Coding and Probabilistic Inference Methods for Data-Dependent Two-Dimensional Channels

Recent advances in magnetic recording systems, optical recording devices and flash memory drives necessitate to study two-dimensional (2-D) coding techniques for reliable storage/retrieval of information. Most channels in such systems introduce errors in messages in response to certain data patterns, and messages containing these patterns are more prone to errors than others. For example, in a single-level cell flash memory channel, inter-cell interference (ICI) is at its maximum when 101 patterns are programmed over adjacent cells in either horizontal or vertical directions. As another example, in two-dimensional magnetic recording channels, 2-D isolated-bits patterns are shown empirically to be the dominant error event, and during the read-back process inter-symbol interference (ISI) and inter-track interference (ITI) arise when these patterns are recorded over the magnetic medium. Shannon in his seminal work, ``A Mathematical Theory of Communications," presented two techniques for reliable transmission of messages over noisy channels, namely error correction coding and constrained coding. In the first method, messages are protected via an error correction code (ECC) from random errors which are independent of input data. The theory of ECCs is well studied, and efficient code construction methods are developed for simple binary channels, additive white Gaussian noise (AWGN) channels and partial response channels. On the other hand, constrained coding reduces the likelihood of corruption by removing problematic patterns before transmission over data-dependent channels. Prominent examples of constraints include a family of binary one-dimensional (1-D) and 2-D $\left(d,k\right)$-run-length-limited (RLL) constraints which improves resilience to ISI timing recovery and synchronization for bandwidth limited partial response channels, where d and k represent the minimum and maximum number of admissible zeros between two successive ones in any direction of array. In principle, the ultimate coding approach for such data-dependent channels is to design a set of sufficiently distinct error correction codewords that also satisfy channel constraints. Designing channel codewords satisfying both ECC and channel constraints is important as it would achieve the channel capacity. However, in practice this is difficult, and we rely on sub-optimal methods such as forward concatenation method (standard concatenation), reverse concatenation method (modified concatenation), and combinations of these approaches. In this dissertation, we focus on the problem of reliable transmission of binary messages over data-dependent 2-D communication channels. Our work is concerned with several challenges in regard to the transmission of binary messages over data-dependent 2-D channels. Design of Two-Dimensional Magnetic Recording (TDMR) Detector and Decoder: TDMR achieves high areal densities by reducing the size of a bit comparable to the size of the magnetic grains resulting in 2-D ISI and very high media noise. Therefore, it is critical to handle the media noise along with the 2-D ISI detection. In this work, we tune the Generalized Belief Propagation (GBP) algorithm to handle the media noise seen in TDMR. We also provide an intuition into the nature of hard decisions provided by the GBP algorithm. Investigation into Harmful Patterns for TDMR channels: This work investigates into the Voronoi based media model to study the harmful patterns over multi-track shingled recording systems. Through realistic quasi micromagnetic simulations studies, we identify 2-D data patterns that contribute to high media noise. We look into the generic Voronoi model and present our analysis on multi-track detection with constrained coded data. We show that 2-D constraints imposed on input patterns result in an order of magnitude improvement in the bit error rate for TDMR systems. Understanding of Constraint Gain for TDMR Channels: We study performance gains of constrained codes in TDMR channels using the notion of constraint gain. We consider Voronoi based TDMR channels with realistic grain, bit, track and magnetic-head dimensions. Specifically, we investigate the constraint gain for 2-D no-isolated-bits constraint over Voronoi based TDMR channels. We focus on schemes that employ the GBP algorithm for obtaining information rate estimates for TDMR channels. Design of Novel Constrained Coding Methods: In this work, we present a deliberate bit flipping (DBF) coding scheme for binary 2-D channels, where specific patterns in channel inputs are the significant cause of errors. The idea is to eliminate a constrained encoder and, instead, embed a constraint into an error correction codeword that is arranged into a 2-D array by deliberately flipping the bits that violate the constraint. The DBF method relies on the error correction capability of the code being used so that it should be able to correct both deliberate errors and channel errors. Therefore, it is crucial to flip minimum number of bits in order not to overburden the error correction decoder. We devise a constrained combinatorial formulation for minimizing the number of flipped bits for a given set of harmful patterns. The GBP algorithm is used to find an approximate solution for the problem. Devising Reduced Complexity Probabilistic Inference Methods: We propose a reduced complexity GBP that propagates messages in Log-Likelihood Ratio (LLR) domain. The key novelties of the proposed LLR-GBP are: (i) reduced fixed point precision for messages instead of computational complex floating point format, (ii) operations performed in logarithm domain, thus eliminating the need for multiplications and divisions, (iii) usage of message ratios that leads to simple hard decision mechanisms

An Optical Grooming Switch for High-Speed Traffic Aggregation in Time, Space and Wavelength

