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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2011/
Module Title: Communications Systems 2
Module Code: ELTR
School: School of Mechanical, Electrical and Process Engineering
Programme Title: B.Eng. in Electronic Engineering B.Eng. (Hons) in Electronic Systems Engineering
Programme Code: CR_EELXE_7_Y CR_EELES_8_Y
External Examiner(s): Dr. A. Donnellan Mr. I. Kennedy
Internal Examiner(s): Dr. O. Gough
Instructions: Answer Four Questions
Duration: 2 Hours
Sitting: Autumn 2012
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)Explain the difference between natural and flat top sampling using time and frequency diagrams. Are there any forms of distortion present and if so explain? [12 marks]
(b) A signal
Vm ( t ) 1. 5 cos 0 t 3 cos 2. 5 0 t
is sampled by a pulse train represented by the following series,
0
() cos n
St an n st
where f 0 is 5kHz.
(i) Determine the minimum sampling rate for the given signal. [2 marks] (ii) Show that the modulated signal consists of multiple side-frequency pairs. [4 marks] (iii) Draw the amplitude spectrum of the sampled signal when the sample rate is twice the minimum rate. [4 marks] (iv) What is the cutoff frequency required for the data recovery filter in this case? [3 marks]
Total : 25 marks
Q4 (a) Explain the principle of Time-Division-Multiplexing (TDM) including the issues of synchronization and switching. How does statistical multiplexing overcome some of TDM’s disadvantages. [ 8 marks] (b) Outline the main features of the STS-1 Frame structure. [8 marks] (c) A PCM-TDM system is required to transmit 24 voice channels. If the input signals are band limited to 10 kHz with 8 bit samples, determine the resulting raw data rate in bits/second. If the signalling/framing overhead amounts to an additional 16 bits/frame, determine the new data rate and estimate the bandwidth requirement. What frame period and slot time is required? [9 marks] Total : 25 marks
Q5. (a)The data sequence ‘1001101011’ is to be transmitted. Sketch the resulting waveforms if the data is transmitted using the following line codes: (i) Unipolar NRZ (ii) Manchester Coding (iii) AMI-RZ [9 marks] (b) Comment on the use of each of the above codes, comment on transmission effects, bandwidth and synchronisation [10 marks]
(c) Encoding schemes such as 4B3T and 4B5B are often used in transmission systems. Explain the nature of one of these mapping schemes, the purpose of using the scheme and name one application of the scheme. [6 marks] Total : 25 marks
Q6. (a) Draw block diagrams for a 16-QAM transmitter and receiver and explain their operation. [12 marks] (b) Draw the constellation for the 16 QAM system, labelling the points with suitable data words, include the decision boundaries in your diagram. [7 marks] (c) A radio channel is implemented with a raised cosine response having a roll-off factor of 0.4 and absolute bandwidth of 200 kHz. If 32-QAM is used to transmit 8 bit samples, determine the maximum sample rate and the maximum data rate. [6 marks] Total : 25 marks
Useful Formulae for Communications Systems 2
Nyquist Sampling: fs =2B
SQNR for sinusoid ( ) 6. 02 1. 76
SQNRdB N b
SQNR M
SQNR for uniformly distributed signal SQNRdB Nb
SQNR M M
Slope overload onset: fs
^2 fA
Delta Modulator SQNR for sinusoid f B
f N
S (^) s q 2
3 8 2
Fourier coefficients (spectral amplitudes) of rectangular pulse train.
sin 2 0
0
n
n
T
a V n
