EE521 Analog and
Digital
Communications
James K. Beard, Ph. D.
jkbeard@temple.edu
Tuesday, March 29, 2005
http://astro.temple.edu/~jkbeard/
April 26, 2005
Week 14
1
April 26, 2005
Week 14
4/19/2005
4/12/2005
4/5/2005
3/29/2005
3/22/2005
3/15/2005
3/8/2005
3/1/2005
2/22/2005
2/15/2005
2/8/2005
2/1/2005
1/25/2005
1/18/2005
Attendance
7
6
5
4
3
2
1
0
2
Essentials
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Text: Bernard Sklar, Digital Communications, Second
Edition
SystemView
Office
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E&A 349
MWF 10:30 AM to 11:30 AM
Hours during Finals Week TBA
Term Projects Due TODAY, April 26
Final Exam
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Tuesday, May 10, 6:00 PM to 8:00 PM
Here in this classroom
Posted within 3 days; you get your grade from Blackboard
April 26, 2005
Week 14
3
Today’s Topics
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Term Project
Review
EE551 in the Fall
Evaluation
 Starts
promptly at 9:00 PM
 Takes 15-20 minutes
 I will leave the room
 Need a student volunteer
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April 26, 2005
Hand out and collect forms
Deliver to office here at Ft. Washington Main Office
Week 14
4
Term Project
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Generate a frequency sweep
 Start frequency:
 End frequency:
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Add noise to obtain a noise floor
Digitize to 16 bits (later relaxed to 8 bits)
Modulate using FSK, BPSK or QPSK
Convert from baseband to a carrier frequency
Model a fading channel with up to 12 paths.
Demodulate and detect
Analyze BER
April 26, 2005
Week 14
5
Modulation Problems with FSK
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16 bits requires 16 X 7+ kHz or 120+ kBPS
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The MFSK token in the Communications library performs
quantization
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It’s a lot of bandwidth
I.F. forced higher than the 450 kHz first mentioned
SystemView sample rate forced to high rates
The bit stream isn’t available directly
The BER token requires that the bit stream be generated
separately
The MFSK input must be at the SystemView sample rate
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Output will be at the inpuput sample rate
Sampled data must be re-sampled or held
April 26, 2005
Week 14
6
Getting the Bit Stream with
MFSK
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Go from input to characters
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Go from characters to bit stream
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Scale and shift characters for MFSK
modulator
April 26, 2005
Week 14
7
Modulation Problems with
MPSK
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No high-level token for MPSK
 Use
two or four bit symbols
 Use Quad Mod, sine function or PM to get
PSK
 Use PSK Demod to get back characters
Gray code required to achieve theoretical
BER
 Character rate 8 to 16 times sample rate
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 Same
April 26, 2005
bandwidth problems as MFSK
Week 14
8
Other Operations
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Convert from baseband to a carrier frequency
 Not
required for channel models
 Channel models take complex input
 Carrier frequency is a parameter
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Modulation to carrier best demodulated to
baseband for the channel model token
 Simplifies
the SystemView sample rate issue
 Prepares the data for demodulation at the output
 Can be done by modulating to a complex I.F. at
baseband
April 26, 2005
Week 14
9
The Channel Models
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Data formats are complex in, complex out
The channel models include phase
 Multiple
paths are summed coherently
 Result is a log normal, Rayleigh, or other fading
channel
 Model is meaningful for complex data
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Generate I.F. centered at zero or use quadrature
demodulator for input to channel modles
April 26, 2005
Week 14
10
BER Measurement
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Inject noise before the channel model
 Sample
the noise output before summing
 You generate an accurate Eb/N0 easily there
 Use a variance in the noise generator that gives a
base (minimum) Eb/N0
 Use an amplifier in dB with linked gain to control the
Eb/N0
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Use of Global Parameter Links, the BER token,
and multiple iterations for BER curves explained
in Appendix A of the Comms library
documentation
April 26, 2005
Week 14
11
Coding/Decoding
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Not specified in the term project scope
Omitting coding
 Avoided
timing and synchronization issues on the
decoder
 Left us with basically a modulation and channel
modeling project
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Coding
 Offers
an insight on the effect of FEC on the BER
curve
 Isn’t the whole picture without interleaving
 Context will be part of next Fall’s EE551
April 26, 2005
Week 14
12
EE521 Analog and
Digital
Communications
Review Topics
jkbeard@temple.edu
April 26, 2005
Week 14
13
Complex Signals
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Base property is distinction of signal at negative
frequencies from signal at positive frequencies
Use in communications systems
 Signal generation steps
 Digital character generation
 Character generation
 Complex FSK, MPSK, GMSK, etc. generation
 Real signal synthesis at I.F. for upconvert
 Demodulation steps
 Complex demodulation used for coherent pilot PLL
 Complex demodulation of PSK, MSK, etc.
April 26, 2005
Week 14
14
Principles of Complex Signals
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Multiplication
z1  z2   x1  j  y1    x2  j  y 2 
 x1  x2  y1  y 2  j   x1  y 2  x2  y1 
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Multiplication between a complex number
and the complex conjugate of another
z1  z   x1  j  y1    x2  j  y 2 
*
2
 x1  x2  y1  y 2  j    x1  y 2  x2  y1 
 z1  z2  j   z1  z2 
April 26, 2005
Week 14
15
Power and Energy of Complex
Signals
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Power
1
Px  lim 
T  T
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T
2
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x  t   x *  t   dt
T
2
Energy
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Ex 
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x  t   x *  t   dt

