| 1. |
- Kakkar, Aditya, 1989-
(författare)
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Frequency Noise in Coherent Optical Systems: Impact and Mitigation Methods
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The increase in capacity demand along with the advancement in digital signal processing (DSP) have recently revived the interest in coherent optical communications and led to its commercialization. However, design and development of robust DSP algorithms for example for carrier phase recovery (CPR) becomes complex as we opt for high order modulation formats such as 16QAM and beyond. Further, electrical-domain dispersion compensation (EDC), while providing many advantages, makes the system more susceptible to laser frequency noise (FN). For instance, in coherent optical links with post-reception EDC, while the transmitter frequency noise causes only phase impairment, the local oscillator (LO) FN in these systems results in a noise enhancement in both amplitude and phase. This noise is commonly known as equalization enhanced phase noise (EEPN). It results in asymmetric requirements for transmitter laser and LO laser. Further, the system design in the presence of lasers with non-white frequency noise becomes increasingly challenging for increased capacity-distance product.The main contributions of this thesis are, firstly, an experimentally validated theory of coherent optical links with lasers having general non-white frequency noise spectrum and corresponding system/laser design criteria and mitigation technique. Secondly, low complexity and high phase noise tolerant CPR for high order modulation formats.The general theory propounded in this thesis elucidates the origin of the laser frequency noise induced noise enhancement in coherent optical links with different DSP configurations. The thesis establishes the existence of multiple frequency noise regimes and shows that each regime results in different set of impairments. The influence of the impairments due to some regimes can ideally be reduced by optimizing the corresponding mitigation algorithms, while other regimes cause irretrievable impairments. Experimentally validated theoretical boundaries of these regimes and corresponding criteria applicable to system/laser design are provided. Further, an EEPN mitigation method and its two possible implementations are proposed and discussed.The thesis also demonstrates an intrinsic limitation of the conventional Blind Phase Search (BPS) algorithm due to angular quantization and provides methods to overcome it. Finally, this thesis proposes and demonstrates single stage and multi-stage carrier phase recovery algorithms for compensation of phase impairments due to the two lasers for higher order circular and square modulations. The proposed methods outperform the state of art algorithms both in performance and in complexity.
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| 2. |
- Rodrigo Navarro, Jaime, 1987-
(författare)
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Phase Noise Tolerant Modulation Formats and DSP Algorithms for Coherent Optical Systems
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- Coherent detection together with multilevel modulation formats has the potential to significantly increase the capacity of existing optical communication systems at no extra cost in signal bandwidth. However, these modulation formats are more susceptible to the impact of different noise sources and distortions as the distance between its constellation points in the complex plane reduces with the modulation index. In this context, digital signal processing (DSP) plays a key role as it allows compensating for the impairments occurring during signal generation, transmission and/or detection relaxing the complexity of the overall system. The transition towards pluggable optical transceivers, offers flexibility for network design/upgrade but sets strict requirements on the power consumption of the DSP thus limiting its complexity. The DSP module complexity however, scales with the modulation order and, in this scenario, low complex yet high performance DSP algorithms are highly desired.In this thesis, we mainly focus on the impact of laser phase noise arising from the transmitter and local oscillator (LO) lasers in coherent optical communication systems employing high order modulation formats. In these systems, the phase noise of the transmitting and LO lasers translate into phase noise in the received constellation impeding the proper recovery of the transmitted data. In order to increase the system phase noise tolerance, we firstly explore the possibility of re-arranging the constellation points in a circularly shaped mQAM (C-mQAM) constellation shape to exploit its inherent phase noise tolerance. Different low-complex carrier phase recovery (CPR) schemes applicable to these constellations are proposed along with a discussion on its performance and implementation complexity. Secondly, the design guidelines of high performance and low complex CPR schemes for conventional square mQAM constellations are presented. We identify the inherent limitation of the state-of-the-art blind phase search (BPS) carrier phase recovery algorithm which hinders its achievable performance and implementation complexity and present a low complex solution to overcome it. The design guidelines of multi-stage CPR schemes for high order modulation formats, where the BPS algorithm is employed at any of the stages, are also provided and discussed. Finally, the interplay between the received dispersed signal and the LO phase noise is analytically investigated to characterize the origin of the equalization enhanced phase noise phenomena.
