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應用於 OFDM 系統之強健化 內部接收機架構設計

應用於 OFDM 系統之強健化 內部接收機架構設計. 指導老師:高永安 學 生:蘇家弘. A Robust Inner Receiver Structure Design for OFDM Systems. Outline. OFDM system block diagram OFDM baseband signal model Inner receiver structure Channel estimation LMS algorithm Selection of  Pilot-based phase estimator

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應用於 OFDM 系統之強健化 內部接收機架構設計

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  1. 應用於OFDM 系統之強健化內部接收機架構設計 指導老師:高永安 學 生:蘇家弘 A Robust Inner Receiver Structure Design for OFDM Systems

  2. Outline • OFDM system block diagram • OFDM baseband signal model • Inner receiver structure • Channel estimation • LMS algorithm • Selection of  • Pilot-based phase estimator • Dynamic simulation by Simulink 5.0 • Conclusion and future work

  3. OFDM system block diagram Eq Eq . . . Eq Up convert CFO SFO n: n-th sample point k: k-th subcarrier l: l-th subcarrier Down convert

  4. Carrier Frequency Offsets • CFO simulation • CFO is due to the oscillator mismatch from up convert and down convert f

  5. CFO calculation for IEEE 802.11a • Maximum quantity of CFO = 20ppm for 5GHz • k: k-th subcarrier, l: l-th OFDM symbol , N=64, n=80

  6. SFO is caused by the oscillator mismatch between A/D & D/A converter SFO simulation Sampling Frequency Offsets TTX TRX WhenTRX>TTX

  7. SFO calculation for IEEE 802.11a • TTX=1/(20MHz 400Hz), TRX=1/(20MHz400Hz) • k: k-th subcarrier, l: l-th OFDM symbol , N=64, n=80

  8. OFDM baseband signal model • OFDM baseband signal after IFFT at the transmitter side • The received OFDM baseband signal before FFT -------- (2) -------- (1) n: n-th sample point k: k-th subcarrier l: l-th subcarrier

  9. OFDM baseband signal model • The received OFDM signal is influenced by channel effect, residual CFO, SFO, initial symbol timing offset and before FFT we can describe (2) as follows: -------- (3) • Td :initial symbol timing offset Hk :frequency response of channel : residual CFO : initial phase offset Ts : sampling clock period at the transmitter Ts’: sampling clock period at the receiver CFO SFO

  10. OFDM baseband signal model • The ICI produced by residual CFO is much smaller compared to Gaussian noise. N’k,lcombine Ik,land Nk,l and (3) can be modified as: -------- (4)

  11. OFDM baseband signal model • The effect of CFO and SFO can be represented as : -------- (5) and

  12. The difference between inner and outer receiver • M. Speth, S. A. Fechtel, G. Fock and H. Meyr, “Optimum Receiver Design for Wireless Broad-band Systems Using OFDM-Part II,” IEEE Trans. Commun., vol. 49, pp.571-578, Apr. 2001. Decoding & demodulation

  13. Inner receiver structure Input signal FFT

  14. Inner receiver structure Training sequence Initial coefficient FFT D a t a Update coefficient of equalizer Phase compensation Frequency Domain Equalizer Pilot Pilot-based phase estimator Phase compensation Hard decision Outer receiver

  15. Channel estimation by least square error • Lk,l : transmitted training sequence Rk,l : received training sequence : equalized training sequence Hk :channel Nk,l : noise : equalizer initial coefficient • Equalized training sequence  l : 2 long training symbol k : 52 subcarrier In 802.11a

  16. Channel estimation by least square error • Error between transmitted signal and equalized signal • Find optimal Heq,k forminimum value of ek  Setting the partial derivative of ek

  17. LMS algorithm • Filtering output: Yk=wkHXk Error estimation: ek=dk-Yk • Tap-weight vector adaptation * After hard decision kis the step size that modified by channel condition

  18. selection of  • Normalized-LMS & time average Training sequence 0 <  < 1

  19. Pilot-based phase estimator Received pilots After giving the appropriate weight Maximum ratio combination (MRC) pilot C’ Im Im A’ C A B B’ ∠2 ∠1 O Re Re O

  20. Simulation by Simulink 5.0

  21. Unequalized signal spacing plot Channel A (Ts=50ns, TRMS=50ns ) SNR=10dB Residual CFO =0.01 SFO=800Hz (Ts=1/(20MHz-400), =1/(20MHz+400 ) Code rate=1/2, QPSK 44 OFDM symbol per packet 1000 packet

