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ELEC 202

SPECIMEN PAPER

SECOND SEMESTER EXAMINATION

REPLACEMENTS 2019/20

COMMUNICATION SYSTEMS

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.

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.

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.

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.

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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.

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.

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.

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.

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