Sample Question Paper for 9210-118
Graduate Diploma in Engineering
Communication systems
Duration: three hours
You should have the
following for this examination
• one answer book
• non-programmable calculator
• pen, pencil, ruler
General instructions
• This paper consists of nine questions
• Answer any five questions.
© The City and Guilds of London Institute
1
a
Define noise in a communication system?
(1 mark)
b
State the relationship between average noise power, temperature and
bandwidth?
(1 mark)
c
Define Noise Figure.
(1 mark)
d
The input Signal to Noise ratio (S/N) of a system is 20dB. The noise figure is
3dB. What is the output S/N? (You may assume the noise temperature of the
system source is 290 K.)
e
Write an equation for the overall noise figure of the system which is shown
in Figure Q1.
F1
G1
F2
-
Noise figure of Amp
Gain of Amp
Noise figure of Mixer
G2
-
Gain of Mixer
(2 marks)
(1 mark)
Figure Q1
f
The values of subsystem noise figures and gains are as follows:
F1 = 1.3dB,
F2=6.1dB,
G1=20dB
(4 marks)
G2=10dB
If the Amp input S/N is 30dB, find the S/N ratio at the mixer output in dB.
(You may assume the noise temperature of the amplifier source is 290 K.)
g
h
Describe the functions of the following layers in the OSI reference model and
state a protocol used in each layer.
i
Physical layer
(2 marks)
ii
Data link layer
(2 marks)
iii
Network layer.
(2 marks)
You have two layer-2 switches and two routers. It is required to install two
LANs at remote locations and connect them by using a private leased data
circuit.
i
Draw a diagram to show how the switches and routers can be
connected to implement this network.
ii
Suggest a suitable layer-2 protocol for the wide area network
segment implemented through the leased circuit.
2
(3 marks)
(1mark)
2
a
What is meant by the polarization of a wave?
(1 mark)
b
Describe the characteristics of an isotropic antenna.
(1 mark)
c
Define the Equivalent Isotropically Radiated Power (EIRP) of an antenna.
(2 marks)
d
i
A power of 100W is radiated from an isotropic antenna. Calculate
the power density at a distance of 5km from the antenna in free
space.
(2 marks)
ii
The power gain of an antenna is 9dB. A power of 100W is fed to
the antenna.
What is the power density at a distance of 5km from the antenna in
free space?
(2 marks)
Prove the following equation for a free space propagation between a
transmit and receive antenna pair.
(6 marks)
e
PR
PR
PT
GT
Ȝ
d
f
-
§ O ·
GT PT ¨
¸
© 4Sd ¹
2
Receive power
Transmit feed power
Transmit antenna gain
Wave length
Distance between the transmit and receive antennas
A satellite in the geostationary orbit is at a height of 36,000km above the
earth. The transmit signal from the satellite is at 4GHz, and the
transmitting antenna gain is 15dB. The earth station has a 45dB gain
antenna. (You may assume that the earth station is located directly
beneath the satellite.)
(6 marks)
If the transmitted power is 100W, what is the received power at the earth
station?
(The velocity of the electromagnetic waves in free space is 3x108ms-1)
3
See next page
3
a
State three advantages of digital signals when compared with analog
signals.
(3 marks)
b
What are the major parts of a Public Switched Telephone Network (PSTN)?
(1 mark)
c
A telephone call is activated between two subscribers who are connected
to two different exchanges in a Public Switched Telephone Network
(PSTN). Draw and label the path between the two subscribers.
(3 marks)
d
An Ethernet LAN has 1km length end-to-end, and its propagation velocity
is 2 x 108 ms-1. The bit rate is 100Mbps. A frame consists of 1800 data bits
and 200 overhead bits. This frame is sent from one end to the other. Find
the following.
e
f
i
Propagation delay
(1mark)
ii
Frame Transmission delay
(1mark)
iii
Maximum data throughput of the system
(3 marks)
An image is 800 pixels with 3 bytes / pixel. It is required to send this image
from a client to a server through an Ethernet network running TCP/IP. The
image can be considered to be application layer data. The IP and TCP
headers are 20 bytes each. The Ethernet frame has 21 header bytes and 4
trail bytes.
Note : An Ethernet frame can carry a maximum of 1500 bytes of payload
data.
i
How many Ethernet frames are required to send this data?
ii
Draw the Ethernet frames. You should include in your drawing, the
TCP header and the IP header in the Ethernet frame as well as the
number of bytes in each part of the frame.
(3 marks)
iii
Calculate the efficiency of the transmission (ratio of useful bytes to
total bytes).
(2 marks)
State two advantages of radio transmission compared to copper cable
transmission.
4
(1 mark)
(2 marks)
4
a
Calculate the loss limited total link length of an optical fibre, which can be
operated without repeaters. The system specifications are given below.