In this book a novel optical switch is designed, developed, and tested. The switch integrates optical switching, transparent traffic aggregation/grooming, and optical regener-ation. Innovative switch subsystems are developed that enable these functionalities, including all-optical OTDM-to-WDM converters. High capacity ring interconnection between metro-core rings, carrying 130 Gbit/s OTDM traffic, and metro-access rings carring 43 Gbit/s WDM traffic is experimentally demonstrated. The developed switch features flexibility in bandwidth provisioning, scalability to higher traffic volumes, and backward compatibility with existing network implementations in a future-proof way

Coding and Probabilistic Inference Methods for Data-Dependent Two-Dimensional Channels

Recent advances in magnetic recording systems, optical recording devices and flash memory drives necessitate to study two-dimensional (2-D) coding techniques for reliable storage/retrieval of information. Most channels in such systems introduce errors in messages in response to certain data patterns, and messages containing these patterns are more prone to errors than others. For example, in a single-level cell flash memory channel, inter-cell interference (ICI) is at its maximum when 101 patterns are programmed over adjacent cells in either horizontal or vertical directions. As another example, in two-dimensional magnetic recording channels, 2-D isolated-bits patterns are shown empirically to be the dominant error event, and during the read-back process inter-symbol interference (ISI) and inter-track interference (ITI) arise when these patterns are recorded over the magnetic medium. Shannon in his seminal work, ``A Mathematical Theory of Communications," presented two techniques for reliable transmission of messages over noisy channels, namely error correction coding and constrained coding. In the first method, messages are protected via an error correction code (ECC) from random errors which are independent of input data. The theory of ECCs is well studied, and efficient code construction methods are developed for simple binary channels, additive white Gaussian noise (AWGN) channels and partial response channels. On the other hand, constrained coding reduces the likelihood of corruption by removing problematic patterns before transmission over data-dependent channels. Prominent examples of constraints include a family of binary one-dimensional (1-D) and 2-D $\left(d,k\right)$-run-length-limited (RLL) constraints which improves resilience to ISI timing recovery and synchronization for bandwidth limited partial response channels, where d and k represent the minimum and maximum number of admissible zeros between two successive ones in any direction of array. In principle, the ultimate coding approach for such data-dependent channels is to design a set of sufficiently distinct error correction codewords that also satisfy channel constraints. Designing channel codewords satisfying both ECC and channel constraints is important as it would achieve the channel capacity. However, in practice this is difficult, and we rely on sub-optimal methods such as forward concatenation method (standard concatenation), reverse concatenation method (modified concatenation), and combinations of these approaches. In this dissertation, we focus on the problem of reliable transmission of binary messages over data-dependent 2-D communication channels. Our work is concerned with several challenges in regard to the transmission of binary messages over data-dependent 2-D channels. Design of Two-Dimensional Magnetic Recording (TDMR) Detector and Decoder: TDMR achieves high areal densities by reducing the size of a bit comparable to the size of the magnetic grains resulting in 2-D ISI and very high media noise. Therefore, it is critical to handle the media noise along with the 2-D ISI detection. In this work, we tune the Generalized Belief Propagation (GBP) algorithm to handle the media noise seen in TDMR. We also provide an intuition into the nature of hard decisions provided by the GBP algorithm. Investigation into Harmful Patterns for TDMR channels: This work investigates into the Voronoi based media model to study the harmful patterns over multi-track shingled recording systems. Through realistic quasi micromagnetic simulations studies, we identify 2-D data patterns that contribute to high media noise. We look into the generic Voronoi model and present our analysis on multi-track detection with constrained coded data. We show that 2-D constraints imposed on input patterns result in an order of magnitude improvement in the bit error rate for TDMR systems. Understanding of Constraint Gain for TDMR Channels: We study performance gains of constrained codes in TDMR channels using the notion of constraint gain. We consider Voronoi based TDMR channels with realistic grain, bit, track and magnetic-head dimensions. Specifically, we investigate the constraint gain for 2-D no-isolated-bits constraint over Voronoi based TDMR channels. We focus on schemes that employ the GBP algorithm for obtaining information rate estimates for TDMR channels. Design of Novel Constrained Coding Methods: In this work, we present a deliberate bit flipping (DBF) coding scheme for binary 2-D channels, where specific patterns in channel inputs are the significant cause of errors. The idea is to eliminate a constrained encoder and, instead, embed a constraint into an error correction codeword that is arranged into a 2-D array by deliberately flipping the bits that violate the constraint. The DBF method relies on the error correction capability of the code being used so that it should be able to correct both deliberate errors and channel errors. Therefore, it is crucial to flip minimum number of bits in order not to overburden the error correction decoder. We devise a constrained combinatorial formulation for minimizing the number of flipped bits for a given set of harmful patterns. The GBP algorithm is used to find an approximate solution for the problem. Devising Reduced Complexity Probabilistic Inference Methods: We propose a reduced complexity GBP that propagates messages in Log-Likelihood Ratio (LLR) domain. The key novelties of the proposed LLR-GBP are: (i) reduced fixed point precision for messages instead of computational complex floating point format, (ii) operations performed in logarithm domain, thus eliminating the need for multiplications and divisions, (iii) usage of message ratios that leads to simple hard decision mechanisms

