联系我们： 手动添加方式: 微信>添加朋友>企业微信联系人>13262280223 或者 QQ: 1483266981

Masters Programmes in
Communications
Broadband Technologies and
Components
18th November 2016
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 question
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
NEXT PAGE
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
For a link distance of 200 km, derive the details of a full link for 1 Gbit/s
considering the following potential elements.
Transmitter DFB laser
Laser Wavelength 1550 nm
Laser Linewidth 2 MHz
Peak transmitter output 2 mW
Fibre: Standard single mode with
Dispersion 16 ps/nm.km @ 1550 nm
Attenuation 0.2 dB/km
Dispersion compensated fibre
Dispersion -38 ps/nm.km @ 1550 nm
Attenuation 0.35 dB/km
EDFA: Saturated power: 16 dBm
Gain: 30 dB
Noise Figure: 5 dB
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%]
NEXT PAGE
Question 2
a) Explain the following aspects of multipath propagation with a brief description
(two or three lines only) for each:
a. What is the mechanism in the communication channel that causes
multipath propagation
[10%]
b. What is the resulting effect of multipath propagation that causes signal
degradation, and why
[10%]
c. A multipath channel is described by the channel impulse response ( )
shown in Figure 1 below. Write the mathematical expression that
describes the channel impulse response.
[10%]
b) A 500 Mbit/s Line of Sight (LoS) link is being set up with a range of 2 km, a
carrier at 10 GHz, and a transmission bandwidth limited to 250 MHz. The
transmit and receive antennas are placed at the same height above sea level,
and each of them has a gain of 10 dB and each is fed by a coaxial cable with
coupling efficiency of 50%. The receiver is operating at room temperature (27
oC) and has a noise figure of 11 dB.
a. Which of the modulation schemes from the ones shown in Figure 2
below, achieves the target data rate and minimises the required
transmit power You should fully justify your selection by appropriate
calculations and clearly state any assumption you make.

[35
%]
Figure 1: Channel impulse response
TURN OVER
b. Using free-space power budget calculations, determine the transmit
power required for your selected modulation scheme, for data
transmission with a maximum error rate of 10-6
, given the BER
performances in Figure 2 below.
[35%]
Figure 2: Bit Error Rate versus SNR for different modulation schemes
NEXT PAGE
Section Two
This section contains 3 questions. Answer 2 questions only.
Please answer each question in a separate answer book.
Question 3
a) For an optical source, derive, showing and explaining all your steps, the
equation relating the spectral linewidth in wavelength units to the spectral
linewidth in frequency units from the equation
c f
Equation (1)
[10%]
b) Making use of the fact that the range of energies of the photons emitted from
an optical source is related to its junction temperature, T, by
E kT
Equation (2)
derive, showing and explaining all of your steps, two equations, one showing
the dependence of the spectral linewidth in frequency units and the other in
wavelength units. [28%]
c) Draw diagrams and use text to clearly the explain the difference, advantages
and limitations between a Distributed Bragg Reflector, DBR and a
Distributed Feedback, DFB laser. [34%]
d) In a long single mode optical transmission system operating at 1.55 micron
wavelength incorporating an erbium doped optical amplifier with a spectral
bandwidth of 5 THz, calculate how many wavelength channels can be
transmitted using
i.) Coarse WDM
ii.) Dense WDM
In both cases make sure you clearly state your assumptions [28%]
TURN OVER
Question 4
a) Explain the purpose of a raised cosine filter for NRZ optical signal transmission
and its impact on the bandwidth.
[30%]
b) Explain what an eye-diagram is and its uses in assessing the performance of
digital systems;
[20%]
c) For an unamplified optical system the received signal has rms noise values on
zeros and ones of 0 = 0.1 V and 1 = 0.25 V respectively with the mean
zero and one levels being = 0.01 V and = 1.01 V respectively.
i) Sketch the eye diagram and noise distribution at the received
[10%]
ii) Calculate the optimum decision threshold
[10%]
iii) Determine the extinction ratio
[10%]
iv) Determine the signal to noise ratio
[20%]
NEXT PAGE
Question 5
a) Describe the three different topologies commonly used in the design of optical
receiver circuitry. Compare the basic characteristics of these topologies.
[20%]
b) Define briefly the following terms:
i) The long wavelength cut-off of a photodetector
ii) Optical receiver sensitivity
[20%]
c) An optical receiver, operating at room temperature of 27o C, is constructed
from a pin photodiode with a responsivity of 0.7 A/W, and a 50Ω load resistor
of 50Ω, followed by a 15 GHz amplifier as in the block diagram below:
The input to the receiver is an equi-probable 10 Gbit/s optical binary. Derive
the receiver sensitivy for a BER of 10-9
.
[60%]
END OF PAPER
-5
Volts
Ground
R = 100Ω
Input
PIN
Voltage gain
= 40 dB