Output power of the optical transmitter
Connector loss at the transmitter
Attenuation of the optical fibre
Average splice loss
Fibre length
Connector loss at receiver
Design margin, including dispersion allowance
Required optical carrier power at receiver
b
2 mW
2dB
0.5dB/km
0.15dB
4 .0km
0.9 dB
6.0dB
2W
The V- number of an optical fibre is denoted by
V
(8 marks)
(6 marks)
§d ·
¸¸ u Numerical Aperture
© O0 ¹
S ¨¨
d = Core diameter of the fibre
Ȝ0 = Median light wave length
The core diameter of a fibre is 50m, and is used with a source having a
median wavelength of 0.80m. The Numerical Aperture is 0.345. Find the
fibre’s V-number and how many modes it will support.
c
i
Briefly describe three types of losses in optical fibres.
(3 marks)
ii
Name and briefly describe two types of light sources.
(1 mark)
iii
Draw the attenuation characteristics of an optical fibre and suggest
two suitable operating wave lengths.
5
(2 marks)
See next page
5
a
The real signals g(t) and x(t) are orthogonal over the interval [t1 , t2] if

t2
t1
(4 marks)
g (t ) x(t ) dt = 0
By using the above definition, show that sin θ and cosθ are orthogonal.
b
(4 marks)
Show that
If and only if n=m,

+T
−T
sin(nωt ) sin(mωt )dt ≠ 0
where T is the period of a sinusoid of frequency ω.
c
Four carriers are BPSK modulated with data at a rate of 2B bits/s. They
can be transmitted via frequency division multiplexing (FDM) or
orthogonal frequency division multiplexing (OFDM). For frequency
division multiplexing, the carriers are bandlimited as illustrated in Figure
Q5. fc is the carrier frequency.
Amplitude A (f)
fc
Frequency (f)
Bandwidth (2B)
Figure Q5
d
i
Compare the effect of Spectral Efficiency in Orthogonal Frequency
Division Multiplexing (OFDM) with that of Frequency Division
Multiplexing (FDM) by illustrating the four carriers on the frequency
axis for both multiplexing techniques. Assume no guard band is
required for FDM.
(4 marks)
ii
Calculate the spectrum saving of OFDM as a percentage compared
to FDM.
(3 marks)
Explain using illustrative sketches why OFDM is more suitable for
frequency selective environments than single carrier systems.
(5 marks)
6
6
It is required to design a Pulse Code Modulator with the following specifications
in order to convert an analog voice signal to digital signal.
Operational Spectrum
Output code size
Peak-Peak Voice input
= 0.3 – 3.4 kHz
= 8 bits
= 2000 mV
a
State Nyquist’s criterion for sampling analogue signals.
b
Draw the spectrum of the signal if the signal is sampled at the following
rates. Also indicate whether each rate is suitable for sampling. Justify your
answer.
i
6kHz
(3 marks)
ii
6.8kHz
(3 marks)
iii
8kHz
Find the voltage difference between two quantization levels, if linear
quantization is used.
(3 marks)
d
If most of the signal components are below 6mV what will be the effect of
this quantization? Suggest one method to minimize this problem.
(2 marks)
e
Calculate the output bit rate if the sampling rate is 8 kHz.
f
Calculate the output bit rate if the sampling rate is 16 kHz and the number
of quantization levels is changed to 1024.
(3 marks)
g
Draw the main blocks of a PCM system and describe the functionality of
each block.
(3marks)
a
What is line coding and why is it needed?
(3 marks)
b
State four properties of a line code.
(3 marks)
c
Draw the line codes of Manchester, AMI and HDB3 for the bit stream
11000000100000111000010. The first bit is the left most bit.
(3 marks)
d
Show that A cos( 2Sft M ) may be expressed as
(5 marks)
c
7
( 1 mark)
(1 mark)
(1 mark)
I cos( 2Sft ) Q sin( 2Sft )
and hence describe the Constellation Diagram of digital modulation
schemes and its usage.
e
Draw a block diagram of a Quadrature Phase Shift Keying (QPSK)
modulator, and show an example of mapping data bits to symbols on the
constellation diagram.
7
(6 marks)
See next page
8
There is a discrete memoryless information source producing independent
messages with following probabilities at a symbol rate of 2000 symbols/sec.
Probability
0.11
0.23
0.47
0.19
Message
M1
M2
M3
M4
Table Q8
9
a
Calculate the self information contained in each message element.
(2 marks)
b
Calculate the self information for the occurrence of pairs of messages
(M1, M2),(M2,M3),(M3,M4) and (M4,M1).
(4 marks)
c
Calculate the entropy H(x) for the information Source.
(4 marks)
d
Calculate the information rate of the source.
(3 marks)
e
Generate a Huffman code for the above message source using the Code
Tree.
(6 marks)
f
Find the average codeword length.
a
Briefly explain the principles behind the following types of error correcting
codes;
(1 mark)
i
Block code
(2 marks)
ii
Convolutional code.
(2 marks)
b
Define parity check bits(q), codeword length(n), message length(k), and
the Hamming distance in relation to Hamming codes.
c
Consider a (7,4) block code with matrix
P
§1
¨
¨1
¨1
¨
¨0
©
0
1
1
1
(4 marks)
1·
¸
1¸
0¸
¸
1 ¸¹
i
Find the check bits for all 16 code-words and show that the
Hamming distance is 3 for the above code.
(6 marks)
ii
Calculate the Syndromes for single error vectors
E1 >1 0 0 0 0 0 0@ and E2 >0 1 0
(6 marks)
8
0 0 0 0@ .