749 research outputs found
Available Techniques for Magnetic Hard Disk Drive Read Channel Equalization
This paper presents an extensive, non-exhaustive, study of available hard disk drive read channel equalization techniques used in the storage and readback of magnetically stored information. The physical elements and basic principles of the storage processes are introduced together with the basic theoretical definitions and models. Both read and write processes in magnetic storage are explained along with the definition of simple key concepts such as user bit density, intersymbol interference, linear and areal density, read head pulse response models, and coding algorithm
Advanced Coding And Modulation For Ultra-wideband And Impulsive Noises
The ever-growing demand for higher quality and faster multimedia content delivery over short distances in home environments drives the quest for higher data rates in wireless personal area networks (WPANs). One of the candidate IEEE 802.15.3a WPAN proposals support data rates up to 480 Mbps by using punctured convolutional codes with quadrature phase shift keying (QPSK) modulation for a multi-band orthogonal frequency-division multiplexing (MB-OFDM) system over ultra wideband (UWB) channels. In the first part of this dissertation, we combine more powerful near-Shannon-limit turbo codes with bandwidth efficient trellis coded modulation, i.e., turbo trellis coded modulation (TTCM), to further improve the data rates up to 1.2 Gbps. A modified iterative decoder for this TTCM coded MB-OFDM system is proposed and its bit error rate performance under various impulsive noises over both Gaussian and UWB channel is extensively investigated, especially in mismatched scenarios. A robust decoder which is immune to noise mismatch is provided based on comparison of impulsive noises in time domain and frequency domain. The accurate estimation of the dynamic noise model could be very difficult or impossible at the receiver, thus a significant performance degradation may occur due to noise mismatch. In the second part of this dissertation, we prove that the minimax decoder in \cite, which instead of minimizing the average bit error probability aims at minimizing the worst bit error probability, is optimal and robust to certain noise model with unknown prior probabilities in two and higher dimensions. Besides turbo codes, another kind of error correcting codes which approach the Shannon capacity is low-density parity-check (LDPC) codes. In the last part of this dissertation, we extend the density evolution method for sum-product decoding using mismatched noises. We will prove that as long as the true noise type and the estimated noise type used in the decoder are both binary-input memoryless output symmetric channels, the output from mismatched log-likelihood ratio (LLR) computation is also symmetric. We will show the Shannon capacity can be evaluated for mismatched LLR computation and it can be reduced if the mismatched LLR computation is not an one-to-one mapping function. We will derive the Shannon capacity, threshold and stable condition of LDPC codes for mismatched BIAWGN and BIL noise types. The results show that the noise variance estimation errors will not affect the Shannon capacity and stable condition, but the errors do reduce the threshold. The mismatch in noise type will only reduce Shannon capacity when LLR computation is based on BIL
Design of adaptive analog filters for magnetic front-end read channels
Esta tese estuda o projecto e o comportamento de filtros em tempo contÃnuo de
muito-alta-frequência. A motivação deste trabalho foi a investigação de soluções de filtragem
para canais de leitura em sistemas de gravação e reprodução de dados em suporte
magnético, com custos e consumo (tamanho total inferior a 1 mm2 e consumo inferior a
1mW/polo), inferiores aos circuitos existentes. Nesse sentido, tal como foi feito neste
trabalho, o rápido desenvolvimento das tecnologias de microelectrónica suscitou esforços
muito significativos a nÃvel mundial com o objectivo de se investigarem novas técnicas
de realização de filtros em circuito integrado monolÃtico, especialmente em tecnologia
CMOS (Complementary Metal Oxide Semiconductor). Apresenta-se um estudo comparativo
a diversos nÃveis hierárquicos do projecto, que conduziu à realização e caracterização
de soluções com as caracterÃsticas desejadas.
Num primeiro nÃvel, este estudo aborda a questão conceptual da gravação e transmissão
de sinal bem como a escolha de bons modelos matemáticos para o tratamento da
informação e a minimização de erro inerente à s aproximações na conformidade aos princÃpios
fÃsicos dos dispositivos caracterizados.
