Oscar E. Agazzi

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.

A 10.3GS/s 6bit (5.1 ENOB at Nyquist) time-interleaved/pipelined ADC using open-loop amplifiers and digital calibration in 90nm CMOS

A 10.3 GS/s ADC with 5 GHz input BW and 6 bit resolution in 90 nm CMOS is presented. The architecture is based on an 8 way interleaved/ pipelined ADC using open-loop amplifiers and digital calibration. The measured performance is 5.8 ENOB (36.6 dB SNDR) for a 100 MHz input signal and 5.1 ENOB (32.4 dB SNDR) for a 5 GHz input (Nyquist) with phase offset correction across the interleaved array.

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.

Maximum likelihood sequence estimation in the presence of chromatic and polarization mode dispersion in intensity modulation/direct detection optical channels

In this paper we investigate maximum likelihood sequence estimation (MLSE) receivers operating on intensity modulated direct detection optical channels. Our study focuses on long haul or metro links spanning several hundred kilometers of single mode fiber with optical amplifiers. We describe the structure of MLSE-based optical receivers operating in the presence of dispersion and amplified spontaneous emission (ASE) noise, and we develop a theory of the error rate of these receivers. Computer simulations show a close agreement between the predictions of the theory and simulation results. We also address some important implementation issues. 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 the dominant source of noise is often ASE noise, which is distributed according to a noncentral chi-square probability density function (pdf). Additionally, optical fibers suffer from chromatic and polarization mode dispersion (PMD). Although the use of MLSE in optical channels has been discussed in earlier literature (J. H. Winter and R. D. Githin, Sept. 1990) (H.F. Haunstein et. al., 2001) no detailed analysis of optical receivers using this technique has been reported so far. This motivates the study reported in this paper.

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

Combination of InP MZM Transmitter and Monolithic CMOS 8-State MLSE Receiver for Dispersion Tolerant 10 Gb/s Transmission

We demonstrate that InP modulators together with 1 sample/bit MLSE gives equivalent performance to linear electro-optic Mach-Zehnder modulators combined with oversampled MLSE, potentially providing significant reduction in power dissipation and footprint.

Background calibration of interleaved analog to digital converters for high-speed communications using interleaved timing recovery techniques

We introduce the interleaved timing recovery (ITR) technique, with the objective of compensating sampling time errors in interleaved arrays of analog to digital converters (ADC) in high-speed communications receivers. When combined with interleaved automatic gain control (IAGC) and offset compensation (IOC), also discussed, this technique effectively compensates the most important components of fixed pattern noise, an impairment of interleaved ADCs that limits their performance and applicability. This technique exploits functions that are already present in the receiver. By modifying these functions, they can be used to compensate the impairments. This technique compares favorably in terms of performance and hardware complexity with previously known analog and digital calibration techniques. Performance is demonstrated by computer simulation of a 12-bit, 8-way interleaved ADC array whose effective resolution is limited to 6 bits before compensation.

Compensation of track and hold frequency response mismatches in interleaved analog to digital converters for high-speed communications

In this paper, we investigate the effect of mismatches in the frequency responses of the track-and-hold (T&H) amplifiers of an interleaved array of analog-to-digital converters (ADC) used as the front-end of a high-speed communications receiver. Furthermore, we introduce digital signal processing (DSP) techniques to compensate the performance loss caused by these mismatches. These techniques take advantage of the specific application of the ADC as a front-end of a digital communication receiver, assumed to be based on a parallel processing implementation of a decision-feedback equalizer (DFE) (Kasturia and Winters, 1991; Parhi, 1999). With a relatively simple modification, the DFE can be transformed into a multiple-input, multiple-output (MIMO) equalizer. The latter effectively compensates the effect of the T&H frequency response mismatch. The performance measure used in this paper is the signal to noise ratio (SNR) at the slicer. An SNR loss of 2.5 dB or more could result from the T/H mismatch. Most of this loss is compensated by the MIMO equalizer. Our conclusions can be easily extended to receiver architectures other than the DFE, for example maximum-likelihood sequence estimation (MLSE) (Agazzi, 2005). As an example of the effectiveness of the techniques introduced here, we present an optical receiver with electronic dispersion compensation (EDC) for the emergent IEEE 802.3aq standard for 10 Gb/s Ethernet over multimode fibers

A new low complexity iterative equalization architecture for high-speed receivers on highly dispersive channels: Decision feedforward equalizer (DFFE)

This paper introduces the decision feedforward equalizer (DFFE), a new low complexity iterative equalization architecture for high speed receivers used on channels with large intersymbol interference (ISI). The decision feedback equalizer (DFE) is one of the preferred receivers for high ISI channels. The high data and symbol rate of current communications systems often require parallel processing receiver implementations [1]. Unfortunately, the complexity of parallel architectures of the DFE grows exponentially with the channel memory [2] and limits their application to low ISI channels. The DFFE avoids the exponential growth of the DFE by using tentative decisions to iteratively cancel the ISI. We show that the DFFE can achieve performance similar to the DFE. These benefits make it an excellent choice for high-speed receivers required to operate over highly dispersive channels.