Maximum Likelihood Sequence Estimation Receiver Research Articles

Toward Cost-Efficient 100G Metro Networks Using IM/DD, 10-GHz Components, and MLSE Receiver

We demonstrate the generation and transmission of 28-Gb/s data employing inexpensive, off-the-shelf, 10-GHz 3-dB bandwidth optical components, intensity modulation (IM), direct detection (DD), and a digital signal processing-based receiver over uncompensated fiber links. We prove that the proposed technology is an enabler for next-generation 100-G cost-efficient point-to-point dispersion compensation fiber (DCF)-free metro networks up to 80 km, as well as for multispan metro-ring networks up to 400 km, consisting of cascaded erbium-doped fiber amplifiers and DCFs as in real-field implementations. Key techniques to enable transmission are the employment of signal predistortion in the transmitter to compensate for the components’ limited bandwidth, a maximum likelihood sequence estimator (MLSE)-based receiver, and a high-performance sampling phase adjustment algorithm. Furthermore, we show that performance can be optimized, while at the same time the complexity of the electronic dispersion compensation part is significantly reduced by exploiting a simplified variant of MLSE that makes use of a reduced number of states. Results in this paper reveal the potentiality of our proposed scheme for a low-cost transition to 100 Gb/s ( $4 \times 28$ Gb/s) wavelength division multiplexed, point-to-point, metro-core, and multispan metro-ring networks, employing inexpensive optical components and the traditional cost-efficient IM/DD scheme.

Blind channel estimation for MLSE receiver in high speed optical communications: theory and ASIC implementation

Blind channel estimation is critical for digital signal processing (DSP) compensation of optical fiber communications links. The overall channel consists of deterministic distortions such as chromatic dispersion, as well as random and time varying distortions including polarization mode dispersion and timing jitter. It is critical to obtain robust acquisition and tracking methods for estimating these distortions effects, which, in turn, can be compensated by means of DSP such as Maximum Likelihood Sequence Estimation (MLSE). Here, a novel blind estimation algorithm is developed, accompanied by inclusive mathematical modeling, and followed by extensive set of real time experiments that verify quantitatively its performance and convergence. The developed blind channel estimation is used as the basis of an MLSE receiver. The entire scheme is fully implemented in a 65 nm CMOS Application Specific Integrated Circuit (ASIC). Experimental measurements and results are presented, including Bit Error Rate (BER) measurements, which demonstrate the successful data recovery by the MLSE ASIC under various channel conditions and distances.

A Comparative Study on Time Domain Sequential Equalizer with MLD and MLSE Equalizer on MIMO Frequency Selective Channels

In this paper, we investigate two receiver structures for spatially multiplexed transmission on MIMO frequency selective fading channels. Those receivers compensate the IAI (Inter-Antenna Interference) and ISI (Inter-Symbol Interference) in the time domain. We first propose the MIMO sequential equalizer in which the block of several receives symbols is processed symbol by symbol by MLD (Maximum Likelihood Detection). Next we investigate the MIMO MLSE (Maximum Likelihood Sequence Estimation) receiver in which the terminated block trellis is decoded by the Viterbi algorithm. The bit error rates of two time domain receivers are examined through computer simulations and we also compare their BER characteristics to those of the conventional MIMO SC-FDE.

XPM and SBS nonlinear effects on MLSE with varying uncompensated dispersion

We investigate the performance of a maximum likelihood sequence estimation (MLSE) receiver at 10.7 Gb/s in the presence of two optical nonlinear impairments, cross-phase modulation (XPM) and stimulated Brillouin scattering (SBS). We find that the tolerance to both nonlinearities decreases with larger levels of uncompensated dispersion. Our results also suggest that the MSLE receiver loses its linear regime advantage in comparison to a standard receiver at some dispersion levels in the presence of the nonlinear effects. We demonstrate that long uncompensated links up to 160 km may show better tolerance to the nonlinear effects with a lower dispersion fiber when using an MLSE receiver.

New Branch Metrics for MLSE Receivers Based on Polarization Diversity for PMD Mitigation

In this paper, we analyze the performance of polarization-diversity receivers employing maximum-likelihood sequence estimation (MLSE) to combat the effects of polarization mode dispersion (PMD) for both the intensity-modulation direct detection (IMDD) and duobinary modulation formats at 43 Gb/s. We propose and study several parametric branch metrics, showing that the use of MLSE receivers based on polarization diversity with appropriate branch metrics allows to cancel out the high penalty incurred by standard MLSE receivers in the presence of PMD.