  22. After channel equalization Applied the proposed inner receiver structure

  23. IEEE 802.11a PER v.s. SNR =0.15 =0.3 Channel A (Ts=50ns, TRMS=50ns ) Residual CFO =0.01 SFO=800Hz (Ts=1/(20MHz-400), =1/(20MHz+400 ) PSDU=256 bytes 1000 packet

  24. Channel B (Ts=50ns, TRMS=100ns ) Channel C (Ts=50ns, TRMS=150ns ) IEEE 802.11a PER v.s. SNR

  25. Channel D (Ts=50ns, TRMS=200ns ) Channel E (Ts=50ns, TRMS=250ns ) IEEE 802.11a PER v.s. SNR

  26. PER v.s. SNR with different  =0.3 Channel A (Ts=50ns, TRMS=50ns ) Residual CFO =0.01 SFO=800Hz (Ts=1/(20MHz-400), =1/(20MHz+400 ) 44 OFDM symbol per packet 1000 packet

  27. Channel C (Ts=50ns, TRMS=150ns ) Channel E (Ts=50ns, TRMS=250ns ) PER v.s. SNR with different 

  28. PER v.s. SNR with different  =0.3 Channel A (Ts=50ns, TRMS=50ns ) Residual CFO =0.01 SFO=800Hz (Ts=1/(20MHz-400), =1/(20MHz+400 ) 200 OFDM symbol per packet 1000 packet

  29. Channel C (Ts=50ns, TRMS=150ns ) Channel E (Ts=50ns, TRMS=250ns ) PER v.s. SNR with different 

  30. PER v.s.  with different channel =0.3 Channel A (Ts=50ns, TRMS=50ns ) Channel C (Ts=50ns, TRMS=150ns ) Channel E (Ts=50ns, TRMS=250ns ) SNR=10dB Residual CFO =0.01 SFO=800Hz (Ts=1/(20MHz-400), =1/(20MHz+400 ) 44 OFDM symbol per packet 1000 packet

  31. PER v.s.  with different channel =0.3 Channel A (Ts=50ns, TRMS=50ns ) Channel C (Ts=50ns, TRMS=150ns ) Channel E (Ts=50ns, TRMS=250ns ) SNR=10dB Residual CFO =0.01 SFO=800Hz (Ts=1/(20MHz-400), =1/(20MHz+400 ) 200 OFDM symbol per packet 1000 packet

  32. Conclusion • A Robust Inner Receiver Structure • Simulink 5.0  pilot-based phase estimator 1. Compensate the residual CFO 2. Assist the LMS equalizer in phase tracking  Dynamic simulation

  33. Future work • At present • Frequency selective fading channel • LMS algorithm • The future work • Slow fading channel • Other adaptive algorithms • Decoding block of Simulink 5.0

  34. Reference • IEEE Std 802.11a-1999, Part 11: Wireless LAN Medium Access Control (MAC) and Physical Layer (PHY) specifications: High Speed Physical Layer in the 5GHz Band. • Yung-An Kao; Chia-Hung Su; Shih-Kai Lee; Chung-Lung Hsiao; Po-Lin Chio, “A robust design of inner receiver structure for OFDM systems,” IEEE Conference. on Consumer Electronics, pp. 377-378, Jan. 2005. • S. Haykin, Adaptive Filter Theory, Englewood Cliffs, NJ: Prentice-Hall, 2002, 4th Ed. • M. Speth, S. A. Fechtel, G. Fock and H. Meyr, “Optimum Receiver Design for Wireless Broad-band Systems Using OFDM-Part I,” IEEE Trans. Commun., vol. 47, pp. 1668-1677, Nov. 1999. • M. Speth, S. A. Fechtel, G. Fock and H. Meyr, “Optimum Receiver Design for Wireless Broad-band Systems Using OFDM-Part II,” IEEE Trans. Commun., vol. 49, pp.571-578, Apr. 2001. • Doufexi, A.; Armour, S.; Butler, M.; Nix, A.; Bull, D.; McGeehan, J.; Karlsson, P., “A comparison of the HIPERLAN/2 and IEEE 802.11a wireless LAN standards,” IEEE Magazine on Comm. Vol. 40, pp.172-180, May 2002. • 黃凡維, 2004, “一揭最小均方差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

  35. Any Questions?

  36. Channel estimation by LSE

  37. Channel estimation by LSE • Applying we obtain and R:real part I:imaginary part

  38. Channel estimation by LSE

  39. Comparison between the MRC pilot and the original pilot multiplication • MRC pilot Only add MRC pilot

  40. Comparison between the MRC pilot and the original pilot addition • Original pilot multiplication After mutual multiplying

  41. MRC pilot 4 pilot multiplying Channel A

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