Discussion For Homework3

Question: HW3, Problem 4, Figure 3 (b), the values of the vectors s1,s2 and s3 are not clear. Can someone tell how to interpret those values? Like s1 = [0 1.5] or something else? similarly others.

Answer[Student]: I spoke with the professor regarding this question and these are the values he provided to me

(1)
\begin{align} {\bf s_1} = [ 0 \hspace{1 mm}, \sqrt2 ] \hspace{3 mm} {\bf s_2} = [ \frac{1}{\sqrt2} \hspace{1 mm}, \frac{1}{\sqrt2} ] \hspace{3 mm} {\bf s_3} = [ \frac{1}{2\sqrt2} \hspace{1 mm}, \frac{3}{2\sqrt2} ] \end{align}

Question
In problem 5, it is required to find the smallest TAW (delay) for no error. I think it should be the maximum TAW (delay) not the smallest one. Is that true?

Answer[Student]: I don't think so. Asking for the smallest value of Tau for which there will be an error means:
Give the smallest value of Tau for which the received signal is closer to any other symbol on they symbol map. This means that there is some small time delay for which the phase of the received message has changed so as to bring the phase of the received message closer to one of the other symbols. This should depend somehow on the carrier frequency.
I think this is a good way of looking at this problem.

Answer[Krishna]: To the student who asked the question, please read the question carefully. It asks for the smallest value of $\tau$ for which there will be an error. I didnt explicitly say this, but $\tau$ is positive.

Question
In problem 6, the question seems to correspond to pi/4-QPSK and not offset QPSK as defined in Proakis. The explanation for Offset QPSK seems to result in a maximum phase difference of pi/2 between symbols whereas in the question, we'll end up with a maximum phase difference of 135 degrees. Could you please clarify this?

Answer[Krishna] Like I mentioned in class, I made a mistake calling it offset QPSK. I should have really called it $\pi/4$ shifted QPSK.

page revision: 7, last edited: 22 Feb 2009 00:57