April 26, 2005
Week 14
16
Sampling and Aliasing
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Sampling a tone at ft at a rate of fs results
in aliasing to frequencies fk
fk  ft  k  fs
The aliasing order k is any integer – zero,
positive or negative
 The base ambiguity region of a sampled
signal on the next slide
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April 26, 2005
Week 14
17
Ambiguity Range for Complex
Signals
fs

2
April 26, 2005
fs

4
0
Week 14
fs
4
fs
2
18
Sampling a Real Signal
This is what we must do with a real R.F.
signal
 The negative frequency image is always
there even with a quadrature demodulator
(why?)
 Study of the figure reveals
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 Nyquist’s
sampling limit
 Why we want to alias to ± fs/4
April 26, 2005
Week 14
19
Three Types of Error Correcting
Codes
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Convolutional codes
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Block codes
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Most often used
Provide spectrum usage within 2 dB of the Shannon limit with
Viterbi decoding
Good for simple codes such as Hamming codes
Simple to understand and use
Provide a basis for understanding other codes
Recursive codes
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Used in Turbo Codes; achieve almost the Shannon limit
May be the codes of the future
Usage is complicated because output does not terminated
April 26, 2005
Week 14
20
Decoding Simple Block Codes
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Works with correct-one, detect-two codes
Find the syndrome for single-bit errors
Match with the syndrome of the received
message
Invert that bit in the received message to form
the corrected message
Check the syndrome for zero
Invert the coding to find the decoded message
April 26, 2005
Week 14
21
EE551Signal
Processing and
Communication Theory
Fall, 2005
CRN 088905
Thursday evenings in Ft. Washington
James K Beard
April 26, 2005
Week 14
22
EE551 Format and Topics
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Base topics from Sklar
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Review of EE521 topics
Ch. 7 and Ch. 8, Channel Coding: Part 2, Part 3
Ch. 9, Modulation and Coding Trade-Offs
Ch. 5, Communications Link Analysis
Ch. 10, Synchronization
Ch. 11, Multiplexing and Multiple Access
Ch. 12, Spread-Spectrum Techniques
Ch. 14, Encryption and Decryption
Ch. 15, Fading Channels
Others TBD
Term Project in SystemView
Seminar format
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Round-table on specified topics every week
You will present your term project
April 26, 2005
Week 14
23
FINAL IS MAY 10
April 26, 2005
Week 14
24
Your Grade for EE521
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Based on
 Quizzes
 Term
project
 Final exam
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Do well on the Final Examination
 First
exam was fair
 Second exam was good
 Nobody helped themselves with the Quiz 2 Backup
 A good Final Exam grade is paramount
April 26, 2005
Week 14
25
Final Exam Procedure
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Show up here at 6:00 PM SHARP on May 10
Your exam will be waiting
Rules
 No
talking
 Questions
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I
Submitted to me on paper
Responses on whiteboard for all
will pick up exams promptly at 8:00 PM
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April 26, 2005
Check off your problems – don’t miss any
If you get done early, check and re-check your work
Week 14
26