1. Linglong Dai and Zhaocheng Wang Tsinghua University, Beijing, China
Spectrum-Efficient Coherent Optical Zero Padding OFDM for Future High-Speed Transport Networks
Linglong Dai and Zhaocheng Wang
Tsinghua University, Beijing, China

2. Spectrum-Efficient CO-OFDM
Outline 1
Background 2
Spectrum-Efficient CO-OFDM 3
Simulation Results 4
Conclusions

3. Development [Shieh’06, Shen’11]
Coherent Optical OFDM
Development [Shieh’06, Shen’11]
Based on mature OFDM in wireline/wireless industry
Practical verification for 100 Gb/s+ optical networks
Polarization division multiplexing (PDM) to double the throughput
Key features [Shieh’08 , Jansen’08]
Tolerance to optical CD, PMD, PDL
Optical channel MIMO Jones matrix

4. Channel Estimation for CO-OFDM
Periodically inserted training symbols
Time-multiplexed single-polarization training symbols [Jansen'08]
Orthogonality in the time domain
Improvements
Using multiple training symbols [Shieh'08]
Intra-symbol frequency-domain averaging (ISFA) within the same training symbol [Liu'08]
A pair of correlated dual-polarization (CDP) training symbols [Liu'09] 4

5. Channel Estimation for CO-OFDM
Problems
All subcarriers used as pilots
 Reduce the spectral efficiency
Large interval between training symbols (e.g. 20 OFDM symbols [Liu'08] )
 Lower the channel tracking capability
Utilize optical channel property:
slow time-variation
slow frequency fluctuation 5

6. Spectrum-Efficient CO-OFDM
Outline 1
Background 2
Spectrum-Efficient CO-OFDM 3
Simulation Results 4
Conclusion 6 6

7. Proposed CO-ZP-OFDM structure
Replace cyclic prefix (CP) by zero padding (ZP)save power
Replace perodical traing symbols by polarization-time-frequency (PTF) coded pilots 7

8. Design of the polarization-time-frequency (PTF) coded pilots
Proposed CO-ZP-OFDM
Design of the polarization-time-frequency (PTF) coded pilots
Four-pilot cluster
Polarization-time orthogonal pilots
Polarization-frequency orthogonal pilots
(e.g., a=b=d=1, c=-1 for BPSK)
Pilot power boosting 8

9. Polarization-Time Channel Estimation
Proposed CO-ZP-OFDM
Polarization-Time Channel Estimation
Slow time-variation of optical channel:
When a=b=d=1, c=-1 is used:
Diversity gain can be achieved due to the noise averaging 9

10. Polarization-Frequency Channel Estimation
Proposed CO-ZP-OFDM
Polarization-Frequency Channel Estimation
Slow frequency fluctuation of optical channels:
When a=b=d=1, c=-1 is used:
Diversity gain can be achieved due to the noise averaging 10

11. Performance Analysis Spectral Efficiency Pilot ratio:
( for N=2048)
Increase spectal efficiency by 6.62% 11

12. Performance Analysis Equivalent OSNR Variation
Equivalent OSNR inrease due to ZP
Equivalent OSNR reduction due to pilot power boosting
0. 51 dB when M/N=1/8
Improved channle estimation without obvious OSNR loss 12

13. Spectrum-Efficient CO-OFDM
Outline 1
Background 2
Spectrum-Efficient CO-OFDM 3
Simulation Results 4
Conclusion 13 13

14. Simulation Setup Simulation setup
Similar to the PMD CO-OFDM system in [Buchali’09]
IFFT size N=2048, Guard interval length M=256
16QAM modulation
LDPC code
Line bandwidth of the optical oscillator is 100 kHz
Large CD and PMD are imposed 14 14

15. Simulation Results (1) Better performance than traditional system 15

16. Simulation Results (2)
Only 0.3 dB away from the ideal B2B case 16 16

17. Spectrum-Efficient CO-OFDM
Outline 1
Background 2
Spectrum-Efficient CO-OFDM 3
Simulation Results 4
Conclusion 17 17

18. Brief Conclusions
The CO-ZP-OFDM transmission scheme based on PTF-coded pilots is proposed for future high-speed optical transport networks;
The optical channel properties are fully exploited to achieve high spectral efficiency and reliable performance;
The PTF-coded pilots enable low-complexity PTF channel estimation with high performance due to the obtained diversity gain and pilot power boosting;
The proposed scheme has higher spectral efficiency than conventional CO-OFDM systems, and the reliable performance under severe CD and PMD conditions has also been demonstrated.

19. Thank you !