1. EE 6331, Spring, 2009 Advanced Telecommunication
                                                           
Zhu Han
Department of Electrical and Computer Engineering
Class 10
Feb. 19th, 2009

2. Outline Review Project 1 Modulation Rayleigh and Ricean Distributions
Basics
Analog modulation
ECE6331 Spring 2009

3. Rayleigh Distributions
Describes the received signal envelope distribution for channels, where all the components are non-LOS:
i.e. there is no line-of–sight (LOS) component.
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4. Ricean Distributions
Describes the received signal envelope distribution for channels where one of the multipath components is LOS component.
i.e. there is one LOS component.
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5. Rayleigh Fading
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6. Rayleigh Fading Distribution
The Rayleigh distribution is commonly used to describe the statistical time varying nature of the received envelope of a flat fading signal, or the envelope of an individual multipath component.
The envelope of the sum of two quadrature Gaussian noise signals obeys a Rayleigh distribution.
 is the rms value of the received voltage before envelope detection, and 2 is the time-average power of the received signal before envelope detection.
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7. Rayleigh PDF 0.6065/s mean = 1.2533s median = 1.177s
variance = s2 s 2s 3s 4s 5s
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8. Ricean Fading Distribution
When there is a dominant stationary signal component present, the small-scale fading envelope distribution is Ricean. The effect of a dominant signal arriving with many weaker multipath signals gives rise to the Ricean distribution.
The Ricean distribution degenerates to a Rayleigh distribution when the dominant component fades away.
The Ricean distribution is often described in terms of a parameter K which is defined as the ratio between the deterministic signal power and the variance of the multipath.
K is known as the Ricean factor
As A0, K  - dB, Ricean distribution degenerates to Rayleigh distribution.
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9. PDF
Probability density function of Ricean distributions: K=-∞dB (Rayleigh) and K=6dB. For K>>1, the Ricean pdf is approximately Gaussian about the mean.
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10. Small-scale fading mechanism
Assume signals arrive from all angles in the horizontal plane 0<α<360
Signal amplitudes are equal, independent of α
Assume further that there is no multipath delay: (flat fading assumption)
Doppler shifts
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11. Small-scale fading: effect of Doppler in a multipath environment
fm, the largest Doppler shift
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12. Carrier Doppler spectrum
Spectrum Empirical investigations show results that deviate from this model Power
Model Power goes to infinity at fc+/-fm
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13. Fading Model: Gilbert-Elliot Model
Fade Period
Signal
Amplitude
Threshold
Time t
Good
(Non-fade)
Bad
(Fade)
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14. Simulating 2-ray multipath
a1 and a2 are independent Rayleigh fading
1 and 2 are uniformly distributed over [0,2)
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15. Simulating multipath with Doppler-induced Rayleigh fading
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16. Review
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17. Project 1 Due 3/3. Electrical version 3 Tasks Write a report!!!
Investigate the large scale fading such as free space, reflection, and diffraction.
Test log-scale propagation loss and shadowing model
Investigate small scale fading with multipath and Doppler shifting using Clarke model.
Write a report!!!
Exam 3/5
50% why why why question
50% likes homework
Cover until last class (class 9)
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18. Project 1 ht=2m, hr=1.5, Freq.=2.4GHz, hf=2.5m, d1=0.5m, d=1-10m
10dB diffraction loss
Light diffraction hf df
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19. Project 1
Two ray model path loss
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20. Project 1
Two ray model phase
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21. Project 1
Diffraction Model Path Loss
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22. Project 1
Diffraction Phase
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23. Project 1
Combined path loss
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24. Project 1
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25. What is modulation
Modulation is the process of encoding information from a message source in a manner suitable for transmission
It involves translating a baseband message signal to a bandpass signal at frequencies that are very high compared to the baseband frequency.
Baseband signal is called modulating signal
Bandpass signal is called modulated signal
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26. Baseband and Carrier Communication
Describes signals and systems whose range of frequencies is measured from 0 to a maximum bandwidth or highest signal frequency
Voice: Telephone 0-3.5KHz; CD KHz
Video: Analog TV 4.5MHz, TV channel is 0-6MHz. Digital, depending on the size, movement, frames per second, …
Example: wire, coaxial cable, optical fiber, PCM phone
Carrier Communication:
Carrier: a waveform (usually sinusoidal) that is modulated to represent the information to be transmitted. This carrier wave is usually of much higher frequency than the modulating (baseband) signal.
Modulation: is the process of varying a carrier signal in order to use that signal to convey information.
Example on the board.
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27. Modulation Techniques
Modulation can be done by varying the
Amplitude
Phase, or
Frequency
of a high frequency carrier in accordance with the amplitude of the message signal.
Demodulation is the inverse operation: extracting the baseband message from the carrier so that it may be processed at the receiver.
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28. Analog/Digital Modulation
Analog Modulation
The input is continues signal
