April 26, 2005Week 14 1 EE521 Analog and Digital Communications James K. Beard, Ph. D....
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Transcript of April 26, 2005Week 14 1 EE521 Analog and Digital Communications James K. Beard, Ph. D....
April 26, 2005 Week 14 1
EE521 Analog and Digital CommunicationsJames K. Beard, Ph. D.
Tuesday, March 29, 2005
http://astro.temple.edu/~jkbeard/
Week 14 2April 26, 2005
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Week 14 3April 26, 2005
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
Week 14 4April 26, 2005
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
Hand out and collect forms Deliver to office here at Ft. Washington Main Office
Week 14 5April 26, 2005
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
Week 14 6April 26, 2005
Modulation Problems with FSK
16 bits requires 16 X 7+ kHz or 120+ kBPS It’s a lot of bandwidth I.F. forced higher than the 450 kHz first mentioned SystemView sample rate forced to high rates
The MFSK token in the Communications library performs quantization 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
Week 14 7April 26, 2005
Getting the Bit Stream with MFSK Go from input to characters
Go from characters to bit stream
Scale and shift characters for MFSK modulator
Week 14 8April 26, 2005
Modulation Problems with MPSK No high-level token for MPSK
Use two or four bit symbolsUse Quad Mod, sine function or PM to get
PSKUse PSK Demod to get back characters
Gray code required to achieve theoretical BER
Character rate 8 to 16 times sample rateSame bandwidth problems as MFSK
Week 14 9April 26, 2005
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
Week 14 10April 26, 2005
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
Week 14 11April 26, 2005
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
Week 14 12April 26, 2005
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
Week 14 14April 26, 2005
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.
Week 14 15April 26, 2005
Principles of Complex Signals
Multiplication
Multiplication between a complex number and the complex conjugate of another
1 2 1 1 2 2
1 2 1 2 1 2 2 1
z z x j y x j y
x x y y j x y x y
*1 2 1 1 2 2
1 2 1 2 1 2 2 1
1 2 1 2
z z x j y x j y
x x y y j x y x y
z z j z z
Week 14 16April 26, 2005
Power and Energy of Complex Signals Power
Energy
2
*
2
1lim
T
x TT
P x t x t dtT
*xE x t x t dt
Week 14 17April 26, 2005
Sampling and Aliasing
Sampling a tone at ft at a rate of fs results in aliasing to frequencies fk
The aliasing order k is any integer – zero, positive or negative
The base ambiguity region of a sampled signal on the next slide
k t sf f k f
Week 14 18April 26, 2005
Ambiguity Range for Complex Signals
02sf
2sf 4
sf
4sf
Week 14 19April 26, 2005
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 revealsNyquist’s sampling limitWhy we want to alias to ± fs/4
Week 14 20April 26, 2005
Three Types of Error Correcting Codes Convolutional codes
Most often used Provide spectrum usage within 2 dB of the Shannon limit with
Viterbi decoding Block codes
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
Week 14 21April 26, 2005
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 22
EE551Signal Processing and Communication Theory
Fall, 2005CRN 088905Thursday evenings in Ft. WashingtonJames K Beard
Week 14 23April 26, 2005
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
Week 14 24April 26, 2005
FINAL IS MAY 10FINAL IS MAY 10
Week 14 25April 26, 2005
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
Week 14 26April 26, 2005
Final Exam Procedure
Show up here at 6:00 PM SHARP on May 10 Your exam will be waiting Rules
No talking Questions
Submitted to me on paper Responses on whiteboard for all
I will pick up exams promptly at 8:00 PM Check off your problems – don’t miss any If you get done early, check and re-check your work