| 1. |
- Chen, Zexin, et al.
(författare)
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Transmitter optimization for PS-QAM signal in high spectral efficiency metro-transmission
- 2023
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Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 41, s. 1-10
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Tidskriftsartikel (refereegranskat)abstract
- Probabilistic constellation shaping (PCS) brings unprecedented flexibility to future optical networks. However, PCS combined with higher-order modulation formats and higher symbol rates puts more demands on the transmitter. For metro-transmission with high spectral efficiency (SE), transmitter impairments are responsible for a major portion of the system constraints. To address this issue, in this work we study the impact of transmitter impairments on probabilistically shaped quadrature amplitude modulation (PS-QAM) signals. First, we investigate the impact of transmitter impairments and radio frequency (RF) amplifier noise on the optimum shaping factor and the modulation depth within a fiber link. Then, we explore how generalized mutual information (GMI) and the shaping factor are affected by the noise from the transmitter. Next, we first jointly optimize the arcsine swing and the clipping ratio, in order to mitigate the transmitter impairments for the PS-QAM signal, when the modulation depth, the noise from the transmitter, the noise from the fiber optical channel, and the shaping factor are fixed. Then, we verify its validity for various configurations of transmission system. The optimized parameters primarily depend on the noise from the transmitter and turn out to be insensitive to the noise from the fiber optical link and the shaping factor. Finally, we experimentally verify the optimization strategy with PS-256QAM in back-to-back (B2B) transmission.
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| 2. |
- He, Zonglong, 1994, et al.
(författare)
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Inter-Channel Interference Cancellation for Long-Haul Superchannel System
- 2024
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Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 42:1, s. 48-56
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Tidskriftsartikel (refereegranskat)abstract
- Optical frequency combs enable superchannel systems with dense channel spacing by providing equally-spaced lines to overcome the relative frequency drifts of free-running lasers, achieving high spectral efficiency. However, strong inter-channel interference is induced in densely spaced systems due to imperfect pulse-shaping, preventing the further improvement of system throughput. By including the spectra information from the adjacent channels, comb-based multi-channel equalization has been previously validated in a back-to-back and 80 km transmission system for linear crosstalk mitigation. In this case, the limiting factor of the system was the effective number of bits (ENOB). Here, we investigate the influence of amplified spontaneous emission noise and the effectiveness of joint equalization in long-haul transmission systems. We show that the performance of the multi-channel equalizer (MCE) is significantly degraded when reducing the optical-signal-to-noise ratio (OSNR), especially for high-order modulation formats, in both noise-loading and long-distance transmission experiments. Moreover, the ASE noise from the neighboring channels can be coupled into the systems by the MCE, degrading the performance improvement. We find that a large-tap MCE can maximize the system performance in the high OSNRs, whereas MCE with a smaller tap should be applied in low OSNRs to alleviate the effect of ASE noise. After transmission of 800 km, an AIR increase of more than 0.1 bits/s/Hz is enabled by the optimized MCE for 16-QAM and 64-QAM. Our results confirm the validity of the joint equalization gain for short-distance transmission but also clearly demonstrate the performance penalty originating from transmission optical noise. Moreover, the equalizer tap number needs to be optimized for each OSNR to maximize the performance of joint equalization. We believe this limitation is also valid for other impairments such as optical noise in the comb and electrical noise in the transceiver.
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| 3. |
- Jamali, Mohammad Vahid, et al.
(författare)
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Statistical studies of fading in underwater wireless optical channels in the presence of air bubble, temperature, and salinity random variations
- 2018
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Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 66:10, s. 4706-4723
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Tidskriftsartikel (refereegranskat)abstract
- Optical signal propagation through underwater channels is affected by three main degrading phenomena, namely, absorption, scattering, and fading. In this paper, we experimentally study the statistical distribution of intensity fluctuations in underwater wireless optical channels with random temperature and salinity variations, as well as the presence of air bubbles. In particular, we define different scenarios to produce random fluctuations on the water refractive index across the propagation path and, then, examine the accuracy of various statistical distributions in terms of their goodness of fit to the experimental data. We also obtain the channel coherence time to address the average period of fading temporal variations. The scenarios under consideration cover a wide range of scintillation index from weak to strong turbulence. Moreover, the effects of beam-expander-and-collimator (BEC) at the transmitter side and aperture averaging lens (AAL) at the receiver side are experimentally investigated. We show that the use of a transmitter BEC and/or a receiver AAL suits single-lobe distributions, such that the generalized Gamma and exponentiated Weibull distributions can excellently match the histograms of the acquired data. Our experimental results further reveal that the channel coherence time is on the order of 10-3 s and larger which implies to the slow fading turbulent channels.
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| 4. |
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| 5. |
- Li, Shen, 1994, et al.