Distributed synchronization algorithms for wireless sensor networks

The ability to distribute time and frequency among a large population of interacting agents is of interest for diverse disciplines, inasmuch as it enables to carry out complex cooperative tasks. In a wireless sensor network (WSN), time/frequency synchronization allows the implementation of distributed signal processing and coding techniques, and the realization of coordinated access to the shared wireless medium. Large multi-hop WSN\u27s constitute a new regime for network synchronization, as they call for the development of scalable, fully distributed synchronization algorithms. While most of previous research focused on synchronization at the application layer, this thesis considers synchronization at the lowest layers of the communication protocol stack of a WSN, namely the physical and the medium access control (MAC) layer. At the physical layer, the focus is on the compensation of carrier frequency offsets (CFO), while time synchronization is studied for application at the MAC layer. In both cases, the problem of realizing network-wide synchronization is approached by employing distributed clock control algorithms based on the classical concept of coupled phase and frequency locked loops (PLL and FLL). The analysis takes into account communication, signaling and energy consumption constraints arising in the novel context of multi-hop WSN\u27s. In particular, the robustness of the algorithms is checked against packet collision events, infrequent sync updates, and errors introduced by different noise sources, such as transmission delays and clock frequency instabilities. By observing that WSN\u27s allow for greater flexibility in the design of the synchronization network architecture, this work examines also the relative merits of both peer-to-peer (mutually coupled - MC) and hierarchical (master-slave - MS) architectures. With both MC and MS architectures, synchronization accuracy degrades smoothly with the network size, provided that loop parameters are conveniently chosen. In particular, MS topologies guarantee faster synchronization, but they are hindered by higher noise accumulation, while MC topologies allow for an almost uniform error distribution at the price of much slower convergence. For all the considered cases, synchronization algorithms based on adaptive PLL and FLL designs are shown to provide robust and scalable network-wide time and frequency distribution in a WSN

Efficient implementation of a dominance protocol for wireless medium access

Mestrado em Engenharia Electrotécnica e de ComputadoresEmbedded computing systems went through extraordinary evolutions during the past two decades, representing nowadays one of the most promising technologies for improving a wide range of application areas such as energy/resource management, safety, health or entertainment. New sensors and actuators are leading to an unprecedented level of interaction between computing systems and their surrounding physical environment. These embedded computers tend to be networked, often wirelessly, and they are becoming denser, of larger scale and more pervasively deployed. Since the wireless channel is a "natural resource" which must be shared between this large number of embedded computers, the medium access control (MAC) protocol significantly influences the performance of the entire system. In particular, satisfying real-time requirements — something that is needed for a computer to tightly interact with its physical environment — plays an important role. One solution was recently proposed by Pereira, Andersson and Tovar. It was a prioritized and collision-free MAC protocol belonging to a family of protocols called dominance/binary countdown protocols. This solution was implemented in commercial-off-the-shelf (COTS) wireless sensor networks (WSN) platforms and the implementation was demonstrated to be working. Unfortunately, those platforms had (for the MAC protocol) unfavourable characteristics which lead to limited efficiency and excessive overhead of the MAC protocol. This work presents a new hardware platform, in the form of a network adapter for common WSN platforms, that allows an efficient implementation of dominance protocols for wireless medium access, allowing the medium access to be performed in less than 5 ms for 216 priority levels, which represents an overhead reduction of more than ten times as compared to the protocol implementation in COTS WSN platforms. Additionally, the overall energy consumption was reduced by approximately 45 % when compared to the theoretical best-case performance of the protocol implementation in COTS WSN platforms. iv This work also allowed, for the first time ever, an aggregate computation scheme for WSN to work exploiting the new efficient implementation of a binary/dominance countdown protocol.No decurso das duas últimas décadas, os sistemas de computação embebidos sofreram uma extraordinária evolução, representando hoje em dia uma das mais promissoras tecnologias para possibilitar melhorias em áreas de aplicação tão diversas como a gestão de energia/recursos, segurança, saúde ou entretenimento. Novos sensores e actuadores conduzem a interacção entre os sistemas de computação e o ambiente físico onde se encontram inseridos até um nível sem precedentes. Estes computadores embebidos tendem a encontrar-se interligados (frequentemente recorrendo a ligações sem fios), sendo instalados de forma intensiva, e cada vez em maior escala. Sendo o meio de transmissão sem fios um “recurso natural” que deve ser partilhado entre este número elevado de computadores embebidos, o protocolo de controlo de acesso ao meio (do Inglês Medium Access Control - MAC) influencia significativamente o desempenho de todo o sistema. Em particular, a satisfação de requisitos temporais — algo que é necessário para uma estreira interacção entre um computador e o meio físico onde se insere — desempenha um papel muito importante. Uma solução foi recentemente proposta por Pereira, Andersson e Tovar, consistindo num protocolo MAC que implementa escalonamento baseado em prioridades e livre de colisões. Esta solução foi implementada em plataformas comercias de redes de sensores sem fios (do Inglês Wireless Sensor Networks - WSN), demonstrando-se funcional. Infelizmente, para a implementação do protocolo MAC, tais plataformas possuem características desfavoráveis, que conduzem a uma eficiência limitada do mesmo. Este trabalho apresenta uma nova platforma de hardware, sob a forma de um adaptador de rede, para platformas de WSN disponíveis comercialmente, que permite que a realização do acesso ao meio decorra em menos de 5 ms (para 216 níveis de prioridades). Tal representa uma redução superior a dez vezes no custo da execução do protocolo, quando comparada com a anterior implementação em platformas comercias de WSN. Adicionalmente, o consumo global de energia foi reduzido em aproximadamente 45 %, quando comparado com o melhor desempenho teórico possível da implementação em plataformas comerciais de WSN. vi Este trabalho permitiu ainda que, pela primeira vez, um esquema de computação agregada para WSN operasse tirando partido da implementação eficiente de um protocolo MAC apresentada neste trabalho

