Mario Rafael Hueda

Maximum-likelihood sequence estimation in dispersive optical channels

This paper discusses the investigation of maximum-likelihood sequence estimation (MLSE) receivers operating on intensity-modulated direct-detection optical channels. The study focuses on long-haul or metro links spanning several hundred kilometers of single-mode fiber with optical amplifiers. The structure of MLSE-based optical receivers operating in the presence of dispersion and amplified spontaneous emission (ASE), as well as shot and thermal noise, are discussed, and a theory of the error rate of these receivers is developed. Computer simulations show a close agreement between the predictions of the theory and simulation results. Some important implementation issues are also addressed. Optical channels suffer from impairments that set them apart from other channels, and therefore they need a special investigation. Among these impairments are the facts that the optical channel is nonlinear, and noise is often non-Gaussian and signal dependent. For example, in optically amplified single-mode fiber links, the dominant source of noise is ASE noise, which after photodetection is distributed according to a noncentral chi-square probability density function. In addition, optical fibers suffer from chromatic and polarization-mode dispersion (PMD). Although the use of MLSE in optical channels has been discussed in previous literature, no detailed analysis of optical receivers using this technique has been reported so far. This motivates the study reported in this paper.

A 90nm CMOS DSP MLSD Transceiver with Integrated AFE for Electronic Dispersion Compensation of Multi-mode Optical Fibers at 10Gb/s

Multi-mode fibers (MMF) are typically used in LAN applications, in links which may reach or exceed 300 meters. Widespread use of electronic dispersion compensation (EDC) for MMF is prompted by the ratification of the 10GBASE-LRM standard. A number of studies have demonstrated the superiority of MLSD for this application. This paper describes an all-DSP single-chip 90nm CMOS MLSD-based EDC transceiver for MMF.

Non-Concatenated FEC Codes for Ultra-High Speed Optical Transport Networks

This paper presents a non-concatenated forward error correction (FEC) code suitable for applications in 100Gb/s optical transport networks (OTN). A typical requirement in this application is a net coding gain (NCG) >10 dB at a bit error rate (BER) of 10^{-15} with an overhead (OH) of ~20%. As discussed in [1], non-concatenated codes are the ultimate frontier in terms of performance for OTN applications, because of their superior performance, lower latency, and lower overhead than concatenated codes. However, a major stumbling block for the use of these codes has been the existence of BER floors at levels significantly higher than the required 10^{-15} (typically 10^{- 10}). In this paper we present a new coding scheme based on a low density parity check (LDPC) code with an expected net coding gain of 11.30dB at 10^{-15}, 20% OH, and a block size of 24576 bits. This represents a significant improvement over the previous state of the art [2], based on a concatenated code with a block size of 74844 bits and 20.5% OH. The code is designed to minimize the BER floor while simultaneously reducing the memory requirements and the interconnection complexity of the iterative decoder [3]. Experimental results obtained with an FPGA-based hardware emulator demonstrate an NCG of 10.70 dB at a BER of 10^{-13} and no error floors. These experimental results are extrapolated to 10^{-15} using importance sampling techniques, resulting in the expected performance stated above. Moreover, we find that fixed-point implementation is the main cause of error floors below 10^{-13}. Based on this finding, we introduce a new low complexity postprocessing technique to push BER floors down to 10^{-15}.

A 90 nm CMOS DSP MLSD Transceiver With Integrated AFE for Electronic Dispersion Compensation of Multimode Optical Fibers at 10 Gb/s

This paper presents the architecture and circuit design of a single chip 32 mm2 90 nm CMOS DSP transceiver for electronic dispersion compensation (EDC) of multimode fibers at 10 Gb/s, based on maximum likelihood sequence detection (MLSD). This is the first MLSD-based transceiver for multimode fibers and the first fully integrated DSP based transceiver for optical channels reported in the technical literature. The digital receiver incorporates equalization, Viterbi detection, channel estimation, timing recovery, and gain control functions. The analog front-end incorporates an 8-way interleaved ADC with self-calibration, a programmable gain amplifier, a phase interpolator, and the transmitter. Also integrated are a XAUI interface, the physical coding sublayer (PCS), and miscellaneous test and control functions. Experimental results using the stressors specified by the IEEE 10 GBASE-LRM standard, as well as industry-defined worst-case fibers are reported. A sensitivity of - 13.68 dBm is demonstrated for the symmetric stressor in a line card application with a 6 inch FR4 interconnect.

