EE521 Analog and Digital Communications
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Transcript EE521 Analog and Digital Communications
EE521 Analog and
Digital
Communications
James K. Beard, Ph. D.
[email protected]
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
Text: Bernard Sklar, Digital Communications, Second
Edition
SystemView
Office
E&A 349
MWF 10:30 AM to 11:30 AM
Hours during Finals Week TBA
Term Projects Due TODAY, April 26
Final Exam
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
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
April 26, 2005
Hand out and collect forms
Deliver to office here at Ft. Washington Main Office
Week 14
4
Term Project
Generate a frequency sweep
Start frequency:
End frequency:
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
16 bits requires 16 X 7+ kHz or 120+ kBPS
The MFSK token in the Communications library performs
quantization
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
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
Go from input to characters
Go from characters to bit stream
Scale and shift characters for MFSK
modulator
April 26, 2005
Week 14
7
Modulation Problems with
MPSK
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
Same
April 26, 2005
bandwidth problems as MFSK
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Other Operations
Convert from baseband to a carrier frequency
Not
required for channel models
Channel models take complex input
Carrier frequency is a parameter
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
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The Channel Models
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
Generate I.F. centered at zero or use quadrature
demodulator for input to channel modles
April 26, 2005
Week 14
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BER Measurement
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
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
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Coding/Decoding
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
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
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EE521 Analog and
Digital
Communications
Review Topics
[email protected]
April 26, 2005
Week 14
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Complex Signals
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
Multiplication
z1 z2 x1 j y1 x2 j y 2
x1 x2 y1 y 2 j x1 y 2 x2 y1
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
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Power and Energy of Complex
Signals
Power
1
Px lim
T T
T
2
x t x * t dt
T
2
Energy
Ex
x t x t dt
*
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Sampling and Aliasing
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
April 26, 2005
Week 14
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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
Nyquist’s
sampling limit
Why we want to alias to ± fs/4
April 26, 2005
Week 14
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Three Types of Error Correcting
Codes
Convolutional codes
Block codes
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
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
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Decoding Simple Block Codes
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
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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
Base topics from Sklar
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
Round-table on specified topics every week
You will present your term project
April 26, 2005
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FINAL IS MAY 10
April 26, 2005
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Your Grade for EE521
Based on
Quizzes
Term
project
Final exam
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
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Final Exam Procedure
Show up here at 6:00 PM SHARP on May 10
Your exam will be waiting
Rules
No
talking
Questions
I
Submitted to me on paper
Responses on whiteboard for all
will pick up exams promptly at 8:00 PM
April 26, 2005
Check off your problems – don’t miss any
If you get done early, check and re-check your work
Week 14
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