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

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

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

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

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

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

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

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

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

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

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

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

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

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

Simulation Results (1) Better performance than traditional system 15

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

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

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.

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