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Automatic Control (AC)
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Communicates & Signal Process (CS)
  
Question 3: Optimization
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Question 2: Signal Processing
 
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Revision as of 15:51, 1 February 2019


ECE Ph.D. Qualifying Exam

Communicates & Signal Process (CS)

Question 2: Signal Processing

August 2017




Problem 1. [50 pts]
Equation 1 below is the formula for reconstructing the DTFT, $ X(\omega) $, from $ N $ equi-spaced samples of the DTFT over $ 0 \leq \omega \leq 2\pi $. $ X_{N}(k) = X(\frac{2\pi k}{N},k=0,1,...,N-1) $ is the N-pt DFT of x[n], which corresponds to N equi-spaced samples of the DTFT of x[n] over $ 0 \leq \omega \leq 2\pi $.

$ X_{r}(\omega)=\sum_{k=0}^{N-1} X_{N}(k) \frac{sin[\frac{N}{2}(\omega - \frac{2 \pi k}{N})]}{N sin[\frac{1}{2} (\omega -\frac{2 \pi k}{N})]} e^{-j\frac{N-1}{2}(\omega - \frac{2 \pi k}{N}) } $
,(1)

(a) Let x[n] be a discrete-time rectangular pulse of length $ L=12 $ as defined below:

$ x[n] = {-1,-1,-1,-1,1,1,1,1,1,1,1,1} $

(i) $ X_{N}(k) $ is computed as a 16-point DFT of x[n] and used in Eqn (1) with N=16. Write a close-form expression for resulting reconstructed spectrum $ X_{r}(\omega) $.
(ii)




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