Some of my questions are not answered by any one. So after few hours or few days that question is not in the list of recent questions. What should I do if I want to get answers for those questions?
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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
Even better than just bumping your question is to edit your question with what you have found out since you asked the question. Presumably, if the question is still important to you, than you have thought about the question again. Maybe that new piece of insight you have had will help others help you.
After asking a question, it is recommended to care about the answers. Sometimes users ask questions for clarifications. If you forget to answer these questions or do not provide the necessary information, the users can get the impression, that you are not really interested in a solution. If somebody creates an answer and you neither accept it nor explain in the comment section, why this does not solve the problem, the impression of a missing interest will grow.
There are some magic keywords, which discourage others to spend time for helping. E.g. in the thread http://www.mathworks.com/matlabcentral/answers/18418-gaussian-mixture-model-maximum-likelihood you answered Walter: "I tried that file and it didn't work.", but you did not explain any details. Ignoring Walter's question "What happened when you tried that file?" is not a good idea in this forum.
Therefore I suggest to step through the list of your former questions, answer all questions for clarifications, accept matching answers or explain why the answer does not solve your problem. Then the contributors will see and feel, that you appreciate their voluntary assistence. We are all human and although this is a forum about a programming language, it is a social interaction which needs a lot of politeness as glue. Matlab Answers is not an answering machine.
Please be aware, that I have not read all your questions and that this answer is not thought for you specifically, but for all readers of this thread. I hope to encourage you to increase your participation in this community.
And just to finish up by stating the obvious, it's possible that either you haven't stated your problem precisely enough for anyone to figure out what you said, or quite simply no one knows. Since there seem to be only a few dozen people who answer questions here, it's quite possible that none of them have experience in the subject area you asked about, particularly ones where people ask things like " Does anyone have any experience doing triple bozon simulation using a modified Las Vegas technique on Chevy manifolds using 4D tensors, as explained in Douglas Adams paper......"? Probably no one, at least no one yet, has ever heard of that esoteric method.