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.

b)   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.

c)    Explain  the  role  of a  regenerative  repeater  in  a  digital  communication  system,  and comment on the effect their use has on digital communications.

d)   Discuss how you would expect the bit error rate of M-ary ASK to vary with M, explaining your reasoning in as much detail as possible, and using suitable illustrations as appropriate.

Total 25

2.    a)    Discuss  3 possible approaches for the demodulation of NBFM, describing in as much detail as possible how each approach works.

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

cos 幼!t − β sin 幼!t ⋅ sin 幼mt

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

c)   A low-cost weather balloon is to be released into the lower atmosphere equipped with the following sensors: camera, temperature, air pressure, humidity and GPS location.  Live pictures are captured at a rate of p per hour (where the value of p is given in the  table on page 3), and the remaining measurements are taken every minute, and

transmitted back to receiving stations on the ground. The on-board processor has no storage, so the data are transmitted at the rate at which they are acquired.

Design  a  suitable  end-to-end  simplex  communication  system  for  the  above  weather balloon, providing as much technical information as possible, showing your calculations and supported by relevant illustrations and sketches, with any pertinent assumptions stated explicitly.

Total 25

3.    a)    Figure Q4 shows how the bit error rate (BER) of selected digital transmission systems is         7

related to the signal-to-noise ratio (Eb/No).

Figure Q3 – Bit error rate of selected digital transmission systems. BBB = bipolar baseband; UBB = unipolar baseband

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 halving the signal power on the BER for the different modulation formats.

b)   A particular baseband communication scenario stipulates a maximum tolerable error rate of 1 bit in 10,000. The choice of signalling format is determined by the signal power available at the transmitter.  Use Figure Q3 to determine which signalling format would require the least power, commenting on the relative requirements of the two: unipolar and bipolar baseband signalling.

Use your knowledge of line coding formats to explain the above answer.

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.

d)   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 before the sampler, or 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.

Total 25

4.    a)      Figure  Q4  (on the  following page) is an extract from the data sheet for an electronic component in the MICROTEL catalogue1 .

i)   Explain the relevance of the different frequencies mentioned.

ii)   The datasheet lists “automotive remote keyless entry” as a possible application of this chip. Design such a communication system, making sure you include relevant block diagrams, signal plots (in the time and frequency domains), suggested antenna dimensions and message scenarios.

iii)   Mention two  similarities  and two differences between the modulation  format used by MICRF002 and DSB-SC in terms of modulation, demodulation and/or spectral shape.

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:

Dynamic range = 2#

Total 25

Figure Q4

5.    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

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 SNR dB, where the value of SNR is provided in the table on page 3.

ii)        Discuss the implications of this, using no more than 200 words.

c)    How much channel bandwidth is required to frequency-division multiplex m user channels, each of bandwidth B kHz, with f$  kHz guard bands between channels, using :

i)         SSB

ii)        DSB

iii)        FM, with modulation index β

Values of m, B , f$    and β are provided in the table on page 3.

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.

Total 25

6.

a)      Sketch the spectrum of a received DSB-LC signal with carrier amplitude Ec  and upper and lower sidebands ELSB  and EUSB  respectively, where ELSB   ≈ EUSB .

b)      Explain why the received signal spectrum may have ELSB  slightly higher or lower than EUSB .

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

ELSB  + EUSB

Ec

d)      Using the values of ELSB , EUSB  and Ec  provided in the table on page 3, find the percentage of the bandpass signal power contained carrier component.

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

Total 25

8.    a)    Broadband and mobile networks are under increased pressure during the current coronavirus

outbreak, as many activities are taking place virtually and remotely. The UK communications regulator Ofcom recently published a number of tips to help families and businesses manage effectively during this time, including the following:

.    Use your landline or Wi-Fi calls if you can.

.    Move your router clear of other devices.

.    Lower the demands on your connection.

.    Try wired rather than wireless.

.    Plug your router directly into your main phone socket.

.    Don’t use the microwave when you’re making video calls, watching HD videos or doing something important online.

.    Disconnect devices that are not in use.

Use your knowledge of communications to explain the reasoning behind each of these tips in as much technical detail as you can.

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.

c)    Consider the following scenario:

You are being detained in a prison cell, with no access to any electronic devices nor to the

outside world. Another engineer is detained in another cell on a different floor. You discover that you are able to communicate with this person by knocking your metal spoon against the  water pipe at night when the complex is silent.

You have with you a hand-drawn map of the prison complex, and you have an urgent need to deliver this to the other person. Using your knowledge of communications, devise a simple

and effective strategy to convey this map to the other prisoner as quickly as possible. The

only items available to you are paper, pencil, a ruler, a metal spoon, a toothbrush, a mirror, a blanket, a pillow, and your lecture notes from ELEC202. You can assume the other person    has similar items available to them, and can respond to you with spoon knocks.

Include as much detail as possible in your answer, but do not exceed 300 words or a single page.

Total 25