A 40nm CMOS single-chip 50Gb/s DP-QPSK/BPSK transceiver with electronic dispersion compensation for coherent optical channels

Optical communication technology in long-haul and metropolitan links is experiencing a transition to coherent techniques and high spectral efficiency modulation formats such as dual-polarization (DP) QPSK, DP-QAM and OFDM. The combination of coherent demodulation and DSP allows costly optical signal-processing hardware used to compensate fiber optic impairments such as chromatic dispersion (CD) and polarization-mode dispersion (PMD) to be replaced by DSP-based techniques [1]. Economic large-scale deployment of coherent systems requires the integration of the optical transceiver functions in CMOS technology.

Performance of MLSE-based receivers in lightwave systems with nonlinear dispersion and amplified spontaneous emission noise

Maximum likelihood sequence estimation (MLSE) has been proposed in earlier literature to combat the effects of nonlinear dispersion in intensity modulation/direct detection (IM/DD) optical channels. We develop a theory of the bit error rate (BER) of MLSE-based IM/DD receivers operating in the presence of nonlinear intersymbol interference (ISI) and amplified spontaneous emission (ASE) noise. Numerical results confirm the predictions of our theory. Based on the new analysis, we also investigate the log-likelihood ratio (LLR) of the received signal yielded by a soft-input/soft-output (SISO) front-end decoder. Our study shows that traditional channel codes designed for transmissions over AWGN channels, in combination with SISO front-end detectors, achieve an asymptotically optimal performance in transmissions over IM/DD optical channels.

Architecture of a Single-Chip 50 Gb/s DP-QPSK/BPSK Transceiver With Electronic Dispersion Compensation for Coherent Optical Channels

The architecture of a single-chip dual-polarization QPSK/BPSK 50 Gigabits per second (Gb/s) DSP-based transceiver for coherent optical communications is presented. The receiver compensates the chromatic dispersion (CD) of more than 3,500 km of standard single-mode fiber using a frequency-domain equalizer. A time-domain four-dimensional MIMO transversal equalizer compensates up to 200 ps of differential group delay (DGD) and 8000 ps 2 of second-order polarization-mode dispersion (SOPMD). Other key DSP functions of the receiver include carrier and timing recovery, automatic gain control, channel diagnostics, etc. A novel low-latency parallel-processing carrier recovery implementation which is robust in the presence of laser phase noise and frequency jitter is proposed. The chip integrates the transmitter, receiver, framer and host interface functions and features a 4-channel 25 Gs/s 6-bit ADC with a figure of merit (FOM) of 0.4 pJ/conversion. Each ADC channel is based on an 8-way interleaved flash architecture. The DSP uses a 16-way parallel processing architecture. Extensive measurement results are presented which confirm the design targets. Measured optical signal-to-noise ratio (OSNR) penalty when compensating 200 ps DGD and 8000 ps 2 is 0.1 dB, while OSNR penalty when compensating 55 ns/nm CD (corresponding to 3,500 km of standard single-mode fiber) is 0.5 dB.

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

Design Tradeoffs and Challenges in Practical Coherent Optical Transceiver Implementations

This tutorial discusses the design and ASIC implementation of coherent optical transceivers. Algorithmic and architectural options and tradeoffs between performance and complexity/power dissipation are presented. Particular emphasis is placed on flexible (or reconfigurable) transceivers because of their importance as building blocks of software-defined optical networks. The paper elaborates on some advanced digital signal processing (DSP) techniques such as iterative decoding, which are likely to be applied in future coherent transceivers based on higher order modulations. Complexity and performance of critical DSP blocks such as the forward error correction decoder and the frequency-domain bulk chromatic dispersion equalizer are analyzed in detail. Other important ASIC implementation aspects including physical design, signal and power integrity, and design for testability, are also discussed.

On the relationship between the block error and channel-state Markov models in transmissions over slow-fading channels

Block-error processes in transmissions over slow-fading channels can be accurately modeled by a two-state Markov chain [the block Markov model (BMM)]. Another line of research has focused on the use of a channel-state Markov model (CSMM) to analyze block transmissions. Although both techniques provide results that agree well with observations, the relationship between both Markov models has not been recognized in the previous literature. In this letter, we show that the BMM for slow-fading channels can be directly derived from the CSMM. In addition, we introduce a greatly simplified channel-modeling methodology. In the new methodology, the BMM is the primary channel characterization tool, and the CSMM becomes essentially an estimation technique that provides parameters for the BMM. Results of packet transmissions in slow-fading channels show that our approach provides significant improvements in both accuracy and simplicity over previously proposed techniques.