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

[Arturo Rinaldi]

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)