Research on equalization based on MLSE

Improvements in silicon processing speeds have resulted in integrated circuits that can sample and digitally process 10 Gbit/s data. Signal processing techniques that have radically altered radio communication can now be applied to optical communications to improve tolerance to impairments such as chromatic dispersion and polarisation mode dispersion, as well as transmitter/receiver imperfections. Whereas optical components tend to be expensive to produce, electronics lends itself to integration and mass production, leading to a reduction in network level cost. For low cost systems using direct detection at the receiver, maximum-likelihood sequence estimation (MLSE) is recognised as being the optimum receiver detection method. We present architecture & results from our first generation digital equaliser chip set (DECS). The equaliser requires no previous knowledge of the transmission channel, can adapt to channel changes, is robust against transient behaviour, and has been demonstrated in conjunction with NRZ. The MLSE algorithm may also be used in conjunction with other technologies such as duobinary or directly-modulated lasers to further increase the tolerance to signal distortion and therefore reduce the network cost. We discuss the challenges in a practical implementation of the MLSE. (12 pages)

Summary form only given. Polarisation mode dispersion (PMD) is a major impairment for high-bit-rate optical systems. The objective of this course is threefold: (1) understand the physical phenomena of PMD, (2) review the current status of understanding of the impact of PMD on optical systems, and (3) PMD mitigation approaches. In this course we will introduce the concept of PMD and its various mathematical description briefly with emphasis on visualizing and comparing different ways of describing PMD. Then, very briefly, we will concentrate on some measurement aspects of PMD in systems. Current understanding of PMD impact on systems will be summarized next. System impact of PMD will be reviewed in three stages. (1) Impact of fiber PMD, 1st and higher order, in high-bit-rate systems in presence of chromatic dispersion. (2) Impact of fiber and discrete component PMD on optical systems. (3) Impact of fiber and component PMD on optical systems in presence of polarization dependent loss. Finally, PMD mitigation approaches will be discussed.

This article proposes a low-complexity maximum likelihood sequence estimation (MLSE) algorithm tailored for non-return-to-zero (NRZ) high-speed transceivers. In particle physics experiments, data transmission volumes are continually increasing, with transceivers assuming pivotal roles. MLSE has garnered significant attention due to its notable advantages in eliminating intersymbol interference (ISI) and its capability to replace decision feedback equalizers (DFEs). However, MLSE complexity exponentially escalates with the increment of traceback length and equalizer order. Hence, reducing MLSE complexity while preserving performance is imperative. This article simplifies MLSE transition metric calculations, obviating the necessity for intricate state computations and result storage. A configurable and highly adaptable transceiver simulation system is developed, utilizing a field-programmable gate array (FPGA), and the proposed algorithm is evaluated using this system. Synthesis results demonstrate that the proposed algorithm significantly reduces resource utilization while maintaining algorithm performance.

Maximum likelihood sequence estimation in high-speed PONs using machine learning-based pre-equalizers

In this article a new quantitative method for polarization mode dispersion (PMD) tolerance testing is presented. Novel PMD sources, test procedures, and evaluation tools have been integrated into a comprehensive test methodology. The new PMD sources provide not only first order PMD, but also higher order PMD states. These sources are stable, able to maintain any desired PMD state for extended bit error tests. The basis of the new PMD tolerance methodology is to remove PMD statistics during the data acquisition, and reintroducing the PMD statistics in the data analysis. Total PMD outage time can then be determined for a fiber span with known mean PMD and defined system power penalty for PMD distortion. We will describe a new PMD source, test and evaluation method in detail, showing simulated test results and the ability to use this method to optimize optical transport systems and network design rules.

This study investigates the effects of polarization mode dispersion (PMD) on high-speed multimode fiber (MMF) optical communication systems, focusing on how differential group delay (DGD) and environmental conditions such as temperature and humidity influence signal quality. The research was conducted in Abuja, Nigeria, using two modulation formats—non-return-to-zero (NRZ) and quadrature amplitude modulation (QAM). Experimental measurements were carried out over fiber lengths ranging from 1 km to 38 km, and data rates from 15 Gbps to 240 Gbps. Key performance metrics such as bit error rate (BER), jitter, and signal-to-noise ratio (SNR) were analyzed in relation to DGD under varying environmental conditions. Results showed that DGD increased from 0.136 ps to 2.78 ps for NRZ and from 0.058 ps to 2.24 ps for QAM across the tested fiber lengths. Corresponding BER values for NRZ ranged from 1.18 × 10-7 to 1.35 × 10-7, while QAM showed slightly better performance with BER values between 1.18 × 10-7 and 1.29 × 10-7. Jitter rose from 4.26 ps to 31.20 ps in NRZ and from 4 ps to 33.54 ps in QAM. SNR values declined as DGD increased, with NRZ dropping from 34.59 dB to 54.56 dB and QAM from 34.83 dB to 52.90 dB. Environmental factors also played a role; temperature increases from 28.34°C to 29.90°C and humidity from 53.1% to 59.5% led to significant increases in DGD. The findings confirm that PMD, though less dominant in MMF than modal dispersion, still significantly affects signal quality in long-distance and high-speed MMF systems. QAM demonstrated greater resilience to PMD compared to NRZ. This study highlights the need for effective dispersion management and modulation format optimization in designing robust optical networks for environmentally dynamic regions like Abuja.

