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Masters Programmes in
Communications
Broadband Technologies and
Components
16th November 2018
Closed Book Exam
10.00am – 12.30pm _____________________________________________________________________
Guidelines:
This paper comprises 2 sections:
Section 1 is 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.
Optical Design question
Question 1
Determine the different elements (type of fibres and fibre length, number of
amplifiers etc…) to build an efficient link operating over 200 km at 10 Gb/s.
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)
EDFA: Saturated power: 16 dBm
Gain: 30 dB
Noise Figure: 5 dB
State and justify any assumptions included in your calculations.
[100%]
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Question 2
a) State and briefly explain the two types of small-scale fading based on the
multipath delay spread.
[30%]
b) Design a microwave line of sight link of range of 5km and a desired data rate of
500 Mbit/s. The link operates on a 2 GHz carrier and you have available two
identical antennas with a gain of 6 dBi each, and each fed by a coaxial cable
with coupling efficiency of 50%. The receiver front end equipment has a noise
figure of 3 dB and is operating at room temperature (27 o
C). The transmission
bandwidth available is limited to 300 MHz.
i. Using appropriate calculations and assuming a double-sided pass-band
transmission, choose a modulation scheme from the ones listed in Fig. 1
below, that achieves the target data rate and maximises the energy
efficiency of your transmitter.

[35%]
ii. Calculate the transmit power in dBW required for your selected
modulation scheme, for data transmission with a maximum error rate of
10-6
, and using free-space power budget calculations and Fig. 1 below.
[35%]
Figure 2.1: Bit Error Rate versus SNR for different modulation schemes
TURN OVER
Section Two
This section contains 3 questions. Answer 2 questions only.
Please answer each question in a separate answer book.
Question 3
a) Draw a graph showing the typical gain and bandwidth of an erbium doped
optical fibre amplifier, EDFA. Take care to add numerical labels of typical
wavelength, bandwidth and gain.
[10%]
b) Briefly list 4 benefits of optical fibre amplifiers as opposed to semiconductor
optical amplifiers, SOAs.
[16%]
c) List 4 noise sources which exist in a receiver once an optically amplified
signal with Amplified Spontaneous Emission, ASE has been directly detected,
and write a mathematical expression for each type of noise in units of photons
squared per Hertz OR in units of receiver current.
[16%]
d) State which noise sources are dependent on the amplitude level of the original
signal entering the optical amplifier and which noise sources have a much
wider bandwidth than the original signal.
[16%]
e) Briefly explain the reason for the differences in the spectral bandwidths of the
different types of noise.
[6%]
f) Discuss the benefits of wavelength division multiplexing compared to time
division multiplexing, taking care to consider both the optical source and the
optical fibre.
[15%]
g) Draw a fully labelled schematic diagram using recognised symbols for an
optical wavelength routing add-drop multiplexer.
[18%]
h) How is an original incoming signal affected by passage through an add-drop
multiplexer when an additional 10 Gb/s signal at a different wavelength is
added
[3%]
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Question 4
a) Discuss the main reasons for the use of raise cosine filtering in an NRZ OOK
optical link. Comment on what roll-off factor you would use in that context.
[30%]
b) In a system with equal noise on 1 and 0 and equiprobable 1s and 0s, demonstrate
that considering a normally distributed noise we have the probability of error
as:
” ≈ 1
√2
,-./0
1
Hint: for a Normal distribution ( ) centred at +A and of widths we have
6
,
8
9
( ) ≈ 1
√2
:

,1
< = 0 0 [40%] c) Using a diagram or otherwise, demonstrate how the Q factor can be determined from an eye diagram. [30%] Question 5 a) Discuss the reasons in terms of output performance and electron behaviour, for biasing a laser well above its threshold point. In your discussion explain stimulated emission of radiation. Plot any graphs you need to help you in your discussion. [25%] b) Calculate the cavity length in microns of a semiconductor Fabry-Perot Laser having a wavelength in free space of 1.55 μm, made from a crystalline material with a refractive index of 3.4 if the strongest longitudinal mode has 999 half wavelengths within the cavity. Comment on whether you consider your answer to be reasonable. [25%] c) An optical receiver is driven by a 125 Mbit/s input signal of an average optical power of -30 dBm. The receiver is constructed from a pin photodiode with a responsivity of 0.8 A/W and a diode capacitance of 0.6 pF; a 1kΩ load resistor and a 6 GHz amplifier as in the block diagram below: Knowing that the amplifier has a very large input resistance (> 100 kΩ) and an
input capacitance of 0.5 pF, answer the following questions and show the details
of how you obtain your answers.:
i- The peak voltage output of the receiver.
[25%]
ii- Would this optical receiver front end require a post-amplification
equaliser Justify your answer by appropriate calculations.
[25%]
END OF PAPER
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