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Advisor: Yung-An Kao Student: Chi-An Young

嶄新的一階頻域 RLS 等化器 結合通道資訊輔助 Viterbi 解碼器應用於 OFDM 系統中 A Novel one-tap frequency domain RLS equalizer combined with Viterbi decoder using channel state information in OFDM systems. Advisor: Yung-An Kao Student: Chi-An Young. Outline . Introduction Motivation OFDM system

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Advisor: Yung-An Kao Student: Chi-An Young

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  1. 嶄新的一階頻域RLS等化器結合通道資訊輔助Viterbi解碼器應用於OFDM系統中A Novel one-tap frequency domain RLS equalizer combined with Viterbi decoder using channel state information in OFDM systems Advisor: Yung-An Kao Student: Chi-An Young

  2. Outline • Introduction • Motivation • OFDM system • Novel one-tap frequency domain RLS equalizer • Conventional LMS and RLS algorithm • Novel equalizer structure • Viterbi decoding with channel state information • Simulation results • Conclusion & Future work

  3. Motivation • To design a receiver in OFDM systems with following consideration: • Channel effects & frequency offset • complexity • performance

  4. Introduction • The advantage of OFDM’s parallel transmission scheme: • makes the bandwidth more effective

  5. Receive Spectrum Transmit Spectrum Channel Spectrum Channel Training Tone Data Tone Introduction • strongly against multi-path channel • frequency selective channel multiple flat fading sub-channels the sub-channel equalization in frequency domain is simple

  6. Carrier frequency offset (CFO) • CFO is due to the oscillator mismatch from up converter and down converter QPSK, IEEE802.11a spec. no noise CFO=0.01x312.5kHz 43 OFDM symbols

  7. Sampling frequency offset (SFO) • SFO is caused by the oscillator mismatch between A/D & D/A converter QPSK, IEEE802.11a spec. no noise SFO=800Hz 43 OFDM symbols

  8. A modified version of RLS algorithm is used an inner receiver structure from[*] CSI is obtain from 1-tap FEQ [*] Y. A. Kao, C. H. Su, S. K. Lee, C. L. Hsiao and P. L. Chio, 2005, “A robust design of inner receiver structure for OFDM systems,” Digest of technical papers, ICCE, pp.377-378. OFDM receiver block signal affect by channel residual frequency offset , noise, etc.. In the proposed FEQ structure, the scale of the signal constellation must be adjusted

  9. Parameter definition • X : transmitted signal in frequency domain • Y : received signal in frequency domain • Z : equalized signal • Z’ : equalized signal (proposed equalizer) • H : channel response • w : weight coefficient of equalizer • e : error signal • d : desired signal • : LMS step size • : RLS forgetting factor • : correlation matrix • k, l : k-th subcarrier and l-th OFDM symbol • n : n-th de-interleaver output

  10. Inner receiver structure The advantage of this structure is the phase compensation.

  11. One-tap frequency domain LMS equalizer • filtering equation: • weight adaptation

  12. One-tap frequency domain LMS equalizer • filtering equation: • weight adaptation: [**]張晉銓, 2005, “一階遞迴最小平方頻域等化器應用於正交分頻多工系 統之特性分析,” 長庚大學電機工程研究所碩士論文.

  13. Division is used in NLMS and RLS algorithm • NLMS: • RLS:

  14. Novel one-tap frequency domain RLS equalizer(1/6) • filtering equation: • definition of θk,l • update ofθk,l

  15. magnitude: phase Novel one-tap frequency domain RLS equalizer(2/6) • Definition of wk,l: • Rewrite RLS filtering equation:

  16. Novel one-tap frequency domain RLS equalizer(3/6) • From the magnitude part of distorted signal is not fully compensated (QPSK modulation is assumed, CNR=15dB): 1-tap FEQ input 1-tap FEQ output

  17. constellation size multiply by Φk,ltimes : signalequalized by conventional RLS FEQ Φk,l Φk,l 1 -Φk,l Φk,l -1 Φk,l Φk,l 1 -1 -Φk,l Φk,l : signal equalized by proposed RLS FEQ Novel 1-tap frequency domain RLS equalizer(4/6) Im Im 3 3 1 Re Re -3 -1 1 3 -3 3 -1 -3 -3

  18. Novel 1-tap frequency domain RLS equalizer(5/6) • The update equation for Φk,l :

  19. Novel 1-tap frequency domain RLS equalizer(6/6) • division operation is not required • calculation of error signal is not used • must adjust the scale of constellation at symbol de-mapping device Proposed 1-tap FEQ P/S Symbol De-mapping … …

  20. decoding error probability may increase Viterbi Decoding with CSI(1/6) • Basic concept of Viterbi decoding: to select the path on code trellis with the minimum Euclidean distance • Viterbi decoding in OFDM system: • in presence of channel fading, each subcarrier is experiencing different channel condition • if we view each subcarrier with the same reliability, except for that the situation will not be reflected

  21. Viterbi Decoding with CSI(2/6) • We use channel state information (CSI) to reflect different sub-channel fading • Concept: • Adding CSI when calculating the Euclidean distance improve reliability on calculating the Euclidean distance

  22. Viterbi decoding using CSI(3/6) • CSI aided Viterbi decoder block diagram[***] BMC: Branch Metric Calculation SAM: State Accumulate Metric SPM: Survival Path Matrix [***]W. C. Lee, H. M. Park, K. J. Kang and K. B. Kim, “Performance analysis of Viterbi decoder using channel state information in COFDM system,” IEEE Transactions on Broadcasting, Vol. 44, no.4, pp.488-496, Dec. 1998.

  23. Viterbi decoding using CSI(4/6) • The calculation of the Euclidean distance: • When SNR is high enough: :possible transmitted signal

  24. the signal constellation that has been adjusted equalized signal: Viterbi decoding using CSI(5/6) • Adding CSI to D :

  25. Viterbi decoding using CSI(6/6) • Considering: • system complexity • system performance • We use to reflect channel condition

  26. Simulation environments & parameters • IEEE 802.11a standard • Transmission packets = 1000 packets • Transmission data per packets = PSDU 256 Bytes • Exponentially decaying Rayleigh fading with sampling period and RMS time • CFO = 3125 Hz • SFO = 800 Hz • = 0.85 • 6-bit soft decision Viterbi decoding

  27. Simulation result PER performance for IEEE 802.11a (no CSI aided) PER performance for IEEE 802.11a (CSI aided)

  28. conclusion • Division in the proposed algorithm is no longer used • By applying this FEQ structure, we can improve the system performance by CSI aided Viterbi decoder

  29. Future work • Apply the equalizer structure to time-variant channel • The optimal solution of CSI • Hardware implementation

  30. Reference • Y. A. Kao, C. H. Su, S. K. Lee, C. L. Hsiao and P. L. Chio, 2005, “A robust design of inner receiver structure for OFDM systems,” Digest of technical papers, ICCE, pp.377-378. • W. C. Lee, H. M. Park, K. J. Kang and K. B. Kim, “Performance analysis of Viterbi decoder using channel state information in COFDM system,” IEEE Transactions on Broadcasting, Vol. 44, no.4, pp.488-496, Dec. 1998. • 張晉銓, 2005, “一階遞迴最小平方頻域等化器應用於正交分頻多工系統之特性分析,”長庚大學電機工程研究所碩士論文.

  31. Thanks for your attention!

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