Constellation Modulator Block Output is incorrect

[Ahmad Oweis]

Hi everyone,

My understanding is that the constellation modulator block should output an RRC pulse in the real and imaginary domains according to the constellation points. The pulse amplitude can either be positive or negative. 

However, if you build a flow chart on GRC and input a vector of zeros to the block, and use non-differential BPSK (so that symbol 0 is mapped to constellation point -1+j0, and symbol 1 to point 1+j0), the output will be a constant DC value of -1 without any pulse. If the input is 1, the output will have a positive amplitude pulse.

This was reported as an issue on GitHub (https://github.com/gnuradio/gnuradio/issues/5449) but the answer was vague "It's the total I/Q signal that matters, since they are not transmitted or processed separately". This did not make a lot of sense to me, I still believe symbol zero should produce a pulse not a DC -1 signal.

Also, if you review issue (https://github.com/gnuradio/gnuradio/issues/4162), you will see that the one who posted it is probably having the same issue (go down to the last comment in the thread and compare the two graphs.)

What do you think? Is the constellation modulator behaving as expected?

Thanks!