High-speed transmission limitations due to Polarization Mode Dispersion

Unified low-complexity decision feedback equalizer with adjustable double radius constraint

Polarization mode dispersion (PMD) is one of the major obstacles in high-speed (above 100 Gbits rate) and long-haul optical communication system. In order to meet the requirements of high-sped optical communication quality, some PMD compensation and mitigation schemes had been developed. Forward error correction (FEC) is useful to increasing the system margin. The combination of distributed fast polarization scrambler (D-FPS) and FEC is an effective method to improving the optical system’s PMD tolerance. The fundamental reason is that D-FPS can accelerate the redistribution of the link PMD to enable FEC to be more effective during the periods that would otherwise have PMD outages. Another advantage of this scheme is that it can mitigate the influence of PMD in multichannel system without feedback control and compensation based on each channel. The principle and performance of scrambling is introduced in this paper and the fundamental idea of improving PMD mitigation by using D-FPS combined with FEC is proposed. Some key factors such as scrambling speed and the number of scrambler which affecting the performance of this scheme are also introduced. Finally, proposing an assumption that the impact for system performance caused by the style of FPSs is distributed along the fiber link, and the mathematical estimation model is established. The combination of D-FPS and FEC would be a promising approach for performance improvement in ultra-high-speed optical communication system.

In this paper, we propose a joint feedforward equalizer (FFE) and an error-based feedforward equalizer (Err-FFE) to eliminate the serious inter-symbol interference and high-frequency colored noise for 100 Gb/s 4-level pulse-amplitude modulation (PAM4) optical systems. The proposed Err-FFE is first designed to replace the joint post-filter (PF) and maximum likelihood sequence estimation (MLSE). The Err-FFE algorithm has only 15% computational complexity compared to the joint PF and MLSE. Moreover, experimental results show that the performance of joint FFE and Err-FFE algorithm is much better than that conventional FFE, and is comparable with that of FFE joint with PF and MLSE. In conclusion, the proposed Err-FFE has both low computational complexity and excellent performance for PAM4-based optical systems, which indicates its potential for applications in low-cost and low-power short-reach optical interconnects.

Analysis and Compensation of Polarization Mode Dispersion in Single Channel, WDM and 32-channel DWDM Fiber Optic System

High data-rate upstream transmission in wavelength division multiplexed passive optical network (WDM-PON) based on reflective semiconductor optical amplifier (RSOA) is limited by severe inter-symbol interference (ISI), owing to low-bandwidth transmitter and fiber dispersion. To overcome the limitations in RSOA-based WDM-PON, we propose a novel receiver based on partial response maximum likelihood (PRML) equalization which combines the use of partial response (PR) signaling with maximum likelihood sequence estimation (MLSE). MLSE has been long considered as the optimal reception technique to overcome various types of impairment in optical transmission such as dispersions. It is demonstrated in this paper that PRML surpasses standard MLSE in ISI and reflection suppression with reduced complexity for RSOA-based WDM-PON system. 150 km unidirectional distance is demonstrated for 10 Gb/s uplink, while bidirectional uplinks up to 50 km and 20 km are achieved for data rate of 10 Gb/s and 20 Gb/s respectively, in WDM-PON using PRML. Furthermore, the impacts of discrete reflections on various equalization techniques are investigated, where PRML also shows superiority over MLSE.

The performance of different electronic equalization and processing schemes for 40- and 10-Gb/s optical transmission over single-mode fiber (SMF) are discussed, from the point of their ability to compensate chromatic dispersion (CD) and polarization mode dispersion (PMD). In addition, the impact of fiber nonlinearity and modulation format on equalization is also investigated. The main objective of this paper is to present an overview and a comparison of the performances rather than a detailed explanation of the principles of the different equalization schemes. The equalizers which will be covered are analog equalizer (feedforward and decision feedback type), maximum likelihood sequence estimator (MLSE), electronic precompensation, coherent/intradyne detection with digital signal processing (DSP) equalization, DSP-based optical orthogonal frequency division multiplexing (OFDM), and turbo equalization.

A maximum a posteriori equalizer is presented for optical communication systems. Assuming that the span of the intersymbol interference (ISI) does not extend beyond the neighboring bits - as is typically the case for the distortion introduced by polarization mode dispersion (PMD) - we derive the conditional probability distribution function in the electrical domain in the presence of PMD and amplifier spontaneous emission dominated noise. Simulation results with an accurate receiver model and all-order PMD show the success of the MAP equalizer in reducing the ISI due to PMD, and that the analytical conditional PDF we derive provides a good match to the actual distribution.

A novel differential-group-delay (DGD) monitoring scheme that can provide feedforward information to facilitate the compensation of polarization mode dispersion (PMD) is demonstrated. Using a polarization scrambler at the beginning of the link, the instantaneous DGD can be monitored at the end of the link by using a second polarization scrambler followed by a fixed 0.8-dB polarization dependent loss (PDL) element or a polarizer. As the PMD value along the link increases, the instantaneous degree-of-polarization will decrease, thereby resulting in a decrease in the measured RMS power fluctuation value through the PDL element. The authors show instantaneous DGD monitoring of up to /spl sim/70 ps for both nonreturn-to-zero (NRZ) and return-to-zero data formats, and they demonstrate PMD compensation of a 10-Gb/s NRZ channel using the monitor output as part of the control feedforward signal.