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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 1 Examinations 2008/
Module Title: Digital Communications
Module Code: ELTR 8008
School: Electrical & Electronic Engineering
Programme Title: Bachelor of Engineering (Honours) in Electronic Systems Engineering - Award
Programme Code: EELES_8_Y
External Examiner(s): Mr. David Denieffe and Dr. Paula O Sullivan Internal Examiner(s): Dr. R. A. Guinee
Instructions: Answer any THREE questions.
Duration: 2 Hours
Sitting: Winter 2008
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 paper. If in doubt please contact an Invigilator.
Q1 (a) Show that Shannon’s Capacity limit for a transmission channel of bandwidth B and fed with signal power S in the presence of white noise power N, from first principles, is given by (^) C = B log 2 ( 1 + NS ) (11.33%)
(b) Examine the limitation of the transmission capacity C with increased channel bandwidth B. Sketch the variation of C with B and comment. (10%)
(c) A QPSK modulator, with an input data rate of 240kBPS, transmits a carrier with power 10-8^ W into an interstellar channel with a white noise power spectral density of 10 -14^ W/Hz. Determine the Shannon channel capacity for the medium used. Determine also the energy per bit to noise density ratio for this modulation scheme. (12%)
Q2 (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 ouput baud rate the Nyquist transmission bandwidth and information density. Compare these results with those for BPSK and 8-PSK systems. (10%)
Q3 (a) Draw the transmitter and receiver block diagram for a Frequency Hop Spread Spectrum (FH_SS) system and explain its operation. Discuss the advantages and disadvantages of this form of spread spectrum communications system. (11.33%) (b) A FH_SS system employs a fast hop with K=10 hops per message bit, M= frequencies, message bit rate 3.0kBPS and final RF multiplication =10. Determine ( i ) the RF signal bandwidth and ( ii ) processing gain in dB. (12%) (c) Determine the pseudonoise (pN) sequence g(t) from the code generator shown in Fig.Q3_(C). Explain from your laboratory work a simple procedure of deriving a fast frequency hop scheme for a message sequence 10110100101 showing the hop frequency value per information digit encoded. (10%)
Fig. Q3_(C)
g(t) 1 0 0