O trabalho principal da tese é focado nos nÃveis hierárquicos da arquitectura do
canal de leitura e da realização em circuito integrado do seu bloco principal – o bloco de
filtragem. Ao nÃvel da arquitectura do canal de leitura, apresenta-se um estudo alargado
sobre as metodologias existentes de adaptação de sinal e recuperação de dados em suporte
magnético. Este desÃgnio aparece no âmbito da proposta de uma solução de baixo custo,
baixo consumo, baixa tensão de alimentação e baixa complexidade, alicerçada em tecnologia
digital CMOS, para a realização de um sistema DFE (Decision Feedback Equalization)
com base na igualização de sinal utilizando filtros integrados analógicos em tempo
contÃnuo.
Ao nÃvel do projecto de realização do bloco de filtragem e das técnicas de implementação
de filtros e dos seus blocos constituintes em circuito integrado, concluiu-se que
a técnica baseada em circuitos de transcondutância e condensadores, também conhecida como filtros gm-C (ou transcondutância-C), é a mais adequada para a realização de filtros
adaptativos em muito-alta-frequência. Definiram-se neste nÃvel hierárquico mais baixo,
dois subnÃveis de aprofundamento do estudo no âmbito desta tese, nomeadamente: a pesquisa
e análise de estruturas ideais no projecto de filtros recorrendo a representações no
espaço de estados; e, o estudo de técnicas de realização em tecnologia digital CMOS de
circuitos de transcondutância para a implementação de filtros integrados analógicos em
tempo contÃnuo.
Na sequência deste estudo, apresentam-se e comparam-se duas estruturas de filtros
no espaço de estados, correspondentes a duas soluções alternativas para a realização de
um igualador adaptativo realizado por um filtro contÃnuo passa-tudo de terceira ordem,
para utilização num canal de leitura de dados em suporte magnético.
Como parte constituinte destes filtros, apresenta-se uma técnica de realização de
circuitos de transcondutância, e de realização de condensadores lineares usando matrizes
de transÃstores MOSFET para processamento de sinal em muito-alta-frequência realizada
em circuito integrado usando tecnologia digital CMOS submicrométrica. Apresentam-se
métodos de adaptação automática capazes de compensar os erros face aos valores nominais
dos componentes, devidos às tolerâncias inerentes ao processo de fabrico, para os
quais apresentamos os resultados de simulação e de medição experimental obtidos.
Na sequência deste estudo, resultou igualmente a apresentação de um circuito passÃvel
de constituir uma solução para o controlo de posicionamento da cabeça de leitura
em sistemas de gravação/reprodução de dados em suporte magnético. O bloco proposto
é um filtro adaptativo de primeira ordem, com base nos mesmos circuitos de transcondutância
e técnicas de igualação propostos e utilizados na implementação do filtro adaptativo
de igualação do canal de leitura.
Este bloco de filtragem foi projectado e incluÃdo num circuito integrado (Jaguar) de
controlo de posicionamento da cabeça de leitura realizado para a empresa ATMEL em
Colorado Springs, e incluÃdo num produto comercial em parceria com uma empresa escocesa
utilizado em discos rÃgidos amovÃveis.This thesis studies the design and behavior of continuous-time very-high-frequency
filters. The motivation of this work was the search for filtering solutions for the readchannel
in recording and reproduction of data on magnetic media systems, with costs and
consumption (total size less than 1 mm2 and consumption under 1mW/pole), lower than
the available circuits. Accordingly, as was done in this work, the rapid development of
microelectronics technology raised very significant efforts worldwide in order to investigate
new techniques for implementing such filters in monolithic integrated circuit, especially
in CMOS technology (Complementary Metal Oxide Semiconductor). We present
a comparative study on different hierarchical levels of the project, which led to the realization
and characterization of solutions with the desired characteristics.
In the first level, this study addresses the conceptual question of recording and
transmission of signal and the choice of good mathematical models for the processing of
information and minimization of error inherent in the approaches and in accordance with
the principles of the characterized physical devices.
The main work of this thesis is focused on the hierarchical levels of the architecture
of the read channel and the integrated circuit implementation of its main block - the filtering
block. At the architecture level of the read channel this work presents a comprehensive
study on existing methodologies of adaptation and signal recovery of data on
magnetic media. This project appears in the sequence of the proposed solution for a lowcost,
low consumption, low voltage, low complexity, using CMOS digital technology for
the performance of a DFE (Decision Feedback Equalization) based on the equalization of
the signal using integrated analog filters in continuous time.