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| 3. |
- El-Taher, A., et al.
(författare)
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Noise characterization and transmission evaluation of unrepeated Raman amplified DP-16QAM link
- 2015
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Ingår i: Optical Fiber Communication Conference, OFC 2015. - Washington, D.C. : The Optical Society. - 9781557529374
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Konferensbidrag (refereegranskat)abstract
- Impairments characterization and performance evaluation of Raman amplified unrepeated DP-16QAM transmissions are conducted. Experimental results indicate that small gain in forward direction enhance the system signal-to-noise ratio for longer reach without introducing noticeable penalty.
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| 4. |
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| 5. |
- Iglesias Olmedo, Miguel, 1987-
(författare)
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Impairment Mitigation in High Capacity and Cost-efficient Optical Data Links
- 2017
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Doktorsavhandling (övrigt vetenskapligt/konstnärligt)abstract
- The work presented in this thesis fits within the broader area of fiber optics communications. This is an important area of research as it provides a breeding ground for the present and future technologies supporting the Internet. Due to the ever-increasing bandwidth demands worldwide, the network infrastructures that make up the Internet are continuously being upgraded. This thesis aims to identify key segments of the Internet that are deemed to become the Internet's bottleneck if new technology does not replace the current one. These are datacenter intra and inter-connects, and metropolitan core area networks. In each category, we provide a comprehensive overview of the state of the art, identify key impairments affecting data transmission, and suggest solutions to overcome them. For datacenter intra and inter-connects, the key impairments are lack of bandwidth from electro-optic devices, and dispersion. Solutions attempting to tackle these impairments must be constrained by cost and power consumption. The provided solution is MultiCAP, an alternative advanced modulation format that is more tolerable to dispersion and provides bandwidth management features, while being flexible enough to sacrifice performance in order to gain simplicity. MultiCAP was the first advanced modulation format to achieve over 100~Gb/s in 2013 for a data-center interconnect and set the world record on data transmission over a single VCSEL in 2014 for a short reach data link. On metro-core networks, the challenge is to efficiently mitigate carrier induced frequency noise generated by modern semiconductor lasers. We point out that, when such lasers are employed, the commonly used laser linewidth fails to estimate system performance, and we propose an alternative figure of merit we name "Effective Linewidth". We derive this figure of merit analytically, explore it by numerical simulations and experimentally validate our results by transmitting a 28~Gbaud DP-16QAM over an optical link.
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| 6. |
- Leong, Miu Yoong, et al.
(författare)
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BCH Codes for Coherent Star DQAM Systems with Laser Phase Noise
- 2017
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Ingår i: Journal of optical communications. - : Walter de Gruyter. - 0173-4911 .- 2191-6322. ; 38:1, s. 47-56
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Tidskriftsartikel (refereegranskat)abstract
- Coherent optical systems have relatively high laser phase noise, which affects the performance of forward error correction (FEC) codes. In this paper, we propose a method for selecting Bose-Chaudhuri-Hocquenghem (BCH) codes for coherent systems with star-shaped constellations and M-ary differential quadrature amplitude modulation (DQAM). Our method supports constellations of any order M which is a power of 2, and includes differential M-ary phase shift keying as a special case. Our approach is straightforward, requiring only short pre-FEC simulations to parameterize a statistical model, based on which we select codes analytically. It is applicable to pre-FEC bit error rates (BERs) of around 10-3. We evaluate the accuracy of our approach using numerical simulations. For a target post-FEC BER of 10-5, codes selected with our method yield BERs within 2× target. Lastly, we extend our method to systems with interleaving, which enables us to use codes with lower overhead.
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| 7. |
- Leong, Miu Yoong, et al.
