ELEC202 SECOND SEMESTER EXAMINATION
SECOND SEMESTER EXAMINATION
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ELEC202
SPECIMEN PAPER
SECOND SEMESTER EXAMINATION
REPLACEMENTS 2019/20
COMMUNICATION SYSTEMS
INSTRUCTIONS TO CANDIDATES
The numbers in the right hand margin represent an approximate guide to the marks available
for that question (or part of a question). Total marks available are 100.
Answer ALL Questions.
This is an open-book test.
Copying any material from another source, or colluding with any other
person in the preparation and production of this work will be considered
suspected academic misconduct and will be dealt with according to
the University’s Academic Integrity Policy.
1. a) The Internet backbone refers to one of the principal data routes between
large, strategically interconnected networks and core routers on the
Internet. It is a very high-speed data transmission line (typically a fiber
optic trunk line) that provides networking facilities to Internet service
providers all around the world.
Using your knowledge of communications, explain why the Internet
backbone consists of cables rather than wireless connections. Use no more
than 200 words.
5
b) An audio message of bandwidth B Hz is sampled at Fs Hz prior to
digitisation, where Fs < 2B. In order to avoid aliasing effects, an anti-
aliasing filter of cut-off frequency Fc Hz can be used, either
i) before the sampler, or
ii) after the sampler
Derive expressions for the fraction of the message bandwidth that can be
successfully recovered following reconstruction in both cases above, and
comment on the effect this filtering will have on the nature of the recovered
audio. Illustrate your answer using spectral sketches.
5
c) Design suitable communication systems (including block diagrams,
modulation schemes or formats, etc) for each of the following scenarios.
Suggest suitable values for relevant parameters, as appropriate, such as
carrier frequency, bandwidth, sampling rate, bit depth, etc. Use no more
than 1 page.
i) An analogue talk radio station.
ii) An analogue television station.
iii) A fibre optic transmission system for telephone signals for a
20 storey apartment building.
iv) A home-made security camera system using a 4G smartphone.
10
d) Explain what is meant by ‘companding’, indicating where and why it is
used. Suggest a suitable transformation function for use in companding,
and illustrate its use with a suitable example. Do not exceed ½ a page.
5
Total
25
2. a) The output of an ultrasonic transducer is to be sampled at a rate of !
samples per second with a G Hz guard band between spectral replicas, and
subsequently transmitted via cable using analogue PAM. Design a suitable
communication system to achieve this, giving expressions for relevant
parameters, and illustrating your design with a suitable block diagram. Use
no more than ½ page.
5
b) Consider a time-division multiplex system with m signals, each
bandlimited to B Hz and quantized using n bits. If the transmission system
can support a maximum signal-to-noise ratio of R dB, derive an expression
for the number (m) of users that can simultaneously share the channel?
Illustrate your findings with a suitable example scenario.
10
c) Show that the NBFM expression for a single tone message can be written
as a scaled version of cos" − sin" ⋅ sin#
Sketch the above signal in the time and frequency domains, and comment
on the bandpass bandwidth.
5
d) Discuss 2 possible approaches for the demodulation of FM, illustrating and
describing in as much detail as possible how each approach works. Use no
more than 300 words and 1 page.
5
Total
25
3. a) Explain, in as much detail as possible, how the bandpass bandwidth of an
FM signal can be estimated. Use no more than ½ a page.
5
b) Explain the structure and function of a coherent detector, and discuss the
various ways in which coherent detectors can be used with different forms
of modulation, including as much mathematical detail as possible. Do not
exceed 1 page.
10
c) Explain what factors govern the maximum rate at which data can be
communicated error-free across a channel and discuss some of the
implications of this. Present your answer in as much detail as possible,
using sketches and diagrams to help illustrate your point where relevant,
and stating any assumptions made. Use no more than 200 words.
5
d) NASA’s New Horizon
space probe has provided us
with stunning high-
resolution 4096-level
grayscale images of, among
other things, the surface of
Pluto, a dwarf planet at the
outer edge of our Solar
System. The probe
encountered Pluto at a distance of 4.6 billion km from Earth, fom where it
transmitted a series of 312.5 kB images back to earth using binary PSK
modulation with a carrier frequency of 8.438 GHz (NB a kB is a thousand
bytes, and one byte is 8 bits). Due to the distance involved and the limited
power on board, the data rate was limited to approximately 1 kbit/sec.
Estimate the time it would take between transmission (from the probe) and
reception (on Earth) of a single such image, and hence estimate how many
images could be received during a month of constant transmission, stating
any assumptions made.
5
Total
4. a) For the binary bit stream segment {… 0, 0, 0, 1, 1, 0, 1, 1 …} sketch the
corresponding bandpass signals using the following modulation formats:
i) BFSK
ii) BPSK
iii) 2-ASK
iv) 4-ASK
5
b) A typical telephone line allows frequencies from 300 to 3300 Hz, and is to
be used for digital data transmission.
i) Find the maximum practical data rate that the line can support,
given a signal-to-noise ratio of 30 dB.
ii) Discuss the implications of this, using no more than 200 words.
10
c) How much channel bandwidth is required to frequency-division multiplex
5 user channels, each of bandwidth 8 kHz, with 1.5 kHz guard bands
between channels, using :
i) SSB
ii) DSB
iii) FM, with b = 8
5
d) An audio (speech) signal is to be sampled and quantised prior to live digital
transmission. The maximum data rate supported by the channel is 130 kbps,
and the available sampling rates and bit depths are as follows:
Sampling rates: 8, 11, 22, 44, 88 and 96 kHz
Bit depths: 2, 4, 8, 16, 24 and 32 bits
Select the digitisation settings that would result in the highest quality
reproduction of the message at the receiver, justifying your choice.