Abstract
We exploit pilot-aided (PA) transmission enabled by single-sideband (SSB) subcarrier modulation of both quadrature signals in the DSP domain to achieve fully feedforward carrier recovery (FFCR) in single-carrier (SC) coherent systems with arbitrary M-QAM constellations. A thorough mathematical description of the proposed PA-FFCR is presented, its linewidth tolerance is assessed by simulations and compared to other FFCR schemes in literature. Also, implementation and complexity issues of PA-FFCR are presented and briefly compared with other CR schemes. Simulation results show that PA-FFCR performs close to the best known CR technique in the literature with less computation complexity. Quantitatively, for 1 dB optical-signal-to-noise-ratio (OSNR) penalty at BER = 3.8 × 10(-3), PA-FFCR tolerates linewidth-symbol-duration products (Δf.Ts) of 1.5 × 10(-4) (4-QAM), 4 × 10(-5) (16-QAM) and 1 × 10(-5) (64-QAM). Finally, we propose the use of maximum likelihood (ML) phase estimation next to pilot phase compensation. This significantly improves tolerable Δf.Ts values to 7.5 × 10(-4) (4-QAM), 1.8 × 10(-4) (16-QAM) and 3.5 × 10(-5) (64-QAM). It turns out that PA-FFCR with ML always performs better or at least the same compared to other CR techniques known in literature with lower complexity in addition to the fact that pilot information can be as well exploited for tasks other than CR e.g., fiber nonlinearity compensation, with no extra complexity.
Highlights
The ever-increasing data rate demand driven by new Internet applications and the exponential growth of electronic processing speeds are the main drivers behind coherent optical communications research [1,2]
We focus on laser phase noise (PN) compensation and extend the idea of PA transmission for feedforward carrier recovery (FFCR) in SC coherent transmission systems employing arbitrary M-ary quadrature amplitude modulation (M-QAM) constellations
We find that at a 1 dB opticalsignal-to-noise-ratio (OSNR) penalty at a BER of 3.8 × 10−3, PA-FFCR tolerates linewidthsymbol-duration products ∆f.Ts of 1.5 × 10−4 (4-QAM), 4 × 10−5 (16-QAM) and 1 × 10−5 (64QAM) which is close to blind phase search (BPS) but with less computation complexity
Summary
The ever-increasing data rate demand driven by new Internet applications and the exponential growth of electronic processing speeds are the main drivers behind coherent optical communications research [1,2]. In [16], Gnauck et al transmitted a 21.4 Gbaud PDM 64-QAM signal over 400 Km of ULAF These highly dense M-QAM constellations have inherently stringent laser linewidth requirements because of the small distances between adjacent constellation points and high performance linewidth tolerant carrier recovery (CR) algorithms are needed. Other FFCR algorithms with less complexity exist in the literature (e.g., Viterbi and Viterbi phase estimation (VVPE) for QPSK [18] and QPSK partitioning for 16-QAM [19]), but with reduced linewidth tolerance. We proposed the use of pilot-aided (PA) transmission to compensate jointly laser phase noise (PN) and fiber nonlinearity (NL) in a SC PDM-QPSK coherent system [20]. We note that pilot information can be exploited for tasks other than CR including fiber nonlinearity compensation [20], with no additional complexity
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