ELEC 202:COMMUNICATION SYSTEMS

1.

a) 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 10 per hour, 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.

Answers are expected to be in the region of 2 pages. 20

b) NASA’s New Horizon space probe has provided us with stunning highresolution 4096-level grayscale images of, among other things, the surface of Pluto, a dwarf planet at the outer edge of our Solar System. In 2015 the probe encountered Pluto at a distance of 4.6 billion km from Earth, from 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 in its entirety, and hence estimate how many such images could be received during a 30 day period, assuming the probe is constantly transmitting, stating any assumptions made. 10

Total 30

2.

a) Determine the modulation index and carrier frequency 5

b) Estimate the effective frequency bandwidth of this signal, and indicate whether this is narrow-band FM or wide-band FM? 5

c) Compare the FM signal above with AM and SSB signals for the same baseband signal. 10

d) State the Shannon-Hartley Theorem, and plot the channel capacity as a function of signal-to-noise ratio for a given channel bandwidth. 5

Total 25

3.

A musical performance containing frequencies of up to 15 kHz is digitised for PCM transmission along an optic fibre.

a) The signal is sampled at a rate of 44.1 kHz, and then quantised into 65,536 levels and subsequently binary coded for PCM transmission. State whether this is critical sampling, over-sampling or under-sampling, and mention any advantages or disadvantages of this choice of sampling rate. 5

b) Determine the minimum number of binary digits required to encode one sample, and the minimum number of binary digits per second required to encode this audio signal. 5

c) Suggest how this digitisation process could be modified to increase the sensitivity to lower amplitudes, but without increasing the bandwidth required to transmit the signal. 5

d) 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.

Total 20

4.

a) A broadband supplier has guaranteed to offer a telephone line with a minimum SNR of 24 dB. What is the maximum noise power, as a fraction of the received signal power (i.e. N/S)? 5

b) Referring to the channel described in part (a) above, given a channel bandwidth of 3 kHz, calculate the maximum realisable error-free data rate. 5

c) An analogue audio signal of bandwidth 10 kHz signal is transmitted in the following manners: - It is modulated using DSB-SC with a particular carrier frequency and transmitted wirelessly - It is suitably sampled and digitised using 8 bits and then modulated using binary ASK at the same carrier frequency, and transmitted wirelessly. Calculate the minimum bandwidth required to transmit each signal. 10

d) 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) 4-ASK 5 Tota

Total 25