1. 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

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 [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]

4. 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)

5. 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

6. 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

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

8. 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

9. 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

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

11. 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

12. 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

13. Receiver Design: Cyclicity Reconstruction
e.g., SPFS

14. 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

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

16. 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

17. 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

18. 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

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

20. 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.

21. References
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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 Recommendation ITU-R M.1225, 1997. 21 21

22. ありがとう
Thank you for your suggestions !

23. Appendix: APFS 23 23