EE521 Analog and Digital Communications

1. EE521 Analog and Digital Communications James K. Beard, Ph. D. jkbeard@temple.edu Tuesday, March 29, 2005 http://astro.temple.edu/~jkbeard/ Week 14

2. Attendance Week 14

3. 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

4. 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

5. 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

6. 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

7. 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

8. 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 bandwidth problems as MFSK Week 14

9. 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

10. 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

11. 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

12. 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

13. EE521 Analog and Digital Communications Review Topics jkbeard@temple.edu Week 14

14. 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

15. Principles of Complex Signals • Multiplication • Multiplication between a complex number and the complex conjugate of another Week 14

16. Power and Energy of Complex Signals • Power • Energy Week 14

17. 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 Week 14

18. Ambiguity Range for Complex Signals Week 14

19. 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 Week 14

20. 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

21. 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 Week 14

22. EE551Signal Processing and Communication Theory Fall, 2005 CRN 088905 Thursday evenings in Ft. Washington James K Beard Week 14

23. 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

24. FINAL IS MAY 10 Week 14

25. 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

26. 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 Week 14