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

[Tom Rondeau]

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