(författare)
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Low-complexity BCH codes with optimized interleavers for DQPSK systems with laser phase noise
- 2017
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Ingår i: Photonic network communications. - : Springer Science and Business Media LLC. - 1387-974X .- 1572-8188. ; 33:3, s. 328-333
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Tidskriftsartikel (refereegranskat)abstract
- The presence of high phase noise in addition to additive white Gaussian noise in coherent optical systems affects the performance of forward error correction (FEC) schemes. In this paper, we propose a simple scheme for such systems, using block interleavers and binary Bose-Chaudhuri-Hocquenghem (BCH) codes. The block interleavers are specifically optimized for differential quadrature phase shift keying modulation. We propose a method for selecting BCH codes that, together with the interleavers, achieve a target post-FEC bit error rate (BER). This combination of interleavers and BCH codes has very low implementation complexity. In addition, our approach is straightforward, requiring only short pre-FEC simulations to parameterize a model, based on which we select codes analytically. We aim to correct a pre-FEC BER of around . We evaluate the accuracy of our approach using numerical simulations. For a target post-FEC BER of , codes selected using our method result in BERs around 3 target and achieve the target with around 0.2 dB extra signal-to-noise ratio.
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| 8. |
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| 9. |
- Pang, Xiaodan, Dr., et al.
(författare)
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Experimental Evaluation of Impairments in Unrepeatered DP-16QAM Link with Distributed Raman Amplification
- 2017
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Ingår i: Photonics. - : MDPI AG. - 2304-6732. ; 4:1
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Tidskriftsartikel (refereegranskat)abstract
- The transmission impairments of a Raman amplified link using dual-polarization 16-quadrature amplitude modulation (DP-16QAM) are experimentally characterized. The impact of amplitude and phase noise on the signal due to relative intensity noise (RIN) transfer from the pump are compared for two pumping configurations: first-order backward pumping and bi-directional pumping. Experimental results indicate that with increased Raman backward pump power, though the optical signal-to-noise ratio (OSNR) is increased, so is the pump-induced amplitude and phase noise. The transmission performance is firstly improved by the enhanced OSNR at a low pump power until an optimum point is reached, and then the impairments due to pump-induced noise start to dominate. However, the introduction of a low pump power in the forward direction can further improve the system's performance.
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| 10. |
- Pang, Xiaodan, et al.
(författare)
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Experimental Study of 1.55-μ m EML-Based Optical IM/DD PAM-4/8 Short Reach Systems
- 2017
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Ingår i: IEEE Photonics Technology Letters. - : Institute of Electrical and Electronics Engineers Inc.. - 1041-1135 .- 1941-0174. ; 29:6, s. 523-526
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Tidskriftsartikel (refereegranskat)abstract
- We experimentally evaluate high-speed intensity modulation/direct detection (IM/DD) transmissions with a 1.55-μ text broadband electro-Absorption modulated laser and pulse amplitude modulations (PAM). We demonstrate 80 Gb/s/ λ PAM-4 and 96 Gb/s/ λ PAM-8 transmissions with low-complexity digital equalizers at the receiver. Performance comparison with different types of equalizers are performed, including linear symbol-spaced feed-forward equalizer (FFE), fractional (half-symbol) spaced FFE and decision feedback equalizer (DFE), with different tap number. It is found that for both cases, a 6-Tap symbol-spaced FFE is sufficient to achieve a stable performance with bit-error-rate below the 7% overhead hard decision forward error correction (7%-OH HD-FEC) threshold over a 4 km standard single mode fiber link. Practical considerations including comparison between adaptive and static equalizer implementation and tolerable fiber chromatic dispersion are discussed.
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| 11. |
- Pang, Xiaodan, Dr., et al.
