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**Transmit Diversity in TDS-OFDM: Space-Time Shifted CAZAC Sequence**

Explore the application of transmit diversity in Time-Domain Synchronous OFDM using Space-Time Shifted CAZAC sequences for improved performance over wireless fading channels.

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**Transmit Diversity in TDS-OFDM: Space-Time Shifted CAZAC Sequence**

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  1. TDS-OFDM Transmit Diversity Based on Space-Time Shifted CAZAC Sequence Linglong Dai, Jintao Wang, Zhaocheng Wang and Jun Wang Tsinghua University, Beijing, China

  2. Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion

  3. Transmit Diversity • Space Diversity[Alamouti 98] • Reliable transmission over wireless fading channels • Without bandwidth or power penalty • Transmit Diversity • Lower hardware complexity • More attractive for broadcasting systems • Transmit Diversity in DVB-T2 [DVB-T2 09]

  4. About TDS-OFDM • TDS-OFDM (Time domain synchronous OFDM )[Song07] • Key technology of the Chinese digital television terrestrial broadcasting standard • Transmit diversity has not been specified • Key feature: PN sequence instead of cyclic prefix (CP) • Higher spectrum efficiency (10% increased) • Faster synchronization (5% required) • Iterative channel estimation and cyclicity reconstruction[Wang05] • Cyclicity of the IDFT block is destroyed

  5. Transmit Diversity for TDS-OFDM • Transmit Diversity for TDS-OFDM • Space-time block coding (STBC) Based [Wang’06] • Space-time-frequency block coding (STFBC) based [Wang’05-1] • Channel Estimation in standard OFDM Two main assumptions: • Ideal channel estimation • Ideal cyclicity reconstruction of the received IDFT block Problem • Not applicable for TDS-OFDM without pilots 5

  6. Transmit Diversity for TDS-OFDM • Channel Estimation for TDS-OFDM Transmit Diversity • Space-time (ST) coded PN sequence [Yang’09-1] • Hypothesis of static channels along consecutive two signal frames • Suitable for strongly frequency-selective but slow fading channels • Space-frequency (SF) coded Training sequence [Yang’09-2] • Assumption of constant CSI over adjacent two subcarriers • Suitable for fast time-varying but weakly frequency-selective channels Problem: • How about the doubly selective channel ? Failed ! 6

  7. Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 7

  8. CAZAC Based Frame Structure • CAZAC (constant amplitude zero autocorrelation ) Sequence • Constant amplitude both in the time and frequency domains • Perfect autocorrelation [Boemer’92] • Space-Time Shifted CAZAC Sequence • K-symbol cyclic shift in space domain • K-symbol cyclic shift in time domain Channel estimation Cyclicity reconstruction

  9. Receiver Design (1): Channel Estimation • Channel Estimation over Doubly Selective Channels • Received CAZAC sequence • Circular correlation between one local sequence with • The CIR estimates and can be directly extracted • Performance is SNR

  10. Receiver Design (2): Joint Cyclicity Reconstruction

  11. Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 11

  12. Simulation Results (4) Parameters • Central Frequency 770 MHz • Transmit Antennas Tx=2 • Modulation QPSK • Symbol rate 7.56 Ms/s • Length N=3780 Nc=256 K=128 • Channels Brazil D • Receiver velocity 140 km/h. Doubly selective! About 2.5 dB SNR gain at BER of

  13. Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 13

  14. Brief Conclusions • A novel frame structure based on CAZAC sequences shifted both in the time and space domains is proposed • The corresponding receiver design including channel estimation and joint cyclicity reconstruction is presented • Improved BER performance over doubly selective channels • Applicable to single-carrier transmission.

  15. References [Alamouti’98] S. M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Select. Areas Commun., vol. 16, no. 8, pp. 1451–1458, Oct. 1998. [DVB-T2’09] Frame Structure, Channel Coding and Modulation for a Second Generation Digital Terrestrial Television Broadcasting System (DVB-T2). European Standard, ETSI EN 302 755, V1.1.1, Sep. 2009. [Song’07] J. Song, Z. Yang, and L. Yang, “Technique review on Chinese digital terrestrial television broadcasting standard and measurements on some working modes,” IEEE Trans. Broadcast., vol. 53, no. 1, pp. 1–7, May 2007. [Wang’06] J. Wang, J. Song, J. Wang, C. Pan, Z. Yang, and L. Yang, “A general SFN structure with transmit diversity for TDS-OFDM system,” IEEE Trans. Broadcast., vol. 52, no. 2, pp. 245–251, Jun. 2006. [Wang’05-1] J. Wang, Z. Yang, C. Pan, J. Song, and L. Yang, “Design of space-time-frequency transmitter diversity scheme for TDS-OFDM system,” IEEE Trans. Broadcast., vol. 51, no. 3, pp. 759–764, Aug. 2005. [Wang’05-2] J. Wang, Z. Yang, C. Pan, and J. Song, “Iterative padding subtraction of the PN sequence for the TDS-OFDM over broadcast channels,” IEEE Trans. Consumer Electron., vol. 51, no. 11, pp. 1148–1152, Nov. 2005. [Gong’03] Y. Gong and K. B. Letaief, “Low complexity channel estimation for space-time wideband OFDM systems,” IEEE Trans. Wireless Commun., vol. 2, no. 5, pp. 876–882, Sep. 2003. [Yang’09-1] F. Yang, K. Peng, J. Wang, J. Song, and Z. Yang, “Training sequence design for low complexity channel estimation in transmit diversity TDS-OFDM system,” IEICE Trans. Commun., vol. E92-B, no. 6, pp. 2308–2311, Jun. 2009. [Yang’09-2] F. Yang, K. Peng, J. Wang, and Z. Yang, “Channel estimation based on space-time-frequency coded training sequence for transmit diversity system,” IEICE Trans. Commun., vol. E92-B, no. 5, pp. 1901–1903, May 2009. [Boemer’92] L. Boemer and M. Antweiler, “Perfect N-phase sequences and arrays,” IEEE J. Select. Areas Commun., vol. 10, no. 4, pp. 782–789, May 1992. [Lee’00] K. F. Lee and D. B. Williams, “Space-frequency transmitter diversity technique for OFDM systems,” in Proc. IEEE 43th Global Telecommunications Conf. (GLOBECOM’00), San Francisco, CA, Nov. 2000, pp. 1473–1477. [ITU’97] Guideline for Evaluation of Radio Transmission Technology for IMT-2000. ITU-R M.1225, 1997. [SET’00] Digital Television Systems-Brazilian Tests-Final Report. SET/ABERT ANATEL SP, May 2000. 15

  16. Thank you for your suggestions !

  17. Simulation Results (1) Parameters • Transmit Antennas Tx=2 • Modulation QPSK • Symbol rate 7.56 MHz • Length N=3780 Nc=256 K=128 • Channels Vehicular A [ITU’97] • Receiver velocity 28 km/h.

  18. Simulation Results (2) Parameters • Transmit Antennas Tx=2 • Modulation QPSK • Symbol rate 7.56 MHz • Length N=3780 Nc=256 K=128 • Channels Vehicular A • Receiver velocity 140 km/h.

  19. Simulation Results (3) Parameters • Transmit Antennas Tx=2 • Modulation QPSK • Symbol rate 7.56 MHz • Length N=3780 Nc=256 K=128 • Channels Brazil D [SET’00] • Receiver velocity 28 km/h.

  20. Performance Analysis • Computational Complexity • SNR loss during the cyclicity reconstruction • Traditional: • Proposed: • Decreased spectral efficiency 0.395 dB 2.88% (K=128, N=3780)

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