopping at
>     QO100 satellite, spread
>     > over 1 MHz, using 10240 different frequencies.
>     > Is this project of general interest?
>
>     Well, that's impossible to say, but honestly: it probably is! And
>     also, you shouldn't care
>     too much :) It's cool in any case!
>     My advise is: Just put it out there.
>
>     But I do have signal theory questions:
>
>     We know that if a signal has a bandwidth of 1 MHz, we can
>     (complex) sample it and contain
>     all its signal content with a sampling rate of 1 MS/s.
>
>     If you're doing a million hops per second, how are you achieving a
>     bandwidth of only 1
>     MHz? That means that every hop only gets a single sample, and you
>     can't signify
>     "frequency" with just a single number.
>
>     So, I might have misunderstood you there, but it would seem what
>     you claim to have done is
>     mathematically not possible :(
>
>     > I create this script using Python to create QAM constellations
>     points.
>     > May be of general interest.
>
>     It's nice, but GNU Radio already comes with that!
>
>     from gnuradio import digital
>     constellation = digital.constellation_16qam()
>     points = constellation.points()
>
>     (and you can just use digital.constellation_16qam().points() in a
>     GRC block parameter, no
>     need to build a string!)
>
>     These are also power-normalized to 1.
>
>     If you don't want normalized (or different sizes of) QAM
>     constellation,
>
>     digital.qam.make_non_differential_constellation(M, gray_coded)
>
>     is your friend;
>
>     digital.qam.make_non_differential_constellation(4096, True)
>
>     makes a nice 4096-QAM, but it's average power isn't 1; you can fix
>     that
>
>     points = digital.qam.make_non_differential_constellation(4096, True)
>     average_pwr = sum(point**2 for point in points) / len(points)
>     print(f"Average power: {average_pwr}; normalization factor hence:
>     {average_pwr**(-1/2)}")
>     normalized_points = [ point * average_pwr**(-1/2) for point in
>     points ]
>
>     Similarly, since you're doing satellite communications, you might
>     be interested in PSKs,
>     an A-PSKs.
>
>     You can create a PSK using
>
>     digital.psk.psk_constellation(m=4, mod_code='gray', differential=True)
>
>     e.g.
>
>     digital.psk.psk_constellation(16, differential=False)
>
>
>     If you don't have GNU Radio but just python,
>
>     str([(i, j) for i in range(-n, n, 2) for j in range (-n, n, 2)])
>
>     does the same as your code, but might be a bit easier to read
>     (again, if you want to use
>     this in GRC, don't do the conversion to `str`; GRC accepts any
>     valid Python in its fields).
>
>     Best regards,
>     Marcus