At the project level of implementation of the filtering block and techniques for implementing
filters and its building components, it was concluded that the technique based
on transconductance circuits and capacitors, also known as gm-C filters is the most appropriate
for the implementation of very-high-frequency adaptive filters. We defined in
this lower level, two sub-levels of depth study for this thesis, namely: research and analysis
of optimal structures for the design of state-space filters, and the study of techniques for the design of transconductance cells in digital CMOS circuits for the implementation
of continuous time integrated analog filters.
Following this study, we present and compare two filtering structures operating in
the space of states, corresponding to two alternatives for achieving a realization of an
adaptive equalizer by the use of a continuous-time third order allpass filter, as part of a
read-channel for magnetic media devices.
As a constituent part of these filters, we present a technique for the realization of
transconductance circuits and for the implementation of linear capacitors using arrays of
MOSFET transistors for signal processing in very-high-frequency integrated circuits using
sub-micrometric CMOS technology. We present methods capable of automatic adjustment
and compensation for deviation errors in respect to the nominal values of the
components inherent to the tolerances of the fabrication process, for which we present
the simulation and experimental measurement results obtained.
Also as a result of this study, is the presentation of a circuit that provides a solution
for the control of the head positioning on recording/playback systems of data on magnetic
media. The proposed block is an adaptive first-order filter, based on the same transconductance
circuits and equalization techniques proposed and used in the implementation
of the adaptive filter for the equalization of the read channel.
This filter was designed and included in an integrated circuit (Jaguar) used to control
the positioning of the read-head done for ATMEL company in Colorado Springs, and
part of a commercial product used in removable hard drives fabricated in partnership with a Scottish company
Study of phase noise in optical coherent systems
Le bruit de phase est un problème important dans la conception de systèmes cohérents optiques. Bien que le bruit de phase soit étudié énormément dans les communications sans fil, certains aspects de bruit de phase sont nouveaux dans des systèmes cohérents optiques. Dans cette thèse, nous explorons les statistiques de bruit de phase dans les systèmes optiques cohérentes et proposons une nouvelle technique pour améliorer la robustesse du système envers le bruit de phase. Notre première contribution traite de l’étude des statistiques de bruit de phase en présence de compensation électronique de la dispersion chromatique (CD) dans des systèmes cohérents. Nous montrons que le modèle proposé précédemment pour l’interaction de CD avec bruit de phase doit être modifié à cause d’un modèle trop simple pour la récupération de phase. Nous dérivons une expression plus précise pour le bruit de phase estimé par la récupération de phase avec décision dirigée (DD), et utilisons cette expression pour modifier les statistiques de décision pour les symboles reçus. Nous calculons le taux d’erreur binaire (BER) pour le format de transmission DQPSK semi-analytiquement en utilisant nos statistiques de décision modifiées et montrons que pour la récupération de phase idéale, le BER semi-analytique est bien assorti avec le BER simulé avec la technique Monte-Carlo (MC). Notre deuxième contribution est l’adaptation d’une technique de codage MLCM pour les systèmes cohérents limités par le bruit de phase et le bruit blanc additif Gaussien (AWGN). Nous montrons que la combinaison d’une constellation optimisée pour le bruit de phase avec MLCM offre un système robuste à complexité modérée. Nous vérifions que la performance de MLCM dans des systèmes cohérents avec constellations 16-aires se détériorés par le bruit de phase non-linéaire et de Wiener. Pour le bruit de phase non-linéaire, notre conception de MLCM démontre une performance supérieure par rapport à une conception de MLCM déjà présente dans la littérature. Pour le bruit de phase de Wiener, nous comparons deux format de transmission, constellations carrées et optimisée pour bruit de phase, et deux techniques de codage, MLCM et codage à débit uniforme. Nos résultats expérimentaux pour BER après codage suivent les mêmes tendances que le BER simulé et confirment notre conception.Phase noise is an important issue in designing today’s optical coherent systems. Although phase noise is studied heavily in wireless communications, some aspects of phase noise are novel in optical coherent systems. In this thesis we explore phase noise statistics in optical coherent systems and propose a novel technique to increase system robustness toward phase noise. Our first contribution deals with the study of phase noise statistics in the presence of electronic chromatic dispersion (CD) compensation in coherent systems. We show that previously proposed model for phase noise and CD interaction must be modified due to an