We propose and experimentally validate a low-complexity time-domain (TD) digital backpropagation (DBP) algorithm for fiber nonlinearity compensation, targeting an optimized hardware implementation. ...To counteract the coherent accumulation of numerical quantization errors between DBP steps, we propose a random step-size distribution along the optical link (with ±5% interval around the optimal step-size). In addition, to further reduce the average quantization bit precision requirements, we propose a partitioned quantization technique, enabling to quantize the finite-impulse response (FIR) filter tail coefficients with significantly lower precision. The proposed low-complexity DBP algorithm is experimentally demonstrated over a 2592 km long-haul wavelength division multiplexing transmission system with 21×32 GBaud PM-16QAM optical channels. Employing the proposed step-size randomization together with dual-time-slot quantization, we demonstrate penalty-free operation at an average of ~4 b per FIR coefficient, leading to a 60% complexity reduction when compared to the standard TD-DBP implementation.
Carrier phase recovery (CPR) is a key digital signal processing (DSP) subsystem in optical fiber communications. In this paper, we review recent advances in CPR algorithms and analyze their ...performance under the impact of different system impairments in a long-haul 256 GBaud setting. We study both single- and dual-stage CPR configurations, in the linear and nonlinear regimes, and at different system baud rates. By exploiting CPR algorithms tailored for digital subcarrier multiplexing (DSCM) systems, we are able to reap the benefits of their intrinsic resilience to the two main performance-limiting effects of optical fiber systems: equalization enhanced phase noise (EEPN) and nonlinear interference (NLI). Overall, the paper sheds light on the potential benefits of DSCM and highlights the need for further research into CPR algorithms that specifically target the effect of nonlinear phase noise (NLPN).
In a subcarrier multiplexing 125 Gbaud DP-PCS-64QAM system with a net bitrate of 800 Gbit/s, we numerically and experimentally study the performance of transmission-aware carrier phase estimation ...(CPE) for long-haul systems. The recently proposed transmission-aware dual-reference subcarrier CPE (DRS-CPE) is assessed against a more conventional per-subcarrier CPE approach and a transmission-agnostic joint CPE in 8- and 16-subcarrier transmission. At laser linewidths of <inline-formula><tex-math notation="LaTeX">\sim</tex-math></inline-formula>150 kHz, we achieve an experimental signal-to-noise ratio gain over a single-carrier of up to 0.5 dB after 1800 km transmission, enabled by the DRS-CPE.
We demonstrate that, in an actual network, maximizing the transmission spectral efficiency requires not only to take into account transceiver impairments, but also to consider WSS filtering and ...flex-grid specifications. In a WDM transmission experiment over a 10-span link covering <inline-formula> <tex-math notation="LaTeX">\sim 38 </tex-math></inline-formula> nm, we study achievable rate, bitrate and spectral efficiency for different symbol rates of 62.5, 75, 87.5, 100 and 112.5Gbaud with corresponding channel spacings of 75, 87.5, 100, 112.5, 125GHz, therefore accounting for few GHz bandguard required for potential WSS filtering effect, as well as being compatible with flex-grid configuration. While maximum achievable information rate per transceiver (wavelength) is achieved for the lowest symbol rate, and operating at larger symbol rate maximizes the bitrate per wavelength, to maximize overall fiber spectral efficiency (and therefore, capacity), operation at intermediate symbol rates, from 75 to 100Gbaud in our study, is required as a trade-off between the penalties from transceiver impairments and WSS filtering.
Using digital backpropagation (DBP) based on the split step Fourier method (SSFM) aided by a memory polynomial (MP) model, we demonstrate an improved DBP approach for fiber nonlinearity compensation. ...The proposed technique (DBP-SSFM&MP) is numerically validated and its performance and complexity are compared against the benchmark DBP-SSFM, considering a single-channel 336 Gb/s PM-64QAM transmission system. We demonstrate that the proposed technique allows to maintain the performance achieved by DBP-SSFM, while decreasing the required number of iterations, by over 60%. For a transmission length of 1600 km we obtain a complexity reduction gain of 50.7% in terms of real multiplications in comparison with the standard DBP-SSFM.
We propose a novel and efficient multiplierless finite-impulse response (FIR)-based filter architecture for chromatic dispersion equalization (CDE) in coherent optical communication systems. After ...quantizing the FIR coefficients, we take advantage of the high multiplicity of their real and imaginary parts, employing the distributive property of multiplication over addition to sharply reduce the number of multiplication operations, obtaining the distributive FIR-CDE (D-FIR-CDE). Furthermore, the implementation of multiplication operations with shifts and additions allows us to obtain a multiplierless D-FIR-CDE (MD-FIR-CDE). The proposed equalizers are experimentally validated in a 100G polarization-multiplexed (PM)-QPSK long-haul optical link and compared against benchmark FIR-CDE and frequency-domain (FD)-CDE implementations. We demonstrate computational resources savings of over 99% in number of multiplication operations and 40% in number of additions, relatively to the FIR-CDE implementation. In addition, the D-FIR-CDE is also shown to compare favorably relatively to the most widely used FD-CDE, achieving significant gains both in terms of required chip area and latency: more than 99% and 30% fewer multipliers and additions, respectively, and a latency reduction of over 90%. We have also experimentally demonstrated that the performance penalty imposed by the coefficient quantization tends to decrease with increasing propagation length, rendering it as an attractive solution for efficient and high-performance chromatic dispersion compensation in long-haul optical fiber links.
We report our recent advances in the implementation of very high-speed optical channels (targeting 400 Gbps and 1 Tbit) based on multi-carrier super-channels supported by intense digital signal ...processing. We present results on modulation format agnostic polarization demultiplexing, computationally efficient chromatic dispersion equalization, and digital compensation of nonlinear impairments.
Digital signal processing (DSP) is nowadays a key enabling technology in coherent optical transmission systems, triggering an increasing research effort on the development and optimization of DSP ...subsystems. The first DSP development step is often based on the use of simulation tools and offline processing of experimental data. Complementarily, real-time implementation is a critical implementation step to assess the performance and feasibility of advanced DSP subsystems. In this work, we address both the offline and real-time stages of development, supported by an experimental optical testbed. Real-time demonstration is enabled by an FPGA processing platform and 1.25 Gsample/s analog-to-digital conversion.
We present an optimized dual-stage CPR method based on pilot symbols and a blind phase search algorithm (BPS) for M -QAM coherent optical transmission systems, targeting an efficient hardware ...implementation. A comprehensive optimization of the key CPR parameters is performed, including, pilot-rate, number of test phases, angle interval and moving average filter size. The optimization process is validated through numerical assessment of the optical communication system performance using 16-QAM, 64-QAM, and 256-QAM modulation formats at 64 GBaud. The optimized dual-stage CPR is demonstrated to operate with a very low number of test phases in the second stage BPS algorithm. In addition, it supports more than 10× higher laser linewidth than the standard CPR based on pilot symbols. Besides, a reduction of more than 90%, in terms of hardware complexity is achieved with respect to the standalone application of the BPS algorithm. The optimized dual-stage CPR is also validated through hardware implementation based on VHDL, and its gate-level complexity is assessed for a commercial off-the-shelf Xilinx Virtex-7 FPGA.
We experimentally demonstrate that symbol-rate optimization (SRO) provides nonlinear gains in multicarrier systems, even with PCS modulation and realistic DSP. Optimized carrier phase recovery is ...crucial to achieving 0.2 dB gain for 1400 km 800G transmission, out of the ∼0.7 dB theoretical maximum gain we measured.
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