Abstract
Polarization dependent loss (PDL) causes imbalanced optical signal to noise ratio (OSNR) of the two polarizations, thus remains one of the major bottlenecks for next-generation polarization-division-multiplexed (PDM) coherent optical transmission systems. In this paper, we investigate Pairwise Coding for adaptive PDL mitigation in PDM coherent optical systems. By pre-coding across two polarizations, the PDL-induced performance degradation can be largely mitigated without any coding overhead. We present details of the coding and de-coding design, and also derive the analytical symbol/bit error rate of the Polarization Pairwise Coding scheme, which can be used to predict the performance gain as well as for optimal rotation angle calculation. Simulation results verify that Pairwise Coding achieves substantial system performance gains over a wide range of PDL values. Compared with other digital coding techniques, Polarization Pairwise Coding shows improved performance than Walsh-Hadamard transform since it maximizes the coordinate diversity; and also Pairwise Coding is computationally much simpler to decode compared with the Golden and Silver Codes, therefore is practical for current 100-Gb/s and future 400-Gb/s and 1-Tb/s digital coherent transceivers.
Highlights
The combination of advanced modulation formats, polarization-division-multiplexing (PDM) and digital coherent receivers enables next-generation high-capacity optical transmission [1]
We present a theoretical analysis for the Pairwise Coding and decoding design, and derive the analytical symbol error rate (SER) and bit error rate (BER) for pairwise coded signals, which leads to accurate prediction of the performance gain and makes the determination of the optimal rotation angle easier
It is worth mentioning that, we have only considered the system performance without forward error correction (FEC) codes, it has been reported that a combination of constellation rotation and FEC codes can achieve further system performance gains over fading wireless channels [25]; there may be some potential benefits by combining advanced FEC techniques with Polarization Pairwise Coding
Summary
The combination of advanced modulation formats, polarization-division-multiplexing (PDM) and digital coherent receivers enables next-generation high-capacity optical transmission [1]. Pairwise Coding was first applied to single-input-single-output systems to improve the performance over fading channels [14], where the I and Q components are interleaved to allow different channel gains, mitigating the imbalanced SNRs of the I and Q components. It was extended to MIMO wireless systems, where pairing of sub-channels with different signal-tointerference-and-noise-ratios (SINR), using the same rotation angle and exchanging the real/imaginary parts between different sub-channels, improved the overall BER performance [15]. This Pairwise Coding improved the receiver sensitivity for direct-detection optical orthogonal-frequency-division multiplexing (OFDM) [16]. By comparing with alternative techniques including the Walsh-Hadamard transform and Golden and Silver Codes, using numerical simulations, we show that Pairwise Coding is a good candidate to be integrated into current 100 Gb/s, future 400 Gb/s and 1 Tb/s digital coherent transceivers
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