Shengjiao Cao

Time-Domain Blind ICI Mitigation for Non-Constant Modulus Format in CO-OFDM

In this letter, we propose a modified time-domain blind intercarrier interference (ICI) mitigation method for non-constant modulus format (e.g., 16-QAM) in coherent optical orthogonal frequency division multiplexing (CO-OFDM) systems. The new algorithm is derived from a blind ICI (BL-ICI) compensation algorithm over constant modulus format for wireless networks. We proposed new power estimation methods for the algorithm to adapt to non-constant modulus format. Our new BL-ICI algorithm is demonstrated to be effective in removing the ICI in a simulated CO-OFDM system over various formats: 8-QAM, 16-QAM, 32-QAM and 64-QAM. It shares the same order of complexity with frequency domain decision-aided ICI compensation method and does not suffer from symbol decision errors.

Decision-Aided, Pilot-Aided, Decision-Feedback Phase Estimation for Coherent Optical OFDM Systems

We propose two phase estimation methods for coherent optical orthogonal frequency-division multiplexing (OFDM) systems, the decision-aided (DA) plus pilot-aided (PA) scheme and DA plus decision feedback (DF) scheme. We further investigate the combinations of DA, PA, and DF in a simulated 40-Gb/s single polarization system. DA+PA helps to increase the phase noise tolerance of DA and reduce the overhead of PA, while DA+DF reduces the overhead to zero and achieves the best performance with one more step of estimation, compensation, and demodulation.

Decision-Aided Joint Compensation of Transmitter IQ Mismatch and Phase Noise for Coherent Optical OFDM

We study the joint effects of transmitter in-phase/quadrature-phase (IQ) mismatch, channel distortion, and laser phase noise in a coherent optical orthogonal frequency-division-multiplexed system. We propose a decision-aided joint compensation scheme, which separates the transmitter IQ mismatch as well as channel distortion and phase noise compensation into two consecutive steps. Simulation results show this novel technique to be effective in mitigating the three effects simultaneously.

Full-range pilot-assisted frequency offset estimation for OFDM systems

We experimentally demonstrated a novel frequency offset compensation method with the widest estimation range by inserting one pilot-tone at the center of the OFDM spectrum.

Decision-aided joint compensation of channel distortion and transmitter IQ imbalance for coherent optical OFDM

We propose a decision-aided joint compensation scheme of channel distortion and transmitter IQ imbalance for coherent optical orthogonal frequency-division-multiplexed (CO-OFDM) system. In this letter, we compare this decision-aided method with Gram-Schmidt orthogonalization procedure (GSOP) and another pilot-assisted (PA) method through computer simulation. It is demonstrated that our decision-aided method is superior to the other two methods.

Pilot-Aided Log-Likelihood Ratio for LDPC Coded MPSK-OFDM Transmission

The pilot-aided log-likelihood ratio (LLR) is derived from low-density parity-check coded coherent optical M-ary phase-shift keying orthogonal frequency division multiplexing transmission system with laser phase noise. Approximations of the metrics, which yield lower computational cost and comparable performance, are also introduced. The bit error ratio performance is tested through simulation for the proposed LLR metrics.

Pre-distortion versus post-equalization for IQ mismatch compensation in CO-OFDM

We compare pre-distortion and post-equalization schemes for compensating IQ mismatch in coherent Optical OFDM systems. It is shown that pre-distortion scheme has much larger tolerance to both phase and amplitude mismatch compared to post-equalization.

Pilot-Tone Assisted Log-Likelihood Ratio for LDPC Coded CO-OFDM System

Pilot-tone assisted log-likelihood ratio (PT-LLR) is derived for low-density parity-check coded, coherent optical orthogonal frequency division multiplexing systems in the presence of linear phase noise. The knowledge of common phase error (CPE) obtained from the pilot-tone is incorporated into the new LLR metric, which eliminates the need for prior CPE estimation and compensation. We compare our metric with the conventional LLR (C-LLR) through extensive simulation using their approximate versions (APT-LLR, AC-LLR). APT-LLR has the same order of complexity as AC-LLR while it outperforms AC-LLR for higher order modulation formats (16-QAM, 64-QAM) at smaller pilot-tone-to-signal power ratios. In addition, we show that with the help of time-domain blind intercarrier interference mitigation, both metrics perform better in the presence of larger laser linewidth.

A performance investigation of correlation-based and pilot-tone-assisted frequency offset compensation method for CO-OFDM

We carry out a comprehensive analysis to examine the performance of our recently proposed correlation-based and pilot-tone-assisted frequency offset compensation method in coherent optical OFDM system. The frequency offset is divided into two parts: fraction part and integer part relative to the channel spacing. Our frequency offset scheme includes the correlation-based Schmidl algorithm for fraction part estimation as well as pilot-tone-assisted method for integer part estimation. In this paper, we analytically derive the error variance of fraction part estimation methods in the presence of laser phase noise using different correlation-based algorithms: Schmidl, Cox and Cyclic Prefix based. This analytical expression is given for the first time in the literature. Furthermore, we give a full derivation for the pilot-tone-assisted integer part estimation method using the OFDM model.

Pilot-aided Log-likelihood Ratio for LDPC coded M-QAM CO-OFDM System

Pilot-aided log-likelihood ratio as well as its approximation are derived for LDPC coded M-QAM CO-OFDM system with consideration of laser phase noise. Our metric performs better than the conventional metric in 16QAM and 64QAM simulation.