Systems with Reduced Complexity Transmit Diversity Scheme for TDS-OFDM Systems with Reduced Complexity Linglong Dai, Zhaocheng Wang, Jintao 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 [1] Reliable transmission over wireless fading channels Without bandwidth or power penalty Transmit Diversity More attractive for broadcasting systems Lower overall hardware complexity Transmit Diversity in DVB-T2 [2]

About TDS-OFDM TDS-OFDM (Time domain synchronous OFDM ) [3] 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)

Transmit Diversity for TDS-OFDM Space-time block coding (STBC) Based [4] Space-time-frequency block coding (STFBC) based [5] Channel Estimation in MIMO-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 [6] Hypothesis of static channels along consecutive two signal frames Suitable for deeply frequency-selective but slow fading channels Space-frequency (SF) coded training sequence (TS) [7] Assumption of constant channel 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

Space Shifted CAZAC Sequences CAZAC (constant amplitude zero autocorrelation ) Sequence Constant amplitude both in the time and frequency domains [9] Perfect autocorrelation Space Shifted CAZAC Sequences for Different Transmit Antennas

CAZAC Based Frame Structure Proposed two flexible frame structures Type 1: Same postfix based frame structure (SPFS) Type 2: Alternate postfix based frame structure (APFS) 9 9

Transmit Diversity for TDS-OFDM Received IDFT block Transmitted Signal channel AWGN 10 10

Receiver Design: Channel Estimation CAZAC-Based Channel Estimation Received CAZAC sequence Circular correlation between one local sequence with The CIR estimates and can be directly extracted Performance is SNR

Receiver Design: Channel Estimation Linear interpolation over fast fading channels The subfreme number M should be small over fast fading channels e.g., M = 8 for slow fading channels e.g., M = 2 for very fast fading channels The tradeoff between spectral efficiency and performance jth superframe (j+1)th superframe mth subframe 12 12

Receiver Design: Cyclicity Reconstruction e.g., SPFS

Performance Analysis Spectral Efficiency 2.88% Higher than conventional schemes when M>3 2.88% SPFS and APFS have similar efficiency, SPFS is preferred due to its flexibility (N=3780,K=256, M=2) Computational Complexity 7.20% of the ST coded PN based solution [6], and 6.71% of the SF coded TS based scheme [7] 14 14

Transmit Diversity in TDS-OFDM Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 15 15

SPFS and APFS have very close performances Simulation Results (1) Parameters: Central Frequency 770 MHz Signal Bandwidth 7.56 MHz Transmit Antennas Tx=2 Modulation QPSK Length N=3780 K=256 M=2 Channels Brazil E [10] Vehicular B [11] Receiver velocity 28/140 km/h SPFS vs. APFS over Brazil E Channel SPFS and APFS have very close performances 16 16

The proposed scheme has the best performance Simulation Results (2) Brazil E (weekly frequency-selective) channel, 140 km/h The proposed scheme has the best performance

Simulation Results (3) Vehicular B (deeply frequency-selective) channel, 140 km/h The SF coded TS based solution [7] can not work The proposed scheme has the best performance The cost is the reduced spectral efficiency of 2.55% 18 18

Transmit Diversity in TDS-OFDM Contents 1 Background 2 Transmit Diversity in TDS-OFDM 3 Simulation Results 4 Conclusion 19 19

Brief Conclusions A simple transmit diversity scheme is proposed for TDS-OFDM systems with reduced complexity; The space shifted CAZAC sequence is proposed for channel estimation; Two types of flexible frame structures called SPFS and APFS are proposed to reconstruct the cyclicity of the received IDFT block without prior channel information; The receiver complexity: less than 10% The better BER performance over the doubly selective channels can be achieved at the cost of the small loss in spectral efficiency.

References M. Alamouti, “A simple transmit diversity technique for wireless communications,” IEEE J. Select. Areas Commun., vol. 16, no. 8, pp. 1451–1458, Oct. 1998. 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. J. Fu, J. Wang, J. Song, C. Pan, and Z. Yang, “A simplified equalization method for dual PN-sequence adding TDS-OFDM systems,” IEEE Trans. Broadcast., vol. 54, no. 4, pp. 825–830, Dec. 2008. 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, Jan. 2006. 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. Consum. Electron., vol. 51, no. 3, pp. 759–764, Aug. 2005. F. Yang, K. Peng, J. Wang, J. Song, and Z. Yang, “Training sequence design for low complexity channel estimation in transmit diversity TDSOFDM system,” IEICE Trans. Commun., vol. E92-B, no. 6, pp. 2308–2311, Jun. 2009. F. Yang, K. Peng, J. Wang, J. Song, 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. B. Muquet, Z. Wang, and G. B. Giannakis, “Cyclic prefixing or zero padding for wireless multicarrier transmissions?” IEEE Trans. Commun., vol. 50, no. 12, pp. 2136–2148, Dec. 2002. 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. Guidelines and Techniques for the Evaluation of DTTB Systems. ITU-R Document 6E/TEMP/131-E, 2003. Guideline for Evaluation of Radio Transmission Technology for IMT-2000. Recommendation ITU-R M.1225, 1997. 21 21

ありがとう Thank you for your suggestions !

Appendix: APFS 23 23