overly simple model of carrier phase recovery. We derive a more accurate expression for the estimated phase noise of decision directed (DD) carrier phase recovery, and use this expression to modify the decision statistics of received symbols. We calculate bit error rate (BER) of a differential quadrature phase shift keying (DQPSK) system semi-analytically using our modified decision statistics and show that for ideal DD carrier phase recovery the semi-analytical BER matches the BER simulated via Monte-Carlo (MC) technique. We show that the semi-analytical BER is a lower bound of simulated BER from Viterbi-Viterbi (VV) carrier phase recovery for a wide range of practical system parameters. Our second contribution is concerned with adapting a multi-level coded modulation (MLCM) technique for phase noise and additive white Gaussian noise (AWGN) limited coherent system. We show that the combination of a phase noise optimized constellation with MLCM offers a phase-noise robust system at moderate complexity. We propose a numerical method to design set-partitioning (mapping bits to symbols) and optimizing code rates for minimum block error rate (BLER).We verify MLCM performance in coherent systems of 16-ary constellations impaired by nonlinear and Wiener phase noise. For nonlinear phase noise, superior performance of our MLCM design over a previously designed MLCM system is demonstrated in terms of BLER. For Wiener phase noise, we compare optimized and square 16-QAM constellations assuming either MLCM or uniform rate coding. We compare post forward error correction (FEC) BER in addition to BLER by both simulation and experiment and show that superior BLER performance is translated into post FEC BER. Our experimental post FEC BER results follow the same trends as simulated BER, validating our design
CONVERGENCE IMPROVEMENT OF ITERATIVE DECODERS
Iterative decoding techniques shaked the waters of the error correction and communications
field in general. Their amazing compromise between complexity and performance
offered much more freedom in code design and made highly complex codes, that were
being considered undecodable until recently, part of almost any communication system.
Nevertheless, iterative decoding is a sub-optimum decoding method and as such, it has
attracted huge research interest. But the iterative decoder still hides many of its secrets,
as it has not been possible yet to fully describe its behaviour and its cost function.
This work presents the convergence problem of iterative decoding from various angles
and explores methods for reducing any sub-optimalities on its operation. The decoding
algorithms for both LDPC and turbo codes were investigated and aspects that contribute
to convergence problems were identified. A new algorithm was proposed, capable of providing
considerable coding gain in any iterative scheme. Moreover, it was shown that
for some codes the proposed algorithm is sufficient to eliminate any sub-optimality and
perform maximum likelihood decoding. Its performance and efficiency was compared to
that of other convergence improvement schemes.
Various conditions that can be considered critical to the outcome of the iterative decoder
were also investigated and the decoding algorithm of LDPC codes was followed
analytically to verify the experimental results
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Digital Signal Processing for Coherent Transceivers Employing Multilevel Formats
Digital coherent transceivers have revolutionized optical fiber communications due to their superior performance offered compared to intensity modulation and direct detection based alternatives. As systems employing digital coherent transceivers seek to approach their information theoretic capacity, the use of multilevel modulation formats combined with appropriate forward error correction becomes essential. Given this context, in this tutorial paper, we therefore explore the digital signal processing (DSP) utilized in a coherent transceiver with a focus on multilevel modulation formats. By way of an introduction, we open by discussing the photonic technology required to realize a coherent transceiver. After discussing this interface between the analog optical channel and the digital domain, the rest of the paper is focused on DSP. We begin by discussing algorithms that correct for imperfections in the optical to digital conversion, including IQ imbalance and timing skew. Next, we discuss channel equalization including means for their realization for both quasi-static and dynamic channel impairments. Synchronization algorithms that correct for the difference between the transmitter and receiver oscillators both optical and electrical are then discussed and issues associated with symbol decoding highlighted. For most of the cases, we start with polarization division multiplexed quadrature phase-shift keying (PDM-QPSK) format as a basis and then discuss the extension to allow for high order multilevel formats. Finally, we conclude by discussing some of the open research challenges in the field.This work was supported in part by the EU project ICONE (608099) and EPSRC through INSIGHT (EP/L026155/2) and UNLOC (EP/J017582/1)
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