1. 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

2. Transmit Diversity in TDS-OFDM
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
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

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 6

7. Transmit Diversity in TDS-OFDM
Contents 1
Background 2
Transmit Diversity in TDS-OFDM 3
Simulation Results 4
Conclusion 7 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. Transmit Diversity in TDS-OFDM
Contents 1
Background 2
Transmit Diversity in TDS-OFDM 3
Simulation Results 4
Conclusion 11 11

12. 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

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

14. 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.

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
[DVB-T2’09] Frame Structure, Channel Coding and Modulation for a Second Generation Digital Terrestrial Television Broadcasting System (DVB-T2). European Standard, ETSI EN , V1.1.1, Sep
[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
[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
[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
[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
[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
[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 ITU-R M.1225, 1997.
[SET’00] Digital Television Systems-Brazilian Tests-Final Report. SET/ABERT ANATEL SP, May 2000. 15 15

16. Thank you for your suggestions !

17. 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.

18. 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.

19. 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.

20. 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)