(författare)
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Key technologies to enable terabit-scale digital radio-over-fiber systems
- 2019
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Ingår i: Proceedings of SPIE - The International Society for Optical Engineering. - : SPIE. - 9781510625327
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Konferensbidrag (refereegranskat)abstract
- With the approach of the 5G era, stringent requirements are imposed on the data transport solutions, including both of the supported transmission reach and the capacity. Radio-over-fiber technologies are considered to be promising candidates to cope with both aspects, owing to the low-loss and broad-bandwidth nature of the optical fibers. Meanwhile with such optical transport solutions, signals can be collected from the distributed remote radio sites and processed in a centralized manner. In this report, we target on the digital radio-over-fiber systems, and discuss about several key technologies, focusing on the aspects of coding and transmission, which could potentially enable terabit-scale data transport.
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| 12. |
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| 13. |
- Verplaetse, Michiel, et al.
(författare)
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Real-Time 100 Gb/s Transmission Using Three-Level Electrical Duobinary Modulation for Short-Reach Optical Interconnects
- 2017
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Ingår i: Journal of Lightwave Technology. - : IEEE. - 0733-8724 .- 1558-2213. ; 35:7, s. 1313-1319
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Tidskriftsartikel (refereegranskat)abstract
- Electrical duobinary modulation is considered as a promising way to realize high capacity because of the low bandwidth requirement on the optical/electrical components and high tolerance toward chromatic dispersion. In this paper, we demonstrate a 100 Gb/s electrical duobinary transmission over 2 km standard single-mode fiber reaching a bit error rate under 7% HD-FEC threshold with the use of PRBS7. This link is tested in real-time without any form of digital signal processing. In-house developed SiGe BiCMOS transmitter and receiver ICs are used to drive an electroabsorption modulated laser and decode the received signal from a PIN-photodiode. The performance of 50 and 70 Gb/s nonreturn-to-zero and electrical duobinary transmission are investigated for comparison.
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| 14. |
- Xu, Tianhua, et al.
(författare)
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Analysis of chromatic dispersion compensation and carrier phase recovery in long-haul optical transmission system influenced by equalization enhanced phase noise
- 2017
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Ingår i: Optik (Stuttgart). - : Elsevier GmbH. - 0030-4026 .- 1618-1336. ; 138, s. 494-508
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Tidskriftsartikel (refereegranskat)abstract
- The performance of long-haul coherent optical fiber transmission system is significantly affected by the equalization enhanced phase noise (EEPN), due to the interaction between the electronic dispersion compensation (EDC) and the laser phase noise. In this paper, we present a comprehensive study on different chromatic dispersion (CD) compensation and carrier phase recovery (CPR) approaches, in the n-level phase shift keying (n-PSK) and the n-level quadrature amplitude modulation (n-QAM) coherent optical transmission systems, considering the impacts of EEPN. Four CD compensation methods are considered: the time-domain equalization (TDE), the frequency-domain equalization (FDE), the least mean square (LMS) adaptive equalization are applied for EDC, and the dispersion compensating fiber (DCF) is employed for optical dispersion compensation (ODC). Meanwhile, three carrier phase recovery methods are also involved: a one-tap normalized least mean square (NLMS) algorithm, a block-wise average (BWA) algorithm, and a Viterbi-Viterbi (VV) algorithm. Numerical simulations have been carried out in a 28-Gbaud dual-polarization quadrature phase shift keying (DP-QPSK) coherent transmission system, and the results indicate that the origin of EEPN depends on the choice of chromatic dispersion compensation methods, and the effects of EEPN also behave moderately different in accordance to different carrier phase recovery scenarios.
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| 15. |
- Xu, Tianhua, et al.
