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Cork Institute of Technology
Bachelor of Engineering (Honours) in Electronic Engineering – Award
National Framework of Qualifications – Level 8
January 2005
Communications Engineering
(Time: 3 Hours)
Instructions Answer FIVE questions, selecting THREE questions from Section A and TWO questions from Section. Use separate answer books for each Section.
Examiners: Mr. R. A. Guinee Dr. B. V. Donovan Prof. C. Burkley Mr. J. Ryan
Section A
Q1 (a) Draw the block diagram of an 8-QAM transmitter and explain its operation. What is the main advantage of this system compared to an 8-PSK system? (5 marks)
(b) Determine the 8-QAM modulator output for the following (QIC) tribits (000) and (001). Use this information to construct the phasor diagram complete with a table of amplitudes and phases. What is the phase margin of error for this system and compare it with that for an 8-PSK system? Determine the error distance. Use the amplitude levels ±0.25 and ±0. in the D/A converters when encoding. (9 marks)
(c) Using the graph in Fig.Q1 determine the minimum bandwidth necessary to achieve an error probability of 10 -7^ for an 8-QAM system operating at 10MBPS with a CNR of 10dB. (6 marks)
P ( e )
Eb /N 0 ( dB )
8-Le vel
Fig.Q1: Error rates of QAM modulation systems.
Q2 (a) Show that for a Linear Block Code the verification procedure used is CH T^ =0 where C is an ( n,k ) block code and H is the parity check matrix. (4 marks)
(b) The parity check bits of an (8,4) block code are generated by c 5 = d 1 + d 2 + d 4 , c 6 = d 1 + d 2 + d 3 c 7 = d 1 + d 3 + d 4 , c 8 = d 2 + d 3 + d 4 where d 1 , d 2 , d 3 and d 4 are the message bits. Determine:
(i) The generator and parity check matrices for this code. (ii) Determine all codewords associated with this code, its minimum weight and error detection and correction capabilities. (7 marks)
(c) If a (7,4) cyclic code has a generator polynomial g(x)=x 3 +x^2 +1 construct the encoder circuit. Determine the code polynomial for the message d(x)=x^3 +x+1 in systematic form. Is the received code polynomial V(x)=1+x+x 2 +x^4 a valid code and if not determine its syndrome. (9 marks)
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/
Determine the average codeword length and compare this with the source entropy in each of the above cases. (7 marks)
(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. (7 marks)
(c) A source produces four symbols A, B, C and D with probabilities 0.4, 0.3, 0.2 and 0.1. Construct an optimal code using the Huffman algorithm for source coded separately. Determine the source entropy and average codeword length. (6 marks)
Q6. In the case of a pulsed marine radar operating at microwave frequencies, show:
(a) how damage to the sensitive receiver, by the high powered transmitted pulse, is avoided, when a common antenna is used for both transmission and reception, (3 marks)
(b) and how the short high-voltage pulses to drive the magnetron can be generated. (3 marks)
What factors in the radar system dictate (a) the transmitted pulse length and (b) the pulse repetition rate? 2 x (2 marks)
A marine radar transmitting at 8GHz and using a common antenna with a gain of 44dB, for transmission and reception, can just discern a small boat of effective target area of 10 m^2 at a range of 15km, without enhancement.
The receiver has a bandwidth of 50MHz and an overall noise figure of 4dB.
Derive the relevant equations from first principles , stating clearly any assumptions
made ….. (5 marks)
and determine the transmitted power. (5 marks)
k = Boltzmann’s constant = 1.38 x 10 -23^ J/K
Gain of Dish/Antenna ≈ 4 π A 0 /λ^2 where A 0 is the ‘effective’ area of the dish.
Q7. Show how Dopplar frequency shift may be used to measure the radial velocity of a moving
target and draw a block diagram of such a radar system showing how an appropriate ‘beat’ frequency could be generated and detected. (4 marks)
The carrier frequency (f (^) o ) of a CW-FM radar is triangle modulated between fo + f (^) d and
f (^) o - f (^) d , at a frequency of fm. Draw the receiver frequency/time graph for an approaching and retreating target (relative to the transmitter graph). (4 marks)
Show by geometrical means or otherwise that the Range and Velocity of the target are as follows:
Range = cfrange /8f (^) d f (^) m and Velocity = cfvel /2f (^) o where frange and fvel are determined from the ascending and descending beat frequencies. 2 x (6 marks)
