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Indira Rajagopal Joydeep Acharya Madhavi V Ratnagiri Sumathi Gopal

Implementation and Performance Evaluation of an OFDM Modem With Variations in Cyclic Prefix Length and Channel Coding for Different Channels. Indira Rajagopal Joydeep Acharya Madhavi V Ratnagiri Sumathi Gopal. Course: Communication Theory (ECE 545); Rutgers University

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Indira Rajagopal Joydeep Acharya Madhavi V Ratnagiri Sumathi Gopal

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  1. Implementation and Performance Evaluation of an OFDM Modem With Variations in Cyclic Prefix Length and Channel Coding for Different Channels Indira Rajagopal Joydeep Acharya Madhavi V Ratnagiri Sumathi Gopal Course: Communication Theory (ECE 545); Rutgers University Professor: Dr. Predrag Spasojevic

  2. Topics • Introduction to OFDM • Description of OFDM system simulated • Channel Simulation • Results • Conclusions • References OFDM Performance

  3. Introduction to OFDM • Orthogonal Frequency Division Multiplexing; • Part of xDSL, IEEE 802.11a standards • Improves Data rates, such as 56Mbps in IEEE 802.11a OFDM Performance

  4. Introduction to OFDM • Resilience to frequency selective channels Courtesy : “Multicarrier Primer” OFDM Performance

  5. S/P S/P Mapping from size N/2 to N IFFT Modulation (M) Channel Coding Inter Leaving Cyclic Prefix Source Data Modulation (M) To Channel Modulation (M) Modulation (M) OFDM Transmitter Bits per OFDM symbol = (IFFT_Size/2) * log2(M) OFDM Performance

  6. P/S S/P Mapping from size N/2 to N IFFT Modulation Channel Coding Inter Leaving Cyclic Prefix Modulation To Channel Source Data Modulation Modulation Channel Coding Courtesy: 802.11a std. OFDM Performance

  7. P/S S/P Mapping from size N/2 to N IFFT Modulation Channel Coding Inter Leaving Cyclic Prefix Modulation To Channel Source Data Modulation Modulation Interleaving Read out b4 b10 b7 b1 b2 b5 b8 b11 Fill in b3 b6 b9 b12 OFDM Performance

  8. P/S S/P Mapping from size N/2 to N IFFT Modulation Channel Coding Inter Leaving Cyclic Prefix Modulation To Channel Source Data Modulation Modulation Interleaving b4 b10 Read out b1 b7 b1 b4 b7 b10 b2 b5 b8 b11 b3 b6 b9 b12 b2 b5 b8 b11 Fill in b3 b6 b9 b12 OFDM Performance

  9. P/S S/P Mapping from size N/2 to N IFFT Modulation Channel Coding Inter Leaving Cyclic Prefix Modulation To Channel Source Data Modulation Modulation Modulation Schemes Used • QPSK (4-QAM) • 16-QAM • 64-QAM OFDM Performance

  10. P/S S/P Mapping from size N/2 to N IFFT Modulation Channel Coding Inter Leaving Cyclic Prefix Modulation To Channel Source Data Modulation Modulation X1 IFFT Block Re(XN/2) X1 X2 Real Valued samples Mapper XN/2-1 Im(XN/2) XN/2-1 X*N/2-1 XN/2 X*1 N N/2 N N/2 to N Mapper and IFFT X*n-k = Xk where k : 1,… n/2 OFDM Performance

  11. Adding cyclic prefix P/S S/P Mapping from size N/2 to N IFFT Modulation Channel Coding Inter Leaving Cyclic Prefix Modulation To Channel Source Data Modulation Modulation x(n) * h(n) = X(k)H(k) Original N Samples Time Added Prefix Cyclic prefix length OFDM Performance

  12. Channel Models • AWGN: r = s + n • Rayleigh Flat fading: r = ρs + n where ρ is Rayleigh distributed • Rayleigh Frequency Selective: r = h*s + nwhere h is channel impulse response OFDM Performance

  13. S/P FFT Mapping from size N to N/2 Thres- holding P/S Demod Remove Cyclic Prefix From Channel Demod De- Inter Leaving Channel De- Coding Received Bits Demod Demod OFDM Receiver OFDM Performance

  14. Thresholding S/P FFT Mapping from size N to N/2 Thres- holding P/S Demod Remove Cyclic Prefix From Channel Demod De- Inter Leaving Channel De- Coding Received Bits Demod Demod OFDM Performance

  15. Thresholding based on decision regions for 16-QAM OFDM Performance

  16. De-Interleaving S/P Mapping from size N to N/2 FFT Thres- holding P/S Demod Remove Cyclic Prefix Channel De- Coding From Channel Demod De- Inter Leaving Received Bits Demod Demod Read out b1 b2 b3 b1 b2 b3 b4 b5 b6 b7 b8 b9 b10 b11 b12 b4 b5 b6 Fill in b7 b8 b9 b10 b11 b12 OFDM Performance

  17. Topics • Introduction to OFDM • Description of OFDM system • Channel Models • Results • Conclusions • References OFDM Performance

  18. Received Signal Constellation for 16-QAM in Presence of AWGN OFDM Performance

  19. Bit Error Rate for AWGN for various IFFT sizes OFDM Performance

  20. Bit Error Rate for AWGN for various Modulation Schemes OFDM Performance

  21. Bit Error Rate for AWGN with and without Coding OFDM Performance

  22. OFDM spectrum OFDM Performance

  23. Coded and uncoded BER for flat fading Rayleigh channel OFDM Performance

  24. Effect of varying cyclic prefix in a flat fading channel OFDM Performance

  25. Conclusions We were able to demonstrate theperformance of an OFDM system in AWGN channel and a Rayleigh channel with flat fading OFDM Performance

  26. Further work • Obtain BER plots of frequency selective channel with and without coding • Demonstrate change in BER with change in length of cyclic prefix, for a frequency selective channel • Analyze any tradeoffs between coding and length of cyclic prefix OFDM Performance

  27. References • IEEE 802.11a standard • J. M. Cioffi, “A multicarrier primer,” ANSI T1E1.4 Committee Contribution, Nov. 1991. • Rappaport, T.S., “Wireless Communications, Principles and Practice”, Second Edition Prentice Hall. • Sklar Bernard, “Digital Communications” , Pearson Education Asia, Second Edition, 2001. OFDM Performance

  28. Acknowledgements • WINLAB students and faculty: Predrag, Leo, Praveen, Hithesh, Ahmed, Lang, Rueheng, Jassi, Amith, and many more. • Zoran Kostic at Thomson Inc. It was FUN! Thank You for your patience OFDM Performance

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