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MSC Examination by course unit
Wednesday, 13 May 2015 10:00 am
ECS701P/U/D Communication Theory Duration: 2 hours 30 minutes
Answer ALL FOUR questions
Calculators are permitted in this examination. Please state on your answer book the name and
type of machine used.
Complete all rough workings in the answer book and cross through any work that is not to be
Possession of unauthorised material at any time when under examination conditions is an
assessment offence and can lead to expulsion from QMUL. Check now to ensure you do not have
any notes, mobile phones, smartwatches or unauthorised electronic devices on your person. If you
do, raise your hand and give them to an invigilator immediately. It is also an offence to have any
writing of any kind on your person, including on your body. If you are found to have hidden
unauthorised material elsewhere, including toilets and cloakrooms it will be treated as being found
in your possession. Unauthorised material found on your mobile phone or other electronic device
will be considered the same as being in possession of paper notes. A mobile phone that causes a
disruption in the exam is also an assessment offence.
Maged Elkashlan
Rob Donnan
© Queen Mary, University of London, 2015
Page 2 ECS701P/D/U (2015)
Question 1
a) Explain the concept of power spectral density and write its mathematical description.
[6 marks]
b) Sketch the block diagram of an FM demodulator and briefly explain the operation of each
element. Assuming the carrier channel is subject to white noise, sketch the Power
Spectral Density of the noise signal at the output of the following points in your block
(i) IF filter
(ii) Phase detector
(iii) Discriminator.
(iv) Post detection filter.
[10 marks]
c) For an FM receiver to operate correctly and achieve optimum detected signal-to-noise
performance a post detection filter is essential. Why is this?
[4 marks]
d) Explain how pre-emphasised FM overcomes the need for an ideal post detection filter.
[5 marks]
Page 3
Turn Over
ECS701P/D/U (2015)
Question 2
a) A single, square-pulse time-signal of unit amplitude, product modulates a cosine carrier of
frequency c  ; evaluate and plot
I. The spectra associated with this AM signal
II. The Power Spectral Density
[8 marks]
b) Show how demodulation of the AM signal in part a) of this Question is achieved by
synchronous detection
[6 marks]
c) Explain the significance of the term ‘synchronous detection’ and consequently the
phenomenon of ‘total fading’, using an appropriate mathematical description.
[6 marks]
d) Briefly outline in relation to Question 1 why AM is inappropriate for satellite communication
[5 marks]
Page 4 ECS701P/D/U (2015)
Question 3
a) Using signal-space representation, prove that the BER of M-ary PSK is given by
[8 marks]
where M = 2 is the modulation order and  is an integer.
b) Using signal-space representation, prove that the BER of M-ary ASK is given by
[8 marks]
where M = 2 is the modulation order and  is an integer.
i) Using signal-space representation, prove that the BER of M-ary QAM is given by
[6 marks]
where M = 2 is the modulation order and  is an integer.
ii) Sketch a comparison of BER versus Eb/N0 for 16-QAM and 64-QAM [3 marks]
  0
2 1
2 1 6 N
Q b

 
  1 0
2 1 6 N
Q b

 
 sin( / ) 0
Q 2 M N
Eb 
ECS701P/D/U (2015) Page 5
Question 4
i) Plot block diagram of transmitter and receiver of direct sequence spread spectrum. [6 marks]
ii) Plot the autocorrelation of a maximal linear spreading code of length N = Ts/Tc, where
Ts is the symbol duration and Tc is the chip duration. [6 marks]
b) A Rake receiver is a receiver that has several branches, with each branch synchronized to a
different multipath component. Plot the block diagram of a Rake receiver.
[6 marks]
c) We assume a multipath channel with one delayed component, as follows
Where and are the amplitudes of the multipath components, and is the delay.
Show that the inter-symbol interference (ISI) in direct sequence spread spectrum is equal to
where is the autocorrelation of the spreading code and is the th symbol.
[7 marks]
End of Paper

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