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CORK INSTITUTE OF TECHNOLOGY INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2011/
Module Title: Digital Communications
Module Code: ELTR
School: Electrical & Electronic Engineering
Programme Title: Bachelor of Engineering in Electronic Engineering – Year 4
Programme Code: EELES_8_Y
External Examiner(s): Dr A. Donnellan, Mr I. Kennedy Internal Examiner(s): Dr R.A. Guinee
Instructions: Answer any three questions.
Duration: 2 hours
Sitting: Winter 2011
Requirements for this examination:
Note to Candidates: Please check the Programme Title and the Module Title to ensure that you have received the correct examination. If in doubt please contact an Invigilator.
Q1 (a) Draw the block diagrams of a QPSK transmitter and receiver and explain their operation. Determine the baud rate and double-sided Nyquist bandwidth in terms of the incoming bit rate to the modulator. (13.33%)
(b) Using the dibit 10, determine the output of the QPSK modulator and use this to fill in the phasor and constellation diagrams. Determine the error phase margin using the phasor diagrams. Determine the error distance d of the QPSK system. (10%)
(c) The input data rate to a QPSK modulator is 10MBPS. Determine the output baud rate the Nyquist transmission bandwidth and information density. Compare these results with those for BPSK and 8-PSK systems. (10%)
Q2 (a) Draw the block diagram of an 8-PSK modulator. Determine the baud rate and double- sided Nyquist bandwidth in terms of the incoming bit rate to the modulator. (10%) (b) Using the (QIC) tribit symbols “000” and “001” from the incoming modulating binary data determine the output of the 8-PSK modulator and use this information to fill in the phasor and constellation diagrams. Determine the error distance and phase margin for this PSK scheme. (11.33%) (c) An 8-PSK modulator operating at 70 MHz has an input bit rate of 10MBPS. Determine the minimum bandwidth required to achieve an error probability of 10-7, using the PSK error performance characteristics in Fig.Q2, with a CNR of 10dB. (12%)
Q3 (a) Distinguish between the following code types, ( i ) distinct, ( ii ) uniquely decodable and ( iii ) instantaneously decodable by giving examples for the following source.
Source Alphabet Symbol Probability A 1/ B 1/ C 1/ D 1/ E 1/ F 1/
Determine the average codeword length and compare this with the source entropy in each of the above cases. (11.5%) (b) Show that for an n -symbol source X represented by an instantaneously decodable code of length L that H ( X ) L where H ( X ) is the source entropy. (11.5%) (c) A source produces five symbols S 1 , S 2 , S 3 , S 4 and S 5 with probabilities 0.1, 0.25, 0.15, 0.2 and 0.3. Construct an optimal code using the Huffman algorithm for this source. Determine the source entropy and average codeword length. (10.33%)
