| 1. |
- He, Zonglong, 1994, et al.
(author)
-
12.2 bit/s/Hz C-band Transmission with High-Gain Low-Complexity 24-Dimensional Geometric Shaping
- 2024
-
In: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; In Press
-
Journal article (peer-reviewed)abstract
- Multidimensional (MD) modulation formats enable a larger minimum Euclidean distance than conventional two-dimensional formats. As a structured geometric shaping method, MD Voronoi constellations (VCs) avoid the use of look-up tables and can be implemented with low-complexity encoding and decoding algorithms, outperforming quadrature amplitude modulation (QAM) formats in terms of mutual information. However, it is challenging to maintain the shaping gain in a practical system requiring bit-mapping and forward error correction (FEC) coding. By using a hybrid labeling and multilevel coding scheme integrated with soft-decision FEC coding, MD VCs can achieve high shaping gain compared to QAM formats after soft-decision (SD) decoding. In practical systems, transceiver impairments significantly degrade the system performance, especially for high-cardinality constellations. To investigate of performance of high spectral-efficiency (SE) MD VCs, we employ a 24-dimensional (24D) VC with uncoded SE of 8 bit/symbol/dimension-pair to a spectral superchannel spanning the full C-band. Specifically, the 24D VC with a record constellation size of 7.9×10 28 is applied to dual-polarization coherent transmission over six time slots. The superchannel is composed of 5.0 Gbaud channels spaced at 5.2 GHz, resulting in an overall spectral efficiency of 12.2 bit/s/Hz and a net throughput of 54.2 Tb/s over the entire C-band after 40 km single-mode fiber. It is the first experimental demonstration of a high-SE MD format providing a significant shaping gain after SD decoding.
|
|
| 2. |
- Mirani, Ali, 1992, et al.
(author)
-
Comparison of Physical Realizations of Multidimensional Voronoi Constellations in Single Mode Fibers
- 2022
-
In: European Conference on Optical Communication, ECOC.
-
Conference paper (peer-reviewed)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
|
|
| 3. |
- Mirani, Ali, 1992, et al.
(author)
-
Experimental Demonstration of 8-Dimensional Voronoi Constellations with 65,536 and 16,777,216 Symbols
- 2021
-
In: 2021 European Conference on Optical Communication, ECOC 2021. - 9781665438681
-
Conference paper (peer-reviewed)abstract
- We experimentally demonstrate high-cardinality, low-complexity Voronoi constellations based on the E8 lattice over multiple time slots with OSNR and launch power gains of up to 1.7 and 2.4 dB for back-to-back and 80 km fiber transmission, respectively, compared to QAM formats.
|
|
| 4. |
- Mirani, Ali, 1992, et al.
(author)
-
Physical Realizations of Multidimensional Voronoi Constellations in Optical Communication Systems
- 2023
-
In: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 41:17, s. 5557-5563
-
Journal article (peer-reviewed)abstract
- Abstract:Multidimensional geometric shaping has been shown to outperform uniform quadrature amplitude modulation (QAM) in optical communication systems but the complexity of symbol decision and bit mapping can often be significant as dimensionality increases. In this paper, a low-complexity geometric shaping method based on multidimensional lattices is investigated both in experiments and simulations. The modulation formats designed based on this method are called Voronoi constellations (VCs) and we study them in 8, 16, and 32 dimensions. We obtain transmission reach improvements of up to 22 and 70% for VCs compared to 4QAM and 16QAM, respectively, in nonlinear long-haul fiber transmission. Moreover, we compare different physical realizations of multidimensional VCs over wavelengths, polarizations, and time slots in both the Gaussian and nonlinear fiber channels. We demonstrate that different physical realizations perform similarly in the fiber-optic back-to-back channel. However, in long-haul transmission systems, spreading the dimensions over time slots can increase the transmission reach up to 4% compared to wavelengths and polarizations. Furthermore, the mutual information and generalized mutual information are estimated and compared to QAM formats at the same spectral efficiencies.
|
|
| 5. |
- Chen, Zexin, et al.
