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# 数字信号代写 | TELECOMMUNICATION PRINCIPLES

### 数字信号代写 | TELECOMMUNICATION PRINCIPLES

SECTION A

Candidates must answer any FIVE parts of Question 1.

Question 1

(a) Sketch the function �(�) = 5 𝛿(� − 1)

(b) Provide a sketch diagram of a 5-level digital signal.

(c) A wireless signal occupies the frequency range from 2.4 GHz to 2.41 GHz.
What minimum sampling frequency is required?

(d) Give two reasons why single mode fibre is generally a better choice than multimode
fibre for long-distance, high data rate links.

(e) Explain briefly the main difference between time-division and frequency–division
multiplexing.

(f) A communications channel has 2 MHz bandwidth and 0 dB signal-to-noise ratio.
What is its theoretical maximum capacity?

(g) Provide a sketch diagram of a phase-locked loop FM demodulator.

(h) A 20-bit binary sequence has a bit-error probability of 0.01. What is the
approximate chance of no errors being detected by basic parity checking?

(i) A certain domestic satellite operates at a fixed distance from Earth. What increase in
satellite transmitter power in dB would be required to double this distance? You may
assume that free-space path loss conditions apply.

(j) A thermal noise-limited transmission system has 10 MHz bandwidth and 10dB signal-
to noise ratio. What variation in Shannon channel capacity, if any, would occur if the
signal-to-noise ratio decreased by 3dB?

Candidates must answer TWO questions from Section B.

Question 2

(a) A High Earth Orbit (HEO) satellite has a typical distance from the Earth’s surface of
40,000 km. The down-path carrier frequency is 2 GHz and 2 MHz downlink
bandwidth is used. What satellite output power in watts will be required to provide
-140 dBm power at the Earth’s surface?

You may assume that the downlink, path loss is given by:

(b) The Hartley-Shannon capacity of the 2 MHz bandwidth downlink in (a) above is to
be 2 Mbit/s. Show that the maximum allowable noise power is -140 dBm at the Earth
station.

(c) Using the noise power calculated in (b) above, determine what satellite output power
in dBW would be required to provide a 20 Mbit/s downlink capacity? 