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ELEC 202

SECOND SEMESTER EXAMINATION

REPLACEMENTS  2020/21

COMMUNICATION SYSTEMS

1.    a)    A carrier signal of frequency f!   is frequency-modulated by a single-tone message signal of frequency fm   such that the largest frequency deviation is Δf.  Estimate the bandpass bandwidth. The values of fm ,  f!  and Δf are as provided in the table on page 3. 5

b)   The amplitude of the message is increased by a factor of p while its frequency is lowered by a factor of q. Estimate the bandwidth of the new modulated signal. The values of p and q are as provided in the table on page 3. 5

c)    Draw the block diagram of a zero-crossing FM detector, and discuss whether this could be  used  to  recover  the  baseband  message  from  a  DSB-SC  signal.  Use  appropriate diagrams, signal sketches and mathematical steps to support and illustrate your answer. 5

d)   Consider the following scenario:

You need to transmit a large amount of plain text documents from one very old computer to another, but there is no way to connect the two computers. Neither computer has any wired  or  wireless  connectivity  options.  Each  computer  has  an  integrated  keyboard, monitor, speaker and microphone, but no serial or parallel ports and no removable storage. You can write and execute software on the computers, but you cannot open or alter the hardware in any way.

You have access to an FM transmitter and receiver, and you have at your disposal all the equipment and components in the Department’s undergraduate laboratories.

Describe  how  you  would  go  about  achieving  this  task,  including  illustrations  and calculations in your design as appropriate. 10

Total

25

2.    a)    A  carrier  signal  of  frequency  f!  is  frequency-modulated  by  a  message  signal  of bandwidth fm such that the largest frequency deviation is Δf. Determine whether this is narrow-band  FM  (NBFM)  or  wide-band  FM  (WBFM)  and  estimate  the  bandpass bandwidth. 5

The values of fm,  f! and Δf are as provided in the table on page 3.

b)    Show that the NBFM expression for a single tone message can be written as a scaled version of

cos wct − β sin wct ⋅ sin wmt

Sketch the  above  signal  in  the  time  and  frequency  domains,  and  comment  on  the bandpass bandwidth. 5

c)    Draw the block diagram of a coherent DSB detector and discuss whether this could be used to recover the baseband message from a NBFM signal. Use appropriate diagrams, signal sketches and mathematical steps to support and illustrate your answer. 5

d)   Consider the following scenario:

You need to transmit a large image data from one very old computer to another, but there is no way to connect the two computers. Neither computer has any wired or wireless

connectivity options. Each computer has an integrated keyboard, monitor, speaker and microphone, but no serial or parallel ports and no removable storage. You can write and execute software on the computers, but you cannot open or alter the hardware in any way.

You have access to an AM radio transmitter and receiver, and you have at your disposal all the equipment and components in the Department’s undergraduate laboratories.

Describe  how  you  would  go  about  achieving  this  task,  including  illustrations  and calculations in your design as appropriate. 10

Total

25

Discuss what the information contained within Figure Q3 means, using communication principles to explain the shape of the curves and their relative positions. Comment on the effects of doubling the signal power on the BER for the different modulation formats.

b)   During the current pandemic, with the increased pressure on the communications networks, we are often advised not to use the microwave (oven) when making video calls, watching HD videos, or doing something important online. Explain whether this advice has any scientific justification, referring in your answer to three different connection types: wired ethernet, Wi-Fi and cellular (mobile) 4G. 5

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, using sketches and diagrams  to help  illustrate  your  point  where relevant,  stating  any  assumptions  made, theorems used and parameters referred to. 5

d)   Explain what is meant by quantization noise, illustrating your answer using a sinusoid quantised using a 2 bit ADC (analogue to digital converter) sampling at ‘ times the Nyquist rate (where ‘ is provided in the table on page 3). 5

e)    Discuss how and why the bit error rate of M-ary ASK varies with M, explaining your reasoning, and using suitable illustrations as appropriate. 5

Total

25

iii)   The data sheet mentions modulation formats QAM, QPSK and BPSK. Briefly discuss the similarities  and  differences  between  these  modulation  formats,  using  illustrations  as appropriate. How can these modulation formats be used on the same medium that delivers the domestic AC power? 5

b)      Define the dynamic range of a quantizer, and show that this depends on the number of bits n used to encode each sample, as follows: 10

Dynamic range (dB) ≈ 6 ⋅ n

c)      Explain the concept of companding and discuss how and why it is used to increase the signal to (quantization) noise ratio of a digital audio signal without increasing the data  rate. 5

Total

25

5.           A spectrum analyser is used to display a bandpass signal as shown in Figure Q5 below,  where the horizontal axis is frequency and the vertical axis is 20log10 E , where E is the magnitude of the signal spectrum at any given frequency.

a)    Explain why a logarithmic scale is often used in such displays. 5

b)   Given that the modulation index can be expressed as a ratio of the (linear) amplitudes of the   sideband and carrier peaks ESB  and EC  as follows: 5

show that the modulation index m can be found from the instrument display by

c)   Using the value of ΔE(dB)  provided in the table on page 3, estimate what percentage of the signal’s total power is contained within the upper sideband. 5

d)   Is this signal over-modulated, under-modulated or perfectly-modulated? Explain your answer 5

e)    What does the peak labelled X represent? Explain where this may have originated from and what affect it may have, if any, on the recovered signal. 5

Total

25

6.           Examine the spectrum analyser screenshot depicted in Figure Q6 below, where both axes are linearly scaled.

a)   Name the modulation format, and describe the regions of the spectrum highlighted and

labelled X and Y. Discuss where these may have originated and how, if at all, they might affect the recovered signal. 5

b)   Find the carrier frequency and bandpass and baseband message bandwidths. 5

c)    Starting from the mathematical representation of this modulation format, show that the modulation index can be expressed as 5

d)   Using the values of LSB , USB  and C  provided in the table on page 3, find the percentage of the bandpass signal power contained in the lower sideband. 5

e)    Is  this  signal  over-modulated,  under-modulated  or  perfectly-modulated?  Explain  your answer. 5

Total

25

Using your knowledge of communications, explain how such a wearable radio could have worked, in as much technical detail as possible. Address issues such as how it could function without batteries, the range of signal frequencies, suitable antenna dimensions, whether it could be used for duplex operation, some of its limitations and whether or not it would work if used today.

c)    Discuss the differences between coherent and non-coherent detectors for amplitude modulated signals, using diagrams, sketches and mathematical expressions to support your answer. 5

Total

25

Design a suitable communication system for the above application (from transducer to data logger), using the constraints given. Include as much detail as possible, including a block diagram, signal plots, details of all relevant parameters (such as antenna size, modulation format etc) with a justification for how they were selected. Do not exceed a single page.

b)   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,  even  though  wireless  signal  propagation  is considerably faster. Use relevant theorems and communication principles to support your answer. 10

Total

25