Channel tracking for optical fiber communication systems: clustering-based approach

As a very effective scheme to realize channel acquisition, clustering-based channel estimation algorithms have been proposed for optical fiber communication systems with a maximum-likelihood sequence estimation receiver. These algorithms can estimate the key channel parameters needed by the Viterbi processor accurately without assuming that the channel memory length is known a priori to the receiver. In this work, a corresponding adaptive channel tracking algorithm is proposed, which is proved to be very powerful in tracking the variation of the communication channel. Cooperation of the clustering-based channel acquisition and channel tracking is realized.

Simple and robust analytically derived variable step-size least mean squares algorithm for channel estimation

A new variable step-size least mean squares (VSS-LMS) algorithm for the estimation of frequency-selective communications channels is herein presented. In contrast to previous works, in which the step-size adaptation is based on the instantaneous samples of the error signal, this algorithm is derived on the basis of analytical minimisation of the ensemble-averaged mean-square weight error. A very simple rule for step-size adaptation is obtained, using a small number of communication system parameters. This is another significant difference from other proposals, in which a large number of control parameters should be tuned for proper use. The algorithm here proposed is shown to be applicable to both time-varying and time-invariant scenarios. While the lack of a termination rule for step-size adaptation is a common characteristic of other schemes, the algorithm here presented adopts a criterion for stopping the step-size adaptation that assures optimal steady-state performance and leads to large computational savings. A simulation-based performance comparison with other VSS-LMS schemes is provided, including their application to maximum likelihood sequence estimation receivers using per survivor processing (MLSE/PSP). The results show that the algorithm proposed in this work has good performance characteristics and a very low computational cost, specially in the application to MLSE/PSP receivers. Besides, this algorithm is shown to be robust to changes in the signal-to-noise ratio (SNR).

Reach Extension of a DPSK Optical Link Using an Optical Filter Demodulator and MLSE Receiver

We have proposed a novel scheme based on chirped fiber Bragg grating (CFBG) as a narrowband optical filter demodulator and a maximum likelihood sequence estimation (MLSE) receiver to attain an enhanced unrepeatered reach in a nonreturn-to-zero differential phase-shift-keying (NRZ-DPSK) transmission link. An extended reach up to 500 km with only 2-dB optical signal-to-noise ratio penalty is found feasible for a 10.7-Gb/s NRZ-DPSK link with the CFBG-MLSE combination.

Clustering-Based Adaptive Channel Estimator for Optical Fiber Communication Systems

In this letter, a new clustering-based adaptive channel estimator is proposed for optical fiber communication systems with the maximum-likelihood sequence estimation (MLSE) receiver. The estimator is very effective in estimating the key channel parameters needed by the Viterbi processor without assuming that the channel memory length is known a priori to the receiver. Moreover, the application of the proposed channel estimator can induce a complexity-reduced MLSE receiver.

An Algorithm for AGC Optimization in MLSE Dispersion Compensation Optical Receivers

Communication systems are designed to provide reliable transmission of information and to cope with various channel imperfections. The amplitude of the received signal is attenuated depending on length and type of the transmission channel. Usually, automatic gain control is placed before the clock recovery in the receiver to combat signal power/swing variations. Optical channel impairments caused by chromatic dispersion, polarization mode dispersion, self-phase modulation, etc. can be efficiently compensated by the use of maximum-likelihood sequence estimation receivers. This paper describes an efficient AGC gain adjustment procedure using a gain control parameter that provides almost the optimum MLSE performance. A gradient algorithm is used to minimize the gain control parameter. Simulation results of 10.7 Gb/s intensity-modulated direct-detection nonreturn-to-zero optical channels disturbed by noise and chromatic dispersion show the advantage of the proposed algorithm.

Long-Haul WDM IMDD Transmission at 10.7 Gbit/s in a Dispersion-Managed Multispan System Using MLSE Receivers

We compare by simulation the performance of a maximum likelihood sequence estimation (MLSE) receiver with that of a standard optimized threshold receiver in a dispersion-managed multispan WDM transmission system at 10.7 Gbit/s in the presence of fiber nonlinearities. We found that the use of MLSE receivers helps to significantly increase the tolerance of the dispersion map design and may improve overall system performance.