A general-purpose pulse sequencer for quantum computing

Thesis (M. Eng.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2005.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Includes bibliographical references (p. 165-170).Quantum mechanics presents a more general and potentially more powerful model of computation than classical systems. Quantum bits have many physically different representations which nonetheless share a common need for modulating pulses of electromagnetic waves. This thesis presents the design and evaluates the implementation of a general-purpose sequencer which supports fast, programmable pulses; a flexible, open design; and feedback operation for adaptive algorithms. The sequencer achieves a timing resolution, minimum pulse duration, and minimum delay of 10 nanoseconds; it has 64 simultaneously-switching, independent digital outputs and 8 digital inputs for triggering or feedback. Multiple devices can operate in a daisy chain to facilitate adding and removing channels. An FPGA is used to implement a firmware network stack and a specialized pulse processor core whose modules are all interconnected using the Wishbone bus standard. Users can write pulse programs in an assembly language and control the device from a host computer over an Ethernet network. An embedded web server provides an intuitive, graphical user interface, while a non-interactive, efficient UDP protocol provides programmatic access to third-party software. The performance characteristics, tolerances, and cost of the device are measured and compared with those of contemporary research and commercial offerings. Future improvements and extensions are suggested. All circuit schematics, PCB layouts, source code, and design documents are released under an open source license.by Paul Tân Thế Phạm.M.Eng

Investigation of the limiting fibre nonlinearities and their suppression in 40Gbit/s optical transmission systems.

This thesis investigates the fundamental limitations to optical transmission at a bit-rate of 40Gbit/s. The signal distortion due to nonlinear effects, noise and dispersion are analysed and techniques for their suppression through dispersion management and optimum choice of modulation format are demonstrated. The high launch powers required for overcoming noise from the amplifiers result in an increase in fibre nonlinearities. Transmission at 40Gbit/s favours the RZ modulation format. However, RZ signals were found to be limited by intra-channel cross phase modulation (IXPM) and intra-channel four-wave-mixing (IFWM). These intra-channel nonlinear effects take place as a result of nonlinear interaction between overlapping pulses of the same wavelength channel. Minimising such pulse overlap by controlling the dispersion-induced pulse broadening during propagation in the fibre was investigated by reducing the fibre local dispersion and by pre-compensating the signal at the transmitter. Dispersion compensation using higher-order-mode devices with high nonlinear tolerance was also investigated, enabling transmission over in-line pre-compensated amplifier spans. In the second part of this thesis, the nonlinear tolerance of the RZ modulation format was increased by use of alternate-polarisation and alternate-phase between adjacent pulses. These techniques were found to improve the transmission performance by approximately 50% and required simple modifications to the transmitter only. These advanced RZ signals were found to be compatible with dispersion management techniques. However, the optimum pre-compensation at the transmitter was found to be dependent on the modulation format and dominant intra-channel effect. A novel modulation format combining alternate-polarisation and phase simultaneously was demonstrated for maximum nonlinear suppression without the use of dispersion management. Finally, a new experimental technique was demonstrated for the investigation of dispersion tolerance. It was found that the choice of optimum modulation format requires a trade-off between nonlinear tolerance and dispersion tolerance. The results of this work can be applied to optimise the design rules of future optical networks