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Masters Programmes in
Communications
Broadband Technologies and
Components
17th November 2017
Closed Book Exam
10.00am – 12.30pm
_____________________________________________________________________
Guidelines:
This paper comprises 2 sections:
Section 1 contains 2 compulsory questions each worth 30%. It is advised that
you spend no longer than 1 hour and 30 minutes on this Section.
Section 2 contains 3 questions of which you must answer 2 questions only
Please answer each question in a separate answer book
The distribution of marks among parts of questions is indicated for guidance
_____________________________________________________________________
UCL DEPARTMENT OF ELECTRONIC &
ELECTRICAL ENGINEERING
Physical Constants
Velocity of light in a vacuum, c = 3 x 108 ms-1
Planck’s constant, h = 6.626 x 10-34 Js
Boltzmann’s Constant, k = 1.38 x 10-23 Joule/Kelvin
Electron Charge, e = 1.602176 x 10-19 C
0 C = 273 K
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TURN OVER
Section 1
This section has two compulsory questions. Each is worth 30% of the total mark.
Please answer each question in a separate answer book.
Question 1
Assess the maximum distance possible for transmission at 10 Gbit/s considering
the following potential elements.
Transmitter DFB laser
Laser Wavelength 1550 nm
Laser Linewidth 100 kHz
Peak transmitter output 10 mW
Fibre: Standard single mode with
Dispersion 17 ps/nm.km @ 1550 nm
Attenuation 0.2 dB/km
Dispersion compensated fibre
Dispersion -34 ps/nm.km @ 1550 nm
Attenuation 0.35 dB/km
Receiver Sensitivity -27 dBm (BER=10-9
at 1550 nm and
modulation of 10 Gbit/s)
State and justify any assumptions included in your calculations.
[100%]
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Question 2
a) Provide the definition of a frequency selective channel with respect to the signal
bandwidth.
[15%]
b) A channel has random complex coefficients in the time domain, where the real
and imaginary parts are statistically independent, and Gaussian distributed with
zero mean. What is the probability distribution that its amplitude belongs to
[15%]
c) Your future employer requires you to design a microwave Line of Sight (LoS)
link to transmit a 500 Mbit/s data stream using a 10 GHz carrier for a range of
3km with the transmit power limited to 10Watt. You have available a receive
antenna with a gain of 10 dB fed by a coaxial cable with coupling efficiency of
30% and a transmit antenna with a gain to be determined. The receiver front
end equipment has a noise figure of 5 dB and is operating at room temperature
(27 oC). The transmission bandwidth available is limited to 300 MHz.
i) Choose an appropriate modulation scheme from the ones shown in Fig.
1 below, that achieves the target data rate and minimises the gain of the
transmit antenna you will use. You should fully justify your selection by
appropriate calculations and clearly state any assumption you make.
[35%]
ii) Using free-space power budget calculations and Fig. 1 below, determine
the transmit antenna gain required for your selected modulation scheme,
for data transmission with a maximum error rate of 10-6
, assuming a
coaxial cable at the transmitter with a coupling efficiency of 30%.
[35%]
TURN OVER
Figure 1: Bit Error Rate versus SNR for different modulation schemes
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Section Two
This section contains 3 questions. Answer 2 questions only.
Please answer each question in a separate answer book.
Question 3
A repeater makes use of optically pumped Erbium Doped Fibre Amplifiers (EDFA) to
amplify the signals in each direction in a bidirectional single mode optical fibre link
operating at a single wavelength of 1.55 μm.
a) Draw a detailed diagram of the system showing all additional components
required in the correct place. Please justify your choices through careful
description of the components functions.
[45%]
b) Compare the optimum value of the decision threshold level in the receiver of a
single mode EDFA amplified system operating with a single 1.55 μm
wavelength with the decision threshold level in the receiver of the same
system when the EDFA is removed.
[20%]
c) In wavelength division multiplexed (WDM) long distance fibre
communication systems, what limits the bandwidth of the communications
channel
[10%]
d) State the definition of Coarse WDM and Dense WDM in a fibre link having
several Erbium Doped Fibre Amplifiers. In such a link, how many channels
are there in Coarse WDM and in Dense WDM if the laser for each wavelength
channel has a 0.1 nm spectral linewidth
[25%]
TURN OVER
Question 4
a) State the different noise sources in a digital system transmission and explain
how they affect the system
[30%]
b) Explain, briefly, the following:
i) Intersymbol interference (ISI) and zero ISI signals;
[10%]
ii) How an eye diagram is constructed and its uses in assessing the
performance of digital systems;
[15%]
iii) Line coding and its applications.
[15%]
c) Using a diagram or otherwise, demonstrate how the Q factor could be derived
from an eye diagram
[30%]
END OF PAPER
Question 5
a) Name the two essential parts of a laser and describe different types of each of
these parts
[25%]
b) The full width half maximum of the spectral emission line of a laser is .
Derive the mathematical equation below
~

2
Where k is Boltzmann’s constant, h is Planck’s constant, c is the speed of light
in a vacuum, T is the absolute temperature and is the wavelength of light in a
vacuum.
[25%]
c) Sketch the basic structure of a PIN photodiode and explain its principle of
operation.
[20%]
d) Define briefly the following terms:
i) The unit of dBΩ in the context of defining the gain of an optical
receiver;
[5%]
ii) Quantum efficiency of a photodiode and the range of values this
quantity may take.
[5%]
e) Describe the three different configurations commonly used in the design of
optical receiver circuitry. Compare the basic characteristics of these topologies.
[20%]