Long-haul 10 Gbit/s linear and non-linear IMDD transmission over uncompensated standard fiber using a SQRT-metric MLSE receiver

We experimentally demonstrated Intensity-Modulated Direct-Detection (IMDD) single-channel 1,040 km linear transmission and 800 km non-linear transmission at 10 Gb/s over standard single-mode (G.652) fiber, without any optical dispersion compensation or mitigation, using a Maximum-Likelihood Sequence-Estimation (MLSE) receiver employing the square-root (SQRT) branch metric with off-line processing. These experiments were designed as to probe the limits of the MLSE approach. They successfully showed that long-haul uncompensated transmission is in principle possible with MLSE, even in the presence of large uncompensated dispersion and strong intra-channel fiber non-linearities, provided that enough complexity can be built into the receiver. In the linear 1,040 km experiment, a Bit Error Rate (BER) of 10(-3) was achieved with an Optical Signal-to-Noise Ratio (OSNR) penalty with respect to back-to-back of 2.9 dB, using two samples per bit and 16,384 trellis states. Several other set-ups were tested as well, including the use of only one sample per bit and fewer trellis states. In the non-linear 800 km experiment, power was ramped up to 12 dBm, exciting substantial Kerr non-linearity, whose induced spectral-broadening exacerbated the effects of the large uncompensated dispersion of the link. Using an MLSE receiver with 1,024 states, we demonstrated a non-linear threshold of 9 dBm. We benchmarked this experiment towards simulations addressing various electrical and optical dispersion compensation strategies. We also carried out an analysis of error run-lengths, on both experiments, which showed that error burstiness may change considerably depending on the number of processor states, OSNR and the amount of non-linearity in the link.

Performance of an MLSE-EDC Receiver With SOA-Induced Nonlinear Impairments

We experimentally investigate the performance of maximum-likelihood sequence estimation (MLSE) electronic dispersion compensation for signals impaired by nonlinear optical effects induced in a semiconductor optical amplifier (SOA). Single- and multichannel effects are investigated. We study 10.7-Gb/s nonreturn-to-zero, low-pass filter duobinary, and differential phase-shift-keying signals detected as optical duobinary, each with different amounts of uncompensated dispersion before amplification by the SOA and detection. The results show performance variability by the MLSE receiver with increasing nonlinearity, from small penalties back-to-back compared to a standard receiver for most formats to advantages of several decibels with uncompensated dispersion.

Optimization of Branch Metric Exponent and Quantization Range in MLSE Receivers for Duobinary Systems

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> We analyze by simulation the performance of an optically amplified uncompensated duobinary system using a 64-state maximum-likelihood sequence estimation (MLSE) receiver (Rx) based on a Euclidean branch metric with variable postdetection nonlinear distortion exponent. We found that the optimum exponent depends on the accumulated dispersion, the Rx analog-to-digital converter (ADC) resolution, and signal clipping. On the other hand, we found performance to be weakly dependent on the exponent value. When using a finite number of ADC resolution bits, drastic signal clipping proved very beneficial in improving the performance of the MLSE processor. Assuming three resolution bits, with joint clipping and exponent optimization, uncompensated transmission at 10.7 Gb/s over 550 km of standard single-mode fiber could be achieved with essentially no penalty with respect to back-to-back. </para>

ISI Mitigation Capability of MLSE Direct-Detection Receivers

We explain why maximum-likelihood sequence estimation (MLSE) receivers for direct-detection optical communication systems do not achieve the single-pulse performance of threshold receivers, in which there is no intersymbol interference (ISI). For this purpose, we propose a simple and intuitive model to approximate the performance of MLSE receivers under the assumption of moderate ISI. A performance analysis of this model shows that an MLSE receiver does not reach the performance of an ISI-free threshold receiver. By simulations we evaluate the range over which the model accurately approximates the MLSE receiver.

Experimental Study of MLSE Receivers in the Presence of Narrowband and Vestigial Sideband Optical Filtering

We report on experimental investigations of real-time maximum-likelihood sequence estimation (MLSE) in the presence of narrowband optical filtering, using 10.7-Gb/s nonreturn-to-zero ON-OFF keying and a fiber grating filter with 6.25-GHz bandwidth. Compared to standard threshold detection, the MLSE eliminates a 10-3 error floor due to narrowband optical filtering and results in DFB a <2-dB optical signal-to-noise ratio penalty at a 10-3 bit-error ratio. Furthermore, we demonstrate the ability of the MLSE to simultaneously compensate for narrowband optical filtering and chromatic dispersion. Finally, we investigate the influence of narrowband filter frequency detuning and show that the well-known effect of increased filtering tolerance given by the vestigial sideband effect observed in standard threshold detection, disappears in the presence of the MLSE.

