Re: Constellation Modulator Block Output is incorrect

[Marcus Müller]

Hi Ahmad,

interesting; haven't tried it here, but I do get correctly that you set 
a samples per symbol of > 1, right?

Generally, it's not my favourite block in the world: while combining the 
constellation mapper and the pulse shaping in the same block does, in 
fact, make sense, the chosen abstraction is kind of detrimental to 
debugability. You really just a "Chunks to Symbols" (or use a "Multiply 
const, const=2" follow by "Add const, const=-1") followed by an "Root 
Raised Cosine Filter" with an interpolation == desired samples per 
symbol to achieve the same.

That has no bearing on whether we should fix bugs with the Constellation 
Modulator, though; we probably should, and we might maybe want to 
consider deprecating it, if it's not that useful, but that would warrant 
a wider discussion.

Best regards,
Marcus

On 2/20/23 10:08, Ahmad Oweis wrote:
> 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!