辅导案例-ELEC 202

  • August 8, 2020

PAPER CODE ………………..ELEC202……………PAGE….1……..OF …………..9……………………..CONTINUED PAPER CODE NO. EXAMINER: Dr W Al-Nuaimy ELEC 202 DEPARTMENT: EE&E [email protected] SECOND SEMESTER EXAMINATION REPLACEMENTS 2019/20 COMMUNICATION SYSTEMS INSTRUCTIONS TO CANDIDATES The numbers in the right hand margin represent an approximate guide to the marks available for that question (or part of a question). Total marks available are 100. Copying any material from another source, or colluding with any other person in the preparation and production of this work will be considered suspected academic misconduct and will be dealt with according to the University’s Academic Integrity Policy. This is an open-book test. Answer ALL Questions. The use of a calculator IS allowed. PAPER CODE ………………..ELEC202……………PAGE….2……..OF …………..9……………………..CONTINUED 1. 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. Use no more than 500 words. 10 Question continues overleaf. PAPER CODE ………………..ELEC202……………PAGE….3……..OF …………..9……………………..CONTINUED Question continued. b) You are tasked with helping a marine biologist design a remote observation system to collect, transmit, receive and record the vocalizations of deep underwater marine life using an analogue hydrophone (underwater transducer) and a floating buoy to transmit the data to a shore-based logging station. Dolphin vocalizations range in frequency from 0.2 to 150 kHz, and whale vocalizations range from 20 to 8000 Hz. The floating transmitter can be set up to transmit using any common analogue or digital modulation format, but the available radio license limits the broadcast to the frequency range between 239.8 and 240.2 MHz, and the hydrophones and digitiser units must be chosen from those below. Available hydrophone models: Model Frequency range (kHz) Max operating depth (m) Omnidirectional Price (£) H1 0.02 to 178 600 Yes 120 H2 0.02 to 80 500 Yes 200 H3 0.2 to 320 800 No 80 Available ADC boxes: Model Max clock (kHz) Bits SNR (dB) Waterproof ADC1 0.02 to 178 16 65 Yes ADC2 0.02 to 80 24 55 Yes ADC3 0.2 to 320 32 45 No 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. 15 Total 25 PAPER CODE ………………..ELEC202……………PAGE….4……..OF …………..9……………………..CONTINUED 2. Two bandpass signals are shown in Figure 2 below: DSB-SC and AM with 100% modulation, for two different message signals. (a) (b) Figure Q2 a) Which of the two signals represents DSB-SC and which represents AM with 100% modulation? Explain your reasoning. 5 b) For each of these bandpass signals, sketch their respective approximate spectra, labelling them “DSB-SC” and “AM”. 5 c) Given the carrier used in both cases has a frequency of 455 kHz, sketch the message signal (in the time domain) in both cases (DSB-SC and AM), clearly indicating the fundamental period on both plots. 5 Question continues overleaf. PAPER CODE ………………..ELEC202……………PAGE….5……..OF …………..9……………………..CONTINUED Question continued. d) Draw a circuit diagram of a suitable non-coherent demodulator for the above AM signal, suggesting suitable values for any components used. Sketch and describe the effects of inappropriate selection of component values. 5 e) If the AM signal were overmodulated, how would this affect the bandpass signal in the time and frequency domains? How would the output of the envelope detector be affected? Use suitable illustrations to support your answer. 5 Total 25 PAPER CODE ………………..ELEC202……………PAGE….6……..OF …………..9……………………..CONTINUED 3. a) A periodic signal () can be approximated using the first 10 terms of its Fourier Series expansion as follows: () ≈ ∑ cos(2000) + sin(200) 10 =1 () is used to frequency-modulate a 50 MHz sinusoid with a modulation index of 5. Find expressions for the maximum frequency and phase deviations of the resulting signal and find an expression for the approximate baseband and bandpass bandwidths. 5 b) Show that the NBFM expression for a single tone message can be written as a scaled version of cos − sin ⋅ sin Sketch the above signal in the time and frequency domains, and comment on the bandpass bandwidth. 5 c) Discuss 3 possible approaches for the demodulation of NBFM, describing in as much detail as possible how each approach works. Use no more than 300 words. 10 Question continues overleaf. PAPER CODE ………………..ELEC202……………PAGE….7……..OF …………..9……………………..CONTINUED Question continued. d) 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. 5 Total 25 PAPER CODE ………………..ELEC202……………PAGE….8……..OF …………..9……………………..CONTINUED 4. Figure Q4 shows how the bit error rate (BER) of selected digital transmission systems is related to the signal-to-noise ratio (Eb/No). Figure Q4 – Bit error rate of selected digital transmission systems. BBB = bipolar baseband; UBB = unipolar baseband a) 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 Q4 to determine which signalling format would require the least power, commenting on the relative requirements of the two: unipolar and bipolar baseband signalling. 5 b) 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. 10 Question continues overleaf. B ER PAPER CODE ………………..ELEC202……………PAGE….9……..OF …………..9……………………..END Question continued. c) The analogue signal () = 10−() is sampled at a frequency of 10 Hz, quantised using a uniform 16 level quantizer and encoded into a stream of binary digits for PCM transmission. For the first 3 samples, starting at = 0, sketch the PCM signal in the time and frequency domains, and provide an estimate for the resulting PCM bandwidth. 5 d) Discuss how the bit error rate of M-ary ASK varies with M, explaining your reasoning in as much detail as possible, and using suitable illustrations as appropriate. Use no more than 100 words. 5 Total 25

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