1. EE521 Analog and Digital Communications
Today some topics will be presented on the whiteboard. The slides will be updated on the web site in a few days.
EE521 Analog and Digital Communications
James K. Beard, Ph. D.
Tuesday, February 15, 2005
February 15, 2005
Week 5

2. Attendance
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3. Essentials Text: Bernard Sklar, Digital Communications, Second Edition
SystemView
Student version included with text
Trial version has 90-day timeout
Office
E&A 709
Tuesday afternoons 3:30 PM to 4:30 PM
Tuesdays before class at Ft. Washington
MWF 10:30 AM to 11:30 AM added
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4. Today’s Topics SystemView Quiz Timeline Term Projects
Bandpass Modulation and Demodulation
Review of Bandpass Modulation and Detection
Coherent and non-coherent detection
Discussion (as time permits)
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5. Quiz timeline Quiz today Follow-up quiz Grade on first quiz Open book
Caclulator
No notes
Follow-up quiz
Take-home
Will require SystemView to complete
Will be deployed on Blackboard this week
Grade on first quiz
Split with take-home depends on grades to today’s quiz
Take-home can be up to 25% of quiz grade
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6. Term Projects Interpret, plan, model Use SystemView
Assignments will be deployed on Blackboard this week
Your preferences and comments are encouraged
Office hours
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7. Bandpass Modulation and Demodulation
Review: Topics from Text Chapter 4
4.1, Why Modulate?
4.2, Digital Bandpass Modulation Techniques
4.3, Detection of Signals in Gaussian Noise
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8. Sklar Chapter 4 Legend: From other sources Essential Information
Message symbols
Optional
Channel symbols
X M I T
Format
Source encode
Encrypt
Channel encode
Multi-plex
Pulse modulate
Bandpass modulate
Fre-quency spread
Multiple access
Digital input
Channel impulse response
Bit stream
Synch-ronization
Digital baseband waveform
Digital bandpass waveform
Channel
Digital output
R C V
Format
Source decode
Decrypt
Channel decode
Demul-tiplex
Detect
Demod-ulate & Sample
Freq-uency despread
Multiple access
To other destinations
Channel symbols
Information
sink
Message symbols
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9. Modulation Input for this stage is baseband encoded Output is IF
Pulse modulation
Bandpass or pulse shaping filtering
Output is IF
Ready to upconvert to RF and transmit
Optional frequency spread may be added
Optional multiple access may be added
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10. Functions of Modulation
Prepare for wireless transmission
Modulate on carrier at RF in band allocated for wireless communication
Formulate digital bandpass waveform
Optional successive steps
Spread-spectrum (Chapter 12, next semester)
Multiple access (Chapter 11, next semester)
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11. Digital Bandpass Modulation
Transition
From Baseband signaling
To bandpass signaling
Operations
Coherent
Special attention to tracking phase
Phase used in demodulation
Non-coherent
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12. Baseband Signaling See Figure 4.1 page 170 PCM waveforms
Non-return-to-zero
Return to zero
Phase encoded
Multilevel binary
M-ary pulse modulation
PAM
PPM
PDM
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13. Bandpass Signaling See Figure 4.1 page 170 Carrier strategies
Coherent
Non-coherent
Modulation types
Phase sift keying (PSK)
Frequency shift keying (FSK)
Amplitude shift keying (ASK)
Continuous phase modulation (CPM)
Hybrids
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14. Phasor Representation of Tones Modulated on a Carrier
Everything referenced to
Carrier frequency
Particular phase of carrier
Physical meaning can be inferred
Quadrature demodulation of tone
L.O. is the reference carrier and phase
Complex result is the phasor
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15. Features of Phasor Concept
Modulation is simple to represent
Phase modulation changes direction of phasor
Amplitude modulation changes length of phasor
Modulation can be posed in terms of phases of the modulation sidebands
Characterization of noise as 2-D Gaussian distribution
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16. Detection in Gaussian Noise
Characterization of Gaussian noise
2-D distribution in phasor plot
Zero mean, sigma proportional to noise amplitude
Phasor concept allows vector addition to represent signal plus noise
Coherent detection
Selection of portions of phasor space
Design is reduced to definition of boundaries in phasor space
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17. Detection Operation One-dimensional case from Figure 3.1 page 108
Two steps
Downconvert and sample
Quadrature demodulator, or
Digital product dectector
Equalize channel and filtering effects
Decision thresholds
The decision threshold input is the predection point
Location of received Eb/N0
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18. Design of Detection Design of matched filter
This is the channel and filter equalization of Step 1
Maximizes received Eb/N0
Design of detection regions
Examples given in book are conceptual
Likelihood ratio defines decision region boundaries
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19. Bandpass Modulation and Detection
Topics from text Chapter 4
Coherent detection
Non-coherent detection
Complex envelope
Error performance
M-ary signalling and performance
Symbol error performance for M-ary systems
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20. Why We’re Doing This The Channel Coding Theory We want to get there
The channel capacity is
When messaging bit rate is below the channel capacity, a coding scheme exists that will achieve an arbitrarily low BER in that channel
We want to get there
We will get quite close
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21. Nearest Neighbor Decision
Weighted distance in vector space
Squared distance d between vectors i and j is given by quadratic form
The smallest distance “wins” the decision
Decision is i == j
Statistic can be chi-square if W is inverse of covariance matrix of noise in the two vectors
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22. Coherent Detection Base block diagram Phase is known
Figure 4.7 page 180
Correlate signal with each of M possible returns
Threshold, or select the largest
Phase is known
PLL on synch pulse or bit
Complex convolution or DPD part of PLL
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23. MPSK Signals and Phasors
Correlate with multiple phases
Provide regions in phasor space for detection decisions
See Figure 4.11 page 189 for example
Demodulate to estimate of phase
See Figure 4.12 page 190 for example
These are equivalent
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24. Coherent Detection of FSK
Use a filter bank
Simultaneously correlate with several cosines
Each represents a frequency in the FSK palette
Phase is same as that of the signal
Decision boundary
Nearest neighbor
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25. Non-Coherent PSK Detection
Defining characteristic is that no phase reference for signal is required or available
Differential PSK
Phase differences are detected
Two pulses are detected and their phases subtracted
No requirement for tracking phase of signal
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26. Non-coherent FSK
Correlation without phase is simply a frequency-selective filters
Decision vector built from frequency filters
The decision vector elements are a series of powers or amplitudes
What is the equivalence to nearest-neighbor detection?
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27. Text and Assignment Text Assignment: From Text
Benard Sklar, Digital Communicatinons ISBN
SystemView User's Manual, Elanix, Inc
Assignment: From Text
Look at Example 4.2 page 193 and be prepared to discuss
Chapter 6, 6.1, 6.2, 6.3
Homework problem 6.1; look at and be prepared to discuss
Browse appendices of text for review and supplementary material
Look at TUARC
K3TU, E&A 918; ask Dr. Silage for access
Websites
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28. Practice Quiz Problems from book homework Quiz will be similar
Problem 1.1 page 51
Problem 2.2 page 101
Problem 3.1 page 162
Quiz will be similar
From homework problems
Modifications to problem statement and parameters
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