You can post a comment to the Question reminding people about the question. Common comments in such cases include "Does anyone have any ideas" and "Bump".
You can, as Greg suggests, try the newsgroup, or another resource such as stackedgen
Your QPSK question is a communications theory question rather than a MATLAB question, so you should have posted it somewhere that discusses communications theory, or find some local resource (e.g., a university) where people know something about communications theory.
Beyond that, it becomes time to hire a consultant (or possibly tutor), if you can find one. Mathworks does offer a consultancy program.
As a general statement (not directed at you in particular): one of the best ways to get a good response is to ask better questions