(author)
-
Transmitter optimization for PS-QAM signal in high spectral efficiency metro-transmission
- 2023
-
In: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; , s. 1-10
-
Journal article (peer-reviewed)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.
|
|
| 6. |
- He, Zonglong, 1994
(author)
-
Advanced Digital Signal Processing for High Spectral Efficiency Multidimensional Transmission
- 2024
-
Doctoral thesis (other academic/artistic)abstract
- Modern coherent optical communication systems utilize all available physical dimensions for data modulation. In general, independent modulation and signal processing are performed in coherent systems, leaving a performance gap to the Shannon limit. This thesis focuses on advanced digital signal processing to achieve joint encoding and joint processing systems with high spectral efficiency (SE). First, we investigate the low-complexity and practical digital predistortion approaches to eliminate the nonlinear transmitter distortion, which is one dominant impairment for the high-SE transponder. Joint processing of multiple wavelength channels in the receiver can eliminate the linear and nonlinear interchannel interference in long-haul superchannel systems. Using the frequency-locked received channels provided by the comb-based superchannel and receiver, two joint processing schemes are studied. We investigate the performance of multichannel equalization, which cancels linear interchannel crosstalk and reduces the guard bands, in the long-haul transmission constrained by the amplified spontaneous emission noise. In addition, we propose a perturbative-based compensation to eliminate both self-phase modulation and cross-phase modulation caused by the interference wavelength channels. Multidimensional (MD) Vononoi constellations generated by a structured geometric shaping method provide shaping gain over conventional quadrature amplitude modulation (QAM) and have low-complexity encoding and decoding algorithms, which are suitable for high-SE applications. The use of MD formats closes the gap to the theoretical Shannon limit. By employing a 24-dimensional VC with a record constellation size of 7.9×10^28 and lookup table based predistortion, we present a 12.2 bit/s/Hz C-band transmission over 40 km single-mode fiber and have the first experimental demonstration of a significant MD shaping gain over QAM formats in the soft-decision coded system.
|
|
| 7. |
- He, Zonglong, 1994, et al.
(author)
-
Experimental Demonstration of Learned Pulse Shaping Filter for Superchannels
- 2022
-
In: 2022 Optical Fiber Communications Conference and Exhibition, OFC 2022 - Proceedings.
-
Conference paper (peer-reviewed)abstract
- We demonstrate a pulse shaping filter enabled by machine learning for spectral superchannels. In contrast to a 1% roll-off root-raised cosine filter, our learned filter reduces the adaptive equalizer length by 47% for the same spectral efficiency.
|
|
| 8. |
- He, Zonglong, 1994, et al.
(author)
-
Inter-Channel Interference Cancellation for Long-Haul Superchannel System
- 2024
-
In: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; 42:1, s. 48-56
-
Journal article (peer-reviewed)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.
|
|
| 9. |
|
|
| 10. |
- He, Zonglong, 1994, et al.
(author)
-
Periodicity-Enabled Size Reduction of Symbol Based Predistortion for High-Order QAM
- 2022
-
In: Journal of Lightwave Technology. - 0733-8724 .- 1558-2213. ; In press
-
Journal article (peer-reviewed)abstract
- We experimentally demonstrate a novel size reduction approach for symbol-based look-up table (LUT) digital predistortion (DPD) of the transmitter impairments taking advantage of the periodicity in the pattern-dependent distortions. Compared to other reduced-size LUT schemes, the proposed method can significantly lessen the storage memory requirements with negligible performance penalty for high-order modulation formats. To further alleviate the storage memory restriction, a twice reduced-size LUT scheme is proposed to provide further size reduction. Importantly, given a targeted memory length, we verify the importance of averaging over sufficient occurrences of the patterns to obtain a well-performing LUT. Moreover, it is necessary to evaluate the performance of LUT-based DPD using random data. Finally, we demonstrate a neural network (NN) based nonlinear predistortion technique, which achieves nearly identical performance to the full-size LUT for all employed constellations and is robust against a change of modulation format. The proposed techniques are verified in a back-to-back transmission experiment of 20 Gbaud 64-QAM, 256-QAM, and 1024-QAM signals considering 3 and 5 symbol memory. The performance of the LUT-based DPD is further validated in a noise loading experiment.
|
|
