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

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

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]

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

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 5

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 6

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

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

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

Receiver Design (2): Joint Cyclicity Reconstruction

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

Tx=2 Doubly selective! Simulation Results (4) 770 MHz 7.56 Ms/s N=3780 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

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

Improved BER performance over doubly selective channels 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.

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 15

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Tx=2 Simulation Results (1) 7.56 MHz N=3780 Nc=256 K=128 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.

Tx=2 Simulation Results (2) 7.56 MHz N=3780 Nc=256 K=128 Vehicular A 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.

Tx=2 Simulation Results (3) 7.56 MHz N=3780 Nc=256 K=128 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.

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