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CORK INSTITUTE OF TECHNOLOGY
INSTITIÚID TEICNEOLAÍOCHTA CHORCAÍ
Semester 2 Examinations 2008/
Module Title: Communications Systems 2
Module Code: ELTR
School: Electrical and Electronic Engineering
Programme Title: B.Eng. in Electronic Engineering
Programme Code: EELXE_7_Y
External Examiner(s): Mr. D. Deneiffe Dr. P. O’Sullivan Internal Examiner(s): Dr. O. Gough
Instructions: Answer four Questions All questions carry equal marks.
Duration: 2 Hours
Sitting: Summer 2009
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 following terms in respect of Pulse Amplitude Modulation
(i) Natural Sampling. [4 marks] (ii) Flat-top Sampling. [3 marks] (iii) Reconstruction Filter. [3 marks] (iv) Aliasing. [4 marks]
(b) Plot the spectrum of the PAM waveform produced by sampling:
V m ( t )= Am cos 2 π fm t
assuming natural sampling with a rectangular pulse train, f (^) m = 250 Hz, sampling period TS = 1 ms and pulse duration 0.25 ms. [7 marks]
What is the cutoff frequency required for the reconstruction filter? [4 marks]
Q2. (a) Draw the block schematic of a simple linear PCM modulator and discuss its operation. Explain the quantization mechanism employed with appropriate sketches and its effect on recovered signal quality. [10 marks]
(b) Show that the quantization noise power for a linear PCM system is given by 12
∆^2
Nq = [10 marks]
(c) A PCM system is required for an audio application. If the audio signal is to be band limited to 12 kHz and the dynamic range must exceed 56 dB determine the resulting data rate for a linear converter. [5 marks]
Q6. (a) Draw block diagrams for a QPSK transmitter and receiver and explain their operation. [12 marks] (b) Using the input pair I=1, Q=1, determine the output of the QPSK modulator and use this to fill in the constellation diagram. Indicate the error phase margin using the diagram. [7 marks] (c) A cable TV data network uses a spare transmission channel of bandwidth of 8 MHz. Determine the maximum data rate that can be achieved using QPSK and 16-QAM modulators assuming raised cosine filtering with roll-off factor of 0.4. [6 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 N b
SQNR M M
Slope overload onset: f s
∆ ≥^2 π fA
Delta Modulator SQNR f B
f N
S (^) s q^2
3 8 2
π
Fourier coefficients (spectral amplitudes) of rectangular pulse train.
2 sin 0
0
n
n
T a V n