(författare)
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Designing Voronoi Constellations to Minimize Bit Error Rate
- 2021
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Ingår i: IEEE International Symposium on Information Theory - Proceedings. - 2157-8095. ; 2021-July, s. 1017-1022
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Konferensbidrag (refereegranskat)abstract
- In a classical 1983 paper, Conway and Sloane presented fast encoding and decoding algorithms for a special case of Voronoi constellations (VCs), for which the shaping lattice is a scaled copy of the coding lattice. Feng generalized their encoding and decoding methods to arbitrary VCs. Less general algorithms were also proposed by Kurkoski and Ferdinand, respectively, for VCs with some constraints on their coding and shaping lattices. In this work, we design VCs with a cubic coding lattice based on Kurkoski's encoding and decoding algorithms. The designed VCs achieve up to 1.03 dB shaping gains with a lower complexity than Conway and Sloane's scaled VCs. To minimize the bit error rate (BER), pseudo-Gray labeling of constellation points is applied. In uncoded systems, the designed VCs reduce the required SNR by up to 1.1 dB at the same BER, compared with the same VCs using Feng's and Ferdinand's algorithms. In coded systems, the designed VCs are able to achieve lower BER than the scaled VCs at the same SNR. In addition, a Gray penalty estimation method for such VCs of very large size is introduced.
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| 6. |
- Li, Shen, 1994, et al.
(författare)
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Low-Complexity Voronoi Shaping for the Gaussian Channel
- 2022
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Ingår i: IEEE Transactions on Communications. - 0090-6778 .- 1558-0857. ; 70:2, s. 865-873
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Tidskriftsartikel (refereegranskat)abstract
- Voronoi constellations (VCs) are finite sets of vectors of a coding lattice enclosed by the translated Voronoi region of a shaping lattice, which is a sublattice of the coding lattice. In conventional VCs, the shaping lattice is a scaled-up version of the coding lattice. In this paper, we design low-complexity VCs with a cubic coding lattice of up to 32 dimensions, in which pseudo-Gray labeling is applied to minimize the bit error rate. The designed VCs have considerable shaping gains of up to 1.03 dB and finer choices of spectral efficiencies in practice compared with conventional VCs. A mutual information estimation method and a log-likelihood approximation method based on importance sampling for very large constellations are proposed and applied to the designed VCs. With error-control coding, the proposed VCs can have higher information rates than the conventional scaled VCs because of their inherently good pseudo-Gray labeling feature, with a lower decoding complexity.
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| 7. |
- Li, Shen, 1994, et al.
(författare)
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Power-Efficient Voronoi Constellations for Fiber-Optic Communication Systems
- 2023
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Ingår i: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 41:5, s. 1298-1308
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Tidskriftsartikel (refereegranskat)abstract
- Voronoi constellations (VCs) are considered as an effective geometric shaping method due to their high power efficiencies and low complexity. In this paper, the performance of 16- and 32-dimensional VCs with a variety of spectral efficiencies transmitted in the nonlinear fiber channel are investigated. Both single-channel and wavelength-division multiplexing systems are considered for the transmission of the VCs, as well as different signal processing schemes, including chromatic dispersion compensation and digital backpropagation. Multiple performance metrics including the uncoded bit error rate, mutual information (MI), and generalized mutual information (GMI) of VCs are evaluated. Compared with quadrature amplitude modulation (QAM) formats, the VCs provide 1.0--2.4 dB launch power gains, up to 0.50 bits/symbol/dimension-pair MI gains, up to around 30% potential reach increase at the same MI, and up to 0.30 bits/symbol/dimension-pair GMI gains in a limited launch power range. The observed performance gains over QAM are found higher than in the back-to-back case. Moreover, a general GMI estimation method for very large constellations using importance sampling is proposed for the first time.
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| 8. |
- Lundberg, Lars, 1989, et al.
(författare)
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Phase-coherent lightwave communications with frequency combs
- 2020
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Ingår i: Nature Communications. - : Springer Science and Business Media LLC. - 2041-1723 .- 2041-1723. ; 11:1
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Tidskriftsartikel (refereegranskat)abstract
- Fiber-optical networks are a crucial telecommunication infrastructure in society. Wavelength division multiplexing allows for transmitting parallel data streams over the fiber bandwidth, and coherent detection enables the use of sophisticated modulation formats and electronic compensation of signal impairments. Optical frequency combs can replace the multiple lasers used for the different wavelength channels. Beyond multiplexing, it has been suggested that the broadband phase coherence of frequency combs could simplify the receiver scheme by performing joint reception and processing of several wavelength channels, but an experimental validation in a fiber transmission experiment remains elusive. Here we demonstrate and quantify joint reception and processing of several wavelength channels in a full transmission system. We demonstrate two joint processing schemes; one that reduces the phase-tracking complexity and one that increases the transmission performance. © 2020, The Author(s).
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| 9. |
- Mirani, Ali, 1992, et al.
(författare)
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Comparison of Physical Realizations of Multidimensional Voronoi Constellations in Single Mode Fibers
- 2022
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Ingår i: European Conference on Optical Communication, ECOC.
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Konferensbidrag (refereegranskat)abstract
- We investigate experimentally and numerically the impact of using different fiber dimensions to spread out the 32-dimensional Voronoi constellations. We find similar performance in experiments and less than 5.4% reach improvements in long-haul transmission simulations by spreading the constellation dimensions over time slots compared to wavelengths
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| 10. |
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