(författare)
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Phase Noise Cancellation in Coherent Communication Systems Using a Radio Frequency Pilot Tone
- 2019
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Ingår i: Applied Sciences. - : MDPI. - 2076-3417. ; 9:21
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Tidskriftsartikel (refereegranskat)abstract
- Long-haul optical fiber communication employing digital signal processing (DSP)-based dispersion compensation can be distorted by the phenomenon of equalization-enhanced phase noise (EEPN), due to the reciprocities between the dispersion compensation unit and the local oscillator (LO) laser phase noise (LPN). The impact of EEPN scales increases with the increase of the fiber dispersion, laser linewidths, symbol rates, signal bandwidths, and the order of modulation formats. In this work, the phase noise cancellation (PNC) employing a radio frequency (RF) pilot tone in coherent optical transmission systems has been investigated. A 28-Gsym/s QPSK optical transmission system with a significant EEPN has been implemented, where the carrier phase recovery (CPR) was realized using the one-tap normalized least-mean-square (NLMS) estimation and the differential phase detection (DPD), respectively. It is shown that the RF pilot tone can entirely eliminate the LPN and efficiently suppress the EEPN when it is applied prior to the CPR.
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| 16. |
- Zhang, Lu, et al.
(författare)
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Digital mobile fronthaul employing differential pulse code modulation with suppressed quantization noise
- 2017
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Ingår i: Optics Express. - : Optical Society of America. - 1094-4087. ; 25:25, s. 31921-31936
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Tidskriftsartikel (refereegranskat)abstract
- A differential pulse code modulation (DPCM) based digital mobile fronthaul architecture is proposed and experimentally demonstrated. By using a linear predictor in the DPCM encoding process, the quantization noise can be effectively suppressed and a prediction gain of 7~8 dB can be obtained. Experimental validation is carried out with a 20 km 15-Gbaud/λ 4-level pulse amplitude modulation (PAM4) intensity modulation and direct detection system. The results verify the feasibility of supporting 163, 122, 98, 81 20-MHz 4, 16, 64, 256 QAM based antenna-carrier (AxC) containers with only 3, 4, 5, 6 quantization bits at a sampling rate of 30.72MSa/s in LTE-A environment. Further increasing the number of quantization bits to 8 and 9, 1024 quadrature amplitude modulation (1024 QAM) and 4096 QAM transmission can be realized with error vector magnitude (EVM) lower than 1% and 0.5%, respectively. The supported number of AxCs in the proposed DPCM-based fronthaul is increased and the EVM is greatly reduced compared to the common public radio interface (CPRI) based fronthaul that uses pulse code modulation. Besides, the DPCM-based fronthaul is also experimentally demonstrated to support universal filtered multicarrier signal that is one candidate waveform for the 5th generation mobile systems.
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| 17. |
- Zhang, Lu, et al.
(författare)
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Digital Radio-Over-Multicore-Fiber System with Self-Homodyne Coherent Detection and Entropy Coding for Mobile Fronthaul
- 2018
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Ingår i: European Conference on Optical Communication, ECOC. - : Institute of Electrical and Electronics Engineers (IEEE).
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Konferensbidrag (refereegranskat)abstract
- We experimentally demonstrate a 28-Gbaud 16-QAM self-homodyne digital radio-over-33.6km-7-core-fiber system with entropy coding for mobile fronthaul, achieving error-free carrier aggregation of 330 100-MHz 4096-QAM 5G-new-radio channels and 921 100-MHz QPSK 5G-new-radio channels with CPRI-equivalent data rate up to 3.73-Tbit/s.
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| 18. |
- Zhang, Lu, et al.
(författare)
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Toward terabit digital radio over fiber systems : Architecture and key technologies
- 2019
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Ingår i: IEEE Communications Magazine. - : Institute of Electrical and Electronics Engineers Inc.. - 0163-6804 .- 1558-1896. ; 57:4, s. 131-137
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Tidskriftsartikel (refereegranskat)abstract
- To support massive deployment of broadband radio applications, such as 5G and high-definition videos for terrestrial televisions, large system capacity and high spectrum efficiency are highly demanded in radio over fiber (RoF) systems. In this article, we propose a terabit digital RoF system capable of providing high-speed transmission, where multicore fiber (MCF) is introduced for the access segment between the central unit and remote unit. Two key technologies that greatly enhance system capacity and spectrum efficiency, namely MCF enabled self-homodyne detection and compressed quantization, are demonstrated.
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