Used in first generation mobile radio systems such as AMPS in USA.
Digital Modulation
The input is time sequence of symbols or pulses.
Are used in current and future mobile radio systems
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29. Goal of Modulation Techniques
Modulation is difficult task given the hostile mobile radio channels
Small-scale fading and multipath conditions.
The goal of a modulation scheme is:
Transport the message signal through the radio channel with best possible quality
Occupy least amount of radio (RF) spectrum.
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30. Frequency versus Amplitude Modulation
Amplitude Modulation (AM)
Changes the amplitude of the carrier signal according to the amplitude of the message signal
All info is carried in the amplitude of the carrier
There is a linear relationship between the received signal quality and received signal power.
AM systems usually occupy less bandwidth then FM systems.
AM carrier signal has time-varying envelope.
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31. Amplitude Modulation
The amplitude of high-carrier signal is varied according to the instantaneous amplitude of the modulating message signal m(t).
AM Modulator
m(t)
sAM(t)
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32. Modulation Index of AM Signal
For a sinusoidal message signal
Index is defined as:
SAM(t) can also be expressed as:
g(t) is called the complex envelope of AM signal.
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33. AM Modulation/Demodulation
Source
Sink
Wireless
Channel
Modulator
Demodulator
Baseband Signal
with frequency fm
(Modulating Signal)
Bandpass Signal
with frequency fc
(Modulated Signal)
Original Signal
with frequency fm
fc >> fm
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34. AM Modulation - Example
1/fmesg
1/fc
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35. AM Spectrum
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36. AM Decoder Rectifier Detector: synchronous
Envelope Detector: asynchronous
AM signal R C +
vc(t) -
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37. AM Broadcasting Limitation History Frequency Long wave: 153-270kHz
Medium wave: 520-1,710kHz, AM radio
Short wave: 2,300-26,100kHz, long distance, SSB, VOA
Limitation
Susceptibility to atmospheric interference
Lower-fidelity sound, news and talk radio
Better at night, ionosphere.
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38. QAM
AM signal BANDWIDTH : AM signal bandwidth is twice the bandwidth of the modulating signal. A 5kHz signal requires 10kHz bandwidth for AM transmission. If the carrier frequency is 1000 kHz, the AM signal spectrum is in the frequency range of 995kHz to 1005 kHz.
QUADRARTURE AMPLITUDE MODULATION is a scheme that allows two signals to be transmitted over the same frequency range.
Equations
Coherent in frequency
and phase. Expensive
TV for analog
Most modems
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39. Angle Modulation
Angle of the carrier is varied according to the amplitude of the modulating baseband signal.
Two classes of angle modulation techniques:
Frequency Modulation
Instantaneous frequency of the carrier signal is varied linearly with message signal m(t)
Phase Modulation
The phase q(t) of the carrier signal is varied linearly with the message signal m(t).
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40. Angle Modulation FREQUENCY MODULATION
kf is the frequency deviation constant (kHz/V)
If modulation signal is a sinusoid of amplitude Am, frequency fm:
PHASE MODULATION
kq is the phase deviation constant
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41. FM Example 4 - + - - + -4 0.5 1 1.5 2 41 Message signal FM Signal- + - - + -4
0.5 1
1.5 2
Message signal
FM Signal
Carrier Signal
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42. FM Index W: the maximum bandwidth of the modulating signal
Df: peak frequency deviation of the transmitter.
Am: peak value of the modulating signal
Example: Given m(t) = 4cos(2p4x103t) as the message signal and a frequency deviation constant gain (kf) of 10kHz/V;
Compute the peak frequency deviation and modulation index!
Answer:
fm=4kHz
Df = 10kHz/V * 4V = 40kHz.
bf = 40kHz / 4kHz = 10
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43. Spectra and Bandwidth of FM Signals
An FM Signal has 98% of the total transmitted power in a RF bandwidth BT
Carson’s Rule
Upper bound
Lower bound
Example: Analog AMPS FM system uses modulation index of Bf = 3 and fm = 4kHz.
Using Carson’s Rule: AMPS has 32kHz upper bound and 24kHz lower bound on required channnel bandwidth.
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44. FM/PM Example (Frequency)
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45. FM Demodulator
Convert from the frequency of the carrier signal to the amplitude of the message signal
FM Detection Techniques
Slope Detection
Zero-crossing detection
Phase-locked discrimination
Quadrature detection
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46. Proportional to the priginal Message Signal
Slope Detector
Vin(t)
Limiter
V1(t)
Differentiator
V2(t)
Vout(t)
Envelope Detector
Proportional to the priginal Message Signal
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47. Comparison of analog modulation systems
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48. Frequency versus Amplitude Modulation
Frequency Modulation (FM)
Most popular analog modulation technique
Amplitude of the carrier signal is kept constant (constant envelope signal), the frequency of carrier is changed according to the amplitude of the modulating message signal; Hence info is carried in the phase or frequency of the carrier.
Has better noise immunity:
atmospheric or impulse noise cause rapid fluctuations in the amplitude of the received signal
Performs better in multipath environment
Small-scale fading cause amplitude fluctuations as we have seen earlier.
Can trade bandwidth occupancy for improved noise performance.
Increasing the bandwith occupied increases the SNR ratio.
The relationship between received power and quality is non-linear.
Rapid increase in quality for an increase in received power.
Resistant to co-channel interference (capture effect).
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49. Satellite FM: XM vs. Sirus
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