首页 > > 详细

代写EEEN3005J代做Java实验、C/C++编程代写

Beijing-Dublin International 
College 
SEMESTER II FINAL EXAMINATION - 2018/2019 
School of Electrical and Electronic Engineering 
EEEN3005J: Communication Theory 
Prof. Peter Kennedy 
Dr. Deepu John 
Time Allowed: 120 minutes 
Instructions for Candidates 
Answer ANY 3 of the 4 questions. All questions carry equal marks. Only the 3 highest scoring 
answers will contribute to final grade. 
BJUT Student ID: UCD Student ID: 
I have read and clearly understand the Examination Rules of both Beijing University of Technology 
and University College Dublin. I am aware of the Punishment for Violating the Rules of Beijing 
University of Technology and/or University College Dublin. I hereby promise to abide by the 
relevant rules and regulations by not giving or receiving any help during the exam. If caught 
violating the rules, I accept the punishment thereof. 
Honesty Pledge: (Signature) 
Instructions for Invigilators 
Non-programmable calculators are permitted. 
No rough-work paper is to be provided for candidates. 
BDIC Semester II Academic Year (2018/2019) 
Question 1: 
Score 
Obtained A baseband signal, g (t) , with bandwidth 10 kHz, is such that its amplitude satisfies |g (t)| < 
0.8 V, and has average power = 0.1 Watts. This signal is connected to a circuit composed of 
a mixer and an adder as shown in Figure 1 below. The resulting signal, s˜(t) , is transmitted 
through a channel as shown. The scaling factor, α , represents a signal power attenuation of 
120 dB, and w (t) is additive white Gaussian noise with single-sided power spectral density 
(PSD) N0= -174 dBm/Hz. The receiver applies an ideal band pass filter having bandwidth 
just sufficient to capture the whole of the received modulated signal. 
[Note that if the power of a signal is Pm mWatts, then its power in dBm is = 10 log10(Pm) 
dBm.] 
Figure 1: Circuit to be considered in question 1 
1. Sketch the time-domain signal, s˜(t), showing numbers and units on both axes. What 
name is given to this type of modulation? 
2. Calculate the signal-to-noise ratio (SNR) in dB at point “A”. Do not count the carrier 
component as contributing to the “signal power” here, as it bears no information. 
3. What do we mean when we say that a random signal is wide-sense stationary? 
4. What is meant by the Energy Spectral Density (ESD) of a deterministic energy sig- 
nal?Consider a signal,x (t), with one-sided ESD given by: 
′ 
x(f) = 
4f2 if 0 < f < 10 kHz 
0 otherwise 
Determine the fraction of the signal’s energy that lies in the frequency range 0 < f < 5 
kHz 
Question 2: 
Score 
Obtained Consider the circuit in Figure 2 where the control signal c (t), that is guaranteed to be either±1 V, is used to control the position of the two switches as shown. 
Figure 2: Circuit to be considered in question 2 
1. Write a simple expression for the circuit’s output, vout(t), in terms of c (t) and s˜(t), 
explaining your answer by clearly describing the operation of the circuit when c (t) = 
+1 V and when c (t) = -1 V. 
2. Let s˜(t) be a bandpass signal with centre frequency fc Hz, and c (t) be a square wave 
signal alternating between ±1 V also with frequency fc Hz. In this case 
Page 2 of 5 
BDIC Semester II Academic Year (2018/2019) 
• sketch a possible power spectral density (PSD) for s˜(t) 
• sketch the PSD of c (t) 
• sketch the resulting PSD of vout(t) 
3. Provide a clear definition of “Double SideBand Suppressed Carrier (DSB-SC)” modu- 
lation. Your answer should contain the mathematical expressions and derivations of a 
DSB-SC signal in the time and frequency domain. You should also include sketches of 
an example DSB-SC signal in both domains. 
4. Explain how the circuit in Figure 2 could be used, in conjunction with some additional 
circuit elements, to demodulate a DSB-SC signal. 
5. The process of demodulating a DSB-SC signal often suffers from local oscillator “phase 
and/or frequency mismatch” errors. Explain the meaning of this statement 
Question 3: 
Score 
Obtained 1. What is the instantaneous frequency fi (t) of an arbitrary signal A (t) cos (φ (t))? 
2. If a modulated signal can be written as Ac cos (2pifct+ θ (t)) give an expression for the 
instantaneous frequency. 
3. What is Frequency Modulation (FM)? your answer should derive an expression for s˜ (t), 
the FM signal, in terms of g (t) the modulating signal, and kf the frequency sensitivity. 
4. Let g (t) be a sine wave with amplitude Am and frequency fm kHz. 
• Derive an expression for s˜ (t) in terms of Am and fm. 
• What is the modulation index β and modify the above expression to include β. 
• Derive an expression for whats called ”the peak frequency deviation” ∆f . Hint: 
∆f is should really be called the ”peak instantaneous frequency deviation” 
• Why is the spectrum of this sinusoidal modulation FM signal, s˜ (t) a line spectra? 
• The Fourier series for s˜ (t) is: 
s˜ (t) = AcJ0 (β) cos (2pifct) +Ac 
∞∑ 
n=1 
J2n (β) [cos {2pi (fc + 2nfm) t}+ cos {2pi (fc − 2nfm) t}] 
+Ac 
∞∑ 
n=1 
J2n−1 (β) [cos {2pi (fc + (2n− 1) fm) t} − cos {2pi (fc − (2n− 1) fm) t}] 
where Jn (β) is the n 
th order Bessel function of the first kind. 
Let fm = 8kHz, and fc = 100MHz: 
– Using Figure 3 sketch the spectra for β = 0.1 
– If Am = 1Volt, what approximate value of frequency sensitivity kf would result 
in the spectral component at fc being zero? You answer should use the correct 
units for kf. 
Page 3 of 5 
BDIC Semester II Academic Year (2018/2019) 
0 1 2 3 4 5 6 7 8 9 10 
−0.5 
0.5 
β 
J n 
(β) 
 
 
J0(β) 
J1(β) 
J2(β) 
J3(β) 
J4(β) 
Figure 3: Some Bessel functions for use in question 3. 
Question 4: 
Score 
Obtained Consider the system shown in Figure 4. 
p(τ)bk ∈ {±1} 
y(t) 
y(kT ) 
every T seconds kT 
Figure 4: System to be considered in question 4 
1. Write an expression for y (t). 
2. Hence, or otherwise, derive (don’t just quote) a time domain criteria on p (τ) such that 
the collection of samples {y (kT )} are all ISI-free. 
3. Derive (don’t just quote) the frequency domain version of the criteria derived in part 2. 
Let T = 1 and P (f), as shown in Figure 5(a), be the Fourier transform of p (τ): 
 
联系我们
  • QQ:99515681
  • 邮箱:99515681@qq.com
  • 工作时间:8:00-23:00
  • 微信:codinghelp
热点文章
程序代写更多图片

联系我们 - QQ: 99515681 微信:codinghelp
© 2014 www.7daixie.com
程序代写网!