Re: [Discuss-gnuradio] Issues estimating the BER of a 16-QAM modulation (and of M-QAM of course)

[Ben Reynwar]

Hi Arturo,

The constellation object scales the provided constellation points so
that the average magnitude of the points is 1.0.   This is causing the
difference between the two sets of points that you noticed.

To avoid the abstract class error, use
"digital.digital_constellation(rotated_const.base(), [], 1, 1)", where
the "base" method is casting the object to the base constellation
class.

Ben

On Tue, Apr 9, 2013 at 3:19 PM, Arturo Rinaldi <address@hidden> wrote:
> Il 06/04/13 15:33, Tom Rondeau ha scritto:
>
> On Fri, Apr 5, 2013 at 1:41 PM, Arturo Rinaldi <address@hidden> wrote:
>
> Hi folks, i have bumped into an error while updating my thesis research to
> the latest version of gnuradio.
> It is a simple tool to estimate the BER of the digital modulation. I had to
> change some lines of code due to the
> fact that now the block constellation_decoder_cb accepts as input a
> "constellation object" and not a complex
> point tuple (i.e. [1+1j,1-1j,-1-1j,-1+1j] and so on) so i modified my code
> in the following way (only the
> deconding part). By asking to the gnuradio mailing list i got the following
> advice :
>
> rot = (3.0/2.0)*sqrt((8.0/5.0)*bit_energy)
>
> rotated_const = map(lambda pt: pt * rot,
> qam.make_nondifferential_constellation(16,gray_coded=True)
> constell = digital.constellation_calcdist(rotated_const,[],1,1).base()   #
> (1) to get the base points and constellation object
> self.slicer = digital.constellation_decoder_cb(constell)
>
> # if self._gray_code:
> #         self.symbol_mapper = gr.map_bb(qam.gray_to_binary[arity])
> #     else:
> #         self.symbol_mapper = gr.map_bb(qam.ungray_to_binary[arity])
>
>
> # unpack the k bit vector into a stream of bits
>
> self.unpack = gr.unpack_k_bits_bb(self.bits_per_symbol())  # (2)
>
> self.connect(self,self.slicer, self.unpack,self)
>
>
> which works fine for M-PSK modulation but since QAM is a mix of both phase
> and amplitude modulation i can't
> recover the exact position of the constellation points. I think the error
> stays in a wrong calculation of the
> euclidean distance. Any suggestions ? i really need help or otherwise the
> functionality of my tool will be
> dramatically reduced ! ! !
>
> Thanks in advance,
>
>                       Arturo
>
> P.S. : The modulator block is shaped in the "old way", by just only passing
> as parameter the constellation
> points to the block
>
> self.chunks2symbols = gr.chunks_to_symbols_bc(rotated_const)
>
> I also use (2) because of my way to compare original bit by decoded bit and
> then calculating the BER. However
> the main issue is the wrong decoding of the constellation points by (1)
>
> Arturo,
>
> Take a look at the constellation_decoder. This uses the constellation
> objects and handles the symbol slicing for you. The slicers are
> defined in constellation.{cc,h}, and if you come up with a
> smarter/cheaper way of slicing, you can add it here. I think this
> might help simplify things for you.
>
> Tom
>
>
> I still can't figure out which is the correct slicer to use. I can't use
> digital.constellation_calcdist because it works only constellations with  a
> small number of points (and for M-PSK is fine). I usually build the qam-16
> constellation with the old source code of gnuradio (the 3.3.0 one, I really
> prefer it......) and when using digital.constellation_rect().points() as
> slicer i get different different points decoded instead of the ones i
> originally sent, this is the code :
>
> arity = 16
>
> rot = (3.0/2.0)*sqrt((8.0/5.0)*1.0)   # base point distance. See "Simon
> Haykin - Communication Sytems"
>
> side = int(sqrt(arity))
> width = 2.0/(side-1)
> #
> #
> #
> rotated_const = map(lambda pt: pt * rot, qam.constellation[arity])   # from
> 3.3.0 qam.py source
> #
> decoded_points =
> digital.constellation_rect(rotated_const,[],4,side,side,width,width).points()
> #
> print "ORIGINAL POINTS\n"
> print rotated_const
> print " "
> print "DECODED POINTS\n"
> print decoded_points
>
> returning :
>
> ORIGINAL POINTS
>
> [(-0.6324555320336758-0.6324555320336758j),
> (-0.6324555320336758-1.8973665961010275j),
> (-1.8973665961010275-0.6324555320336758j),
> (-1.8973665961010275-1.8973665961010275j),
> (-0.6324555320336758+0.6324555320336758j),
> (-0.6324555320336758+1.8973665961010275j),
> (-1.8973665961010275+0.6324555320336758j),
> (-1.8973665961010275+1.8973665961010275j),
> (0.6324555320336758-0.6324555320336758j),
> (0.6324555320336758-1.8973665961010275j),
> (1.8973665961010275-0.6324555320336758j),
> (1.8973665961010275-1.8973665961010275j),
> (0.6324555320336758+0.6324555320336758j),
> (0.6324555320336758+1.8973665961010275j),
> (1.8973665961010275+0.6324555320336758j),
> (1.8973665961010275+1.8973665961010275j)]
>
> DECODED POINTS
>
> ((-0.33385056257247925-0.33385056257247925j),
> (-0.33385056257247925-1.0015517473220825j),
> (-1.0015517473220825-0.33385056257247925j),
> (-1.0015517473220825-1.0015517473220825j),
> (-0.33385056257247925+0.33385056257247925j),
> (-0.33385056257247925+1.0015517473220825j),
> (-1.0015517473220825+0.33385056257247925j),
> (-1.0015517473220825+1.0015517473220825j),
> (0.33385056257247925-0.33385056257247925j),
> (0.33385056257247925-1.0015517473220825j),
> (1.0015517473220825-0.33385056257247925j),
> (1.0015517473220825-1.0015517473220825j),
> (0.33385056257247925+0.33385056257247925j),
> (0.33385056257247925+1.0015517473220825j),
> (1.0015517473220825+0.33385056257247925j),
> (1.0015517473220825+1.0015517473220825j))
>
> This issue, generally speaking, also breaks the compatibility with my
> ASK-based modulation code. I really put an effort writing these source codes
> on my own and now all my work is almost wasted.....:(.
>
> A perfect solution would be to build a pure digital_constellation_sptr
> object and pass it to the decoder but when i try to do it with
>
> digital.digital_constellation(rotated_const, [], 1, 1)
>
> but i obviously get an abstract class error. Do you mind to insert the old
> constellation_decoder_cb module as a deprecated one in the next release ? It
> would really solve my problems and my thesis research would be working again
> as it should !
>
> I hope you can help me out......thx in advance
>
> Kind Regards,
>
>                 Arturo
>
> P.S. : Would it be possible to get the source code of the old
> constellation_decoder_cb from the 3.3.0 tarball and build as a new block
> with gr-how-to-make-block by modyfing the *.i, *.cc and *.h files ?
>
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