Parametric Estimation of IM/DD Optical Channels Using New Closed-Form Approximations of the Signal PDF

In this paper, we propose new closed-form approximations to the probability-density function (pdf) of the received signal in intensity-modulation/direct-detection (IM/DD) optical channels. These approximations greatly simplify the problem of channel estimation. This is an important problem in the implementation of maximum-likelihood sequence-estimation (MLSE) receivers for electronic dispersion compensation (EDC) of optical fibers, which has been a topic of great interest in recent years. The approximations proposed here are also useful in the analysis of the error rate of these receivers. It is well known that noise in IM/DD optical channels is strongly non-Gaussian and signal dependent. Except in the simplest situations, the pdf of the signal corrupted by noise does not have a closed-form expression. This leads to difficulties in the calculation of the probability of error on these channels and, more importantly, in the implementation of receivers that exploit knowledge of the signal pdf to minimize the error rate, for example, those based on MLSE techniques. To limit the complexity of channel estimation (an important requirement in real-time adaptive EDC receivers), it is important that the functional form assumed for the pdf be as simple as possible, while providing a good match with the actual statistical properties of the channel. In this paper, we introduce a new generic functional form for the pdf that accurately models the behavior of the received signal. Based on this expression, we introduce a channel-estimation approach that dramatically simplifies the analysis and design of MLSE receivers for IM/DD channels. Simulations show an excellent agreement between the results based on the approximations proposed here and the exact expressions for the pdf

ERROR PROBABILITY OF SPACE-TIME BLOCK CODED SYSTEMS IN MULTIPATH RAYLEIGH CHANNEL: MLSE AND SUBOPTIMUM DECODING

In this paper, we address the error probability performance of space-time block coded system (STBC) in multipath Rayleigh fading channel. We consider STBC with rate one for two transmit antennae and rate half for three transmit antennae, and present an optimal maximum-likelihood-sequence-estimation (MLSE) and a block-by-block suboptimum decoding receiver structures. The proposed receiver schemes are suitable for short-range wireless communication or wireless personal area network (WPAN), where the delay spread is usually smaller than one symbol period. A reduced-state MLSE receiver with new trellis structure is first developed for the multiple-input multiple-output (MIMO) scenario assuming availability of ideal channel state information (CSI). Then a less complex block-by-block suboptimum decoding algorithm is also developed for the unequalized multipath fading channel, as an alternative to the computationally intensive MLSE. The suboptimum receiver symbol timing is optimized in order to obtain minimum error probability for the general multipath power delay profiles. From our simulation results, we show that our proposed MLSE receiver can mitigate the multipath induced intersymbol interference (ISI) effectively and at bit error rate (BER) of 10-3, a two-transmit and two-receive (2, 2) STBC system gains as much as 11.5 dB in SNR over a single-antenna uncoded system. We also verify that the MLSE receiver always outperforms the suboptimum receiver, as expected. Error floors appear in the BER performance curves of the suboptimum receiver because ISI is not equalized, whereas no such floors appear with the MLSE receiver.

Performance of MLSE Receiver in a Dispersion-Managed Multispan Experiment at 10.7 Gb/s Under Nonlinear Transmission

We report the first comparison of the performance of a maximum likelihood sequence estimation (MLSE) receiver with that of a standard threshold receiver in a dispersion-managed eight-span transmission experiment at 10.7 Gb/s in the presence of fiber nonlinearities. We find a loss of performance advantage of the MLSE receiver for compensating fiber chromatic dispersion under highly nonlinear transmission conditions

An MLSE Receiver for Electronic Dispersion Compensation of OC-192 Fiber Links

A maximum-likelihood sequence estimation (MLSE) receiver is fabricated to combat dispersion/intersymbol interference (chromatic and polarization mode), noise (optical and electrical), and nonlinearities (e.g., fiber, receiver photodiode, or laser) in OC-192 metro and long-haul links. The MLSE receiver includes a variable gain amplifier with 40-dB gain range and 7.5-GHz 3-dB bandwidth, a 12.5-Gb/s 4-bit analog-to-digital converter, a dispersion-tolerant phase-locked loop, a 1:8 demultiplexer, and a digital equalizer implementing the MLSE algorithm. The MLSE receiver achieves more than 50% reach extension at signal-to-noise levels of interest as compared to conventional clock data recovery systems