Statistical mechanics of typical set decoding

The performance of "typical set (pairs) decoding" for ensembles of Gallager's linear code is investigated using statistical physics. In this decoding method, errors occur, either when the information transmission is corrupted by atypical noise, or when multiple typical sequences satisfy the parity check equation as provided by the received corrupted codeword. We show that the average error rate for the second type of error over a given code ensemble can be accurately evaluated using the replica method, including the sensitivity to message length. Our approach generally improves the existing analysis known in the information theory community, which was recently reintroduced in IEEE Trans. Inf. Theory 45, 399 (1999), and is believed to be the most accurate to date. © 2002 The American Physical Society.

Statistical physics of low density parity check error correcting codes

We study the performance of Low Density Parity Check (LDPC) error-correcting codes using the methods of statistical physics. LDPC codes are based on the generation of codewords using Boolean sums of the original message bits by employing two randomly-constructed sparse matrices. These codes can be mapped onto Ising spin models and studied using common methods of statistical physics. We examine various regular constructions and obtain insight into their theoretical and practical limitations. We also briefly report on results obtained for irregular code constructions, for codes with non-binary alphabet, and on how a finite system size effects the error probability.

Typical performance of low-density parity-check codes over general symmetric channels

Typical performance of low-density parity-check (LDPC) codes over a general binary-input output-symmetric memoryless channel is investigated using methods of statistical mechanics. The binary-input additive-white-Gaussian-noise channel and the binary-input Laplace channel are considered as specific channel noise models.

CDMA multiuser detection, neural networks, and statistical mechanics

A novel approach, based on statistical mechanics, to analyze typical performance of optimum code-division multiple-access (CDMA) multiuser detectors is reviewed. A `black-box' view ot the basic CDMA channel is introduced, based on which the CDMA multiuser detection problem is regarded as a `learning-from-examples' problem of the `binary linear perceptron' in the neural network literature. Adopting Bayes framework, analysis of the performance of the optimum CDMA multiuser detectors is reduced to evaluation of the average of the cumulant generating function of a relevant posterior distribution. The evaluation of the average cumulant generating function is done, based on formal analogy with a similar calculation appearing in the spin glass theory in statistical mechanics, by making use of the replica method, a method developed in the spin glass theory.

A statistical-mechanical analysis of coded CDMA with regular LDPC codes

We analyze, using the replica method of statistical mechanics, the theoretical performance of coded code-division multiple-access (CDMA) systems in which regular low-density parity-check (LDPC) codes are used for channel coding.

Statistical mechanics of broadcast channels using low-density parity-check codes

We investigate the use of Gallager's low-density parity-check (LDPC) codes in a degraded broadcast channel, one of the fundamental models in network information theory. Combining linear codes is a standard technique in practical network communication schemes and is known to provide better performance than simple time sharing methods when algebraic codes are used. The statistical physics based analysis shows that the practical performance of the suggested method, achieved by employing the belief propagation algorithm, is superior to that of LDPC based time sharing codes while the best performance, when received transmissions are optimally decoded, is bounded by the time sharing limit.

Enhancing performance through the introduction of customer orientation into the building components industry

Purpose: This paper aims to describe an investigation into how company performance can be improved by integrating internal and external customers and technology. The approach was developed, implemented and evaluated in the operations of the building components industry. The research was carried out in the precast concrete division of a Singapore company. Design/methodology/ approach: For the purpose of undertaking the investigation an exploratory case study approach was used. This was divided into conceptual and action research stages. The action research was also used to implement the changes in the company. Questionnaire surveys were carried out among company employees and external customers to assess the effect of these changes. Results of the investigation were derived using content and statistical analysis. Triangulation between three sources was used for validating the data. Findings: The exploratory case study strategy resulted in rich research data, which provided evidence of the changes taking place and integration happening, leading to improved performance. The action research approach proved a powerful tool where the uncertainty of outcomes makes it near impossible to make accurate forecasts. Another output of the research was the development of an "integrated customer orientation" (ICO) model. Research limitations/implications: The research in this paper used a single site action research investigation so should be interpreted within the specific company and industry context. There are implications for theory and practice in a number of areas of production and marketing as well as contributions to understanding about productivity improvement and organisational development. The investigation also fulfils the dual objectives of action research by contributing to both knowledge and practice. Originality/value: The paper describes a unique approach towards improving productivity, quality and service through the use of action research to implement changes, as well as providing the research evidence to evaluate both the process of implementation and results achieved. © Emerald Group Publishing Limited.

Malmquist indexes using a geometric distance function (GDF)

Traditional approaches to calculate total factor productivity change through Malmquist indexes rely on distance functions. In this paper we show that the use of distance functions as a means to calculate total factor productivity change may introduce some bias in the analysis, and therefore we propose a procedure that calculates total factor productivity change through observed values only. Our total factor productivity change is then decomposed into efficiency change, technological change, and a residual effect. This decomposition makes use of a non-oriented measure in order to avoid problems associated with the traditional use of radial oriented measures, especially when variable returns to scale technologies are to be compared.

The impact of leadership and quality climate on hospital performance

Objective. To explore the relationship between leadership effectiveness and health-care trust performance, taking into account external quality measures and the number of patient complaints; also, to examine the role of care quality climate as a mediator. Design. We developed scales for rating leadership effectiveness and care quality climate. We then drew upon UK national indices of health-care trust performance—Commission for Health Improvement star ratings, Clinical Governance Review ratings and the number of patient complaints per thousand. We conducted statistical analysis to examine any significant relationships between predictor and outcome variables. Setting. The study is based on 86 hospital trusts run by the National Health Service (NHS) in the UK. The data collection is part of an annual staff survey commissioned by the NHS to explore the quality of working life. Participants. A total of 17 949 employees were randomly surveyed (41% of the total sample). Results. Leadership effectiveness is associated with higher Clinical Governance Review ratings and Commission for Health Improvement star ratings for our sample (ß = 0.42, P < 0.05; ß = 0.37, P < 0.05, respectively), and lower patient complaints (ß = –0.57, P < 0.05). In addition, 98% of the relationship between leadership and patient complaints is explained by care quality climate. Conclusions. Results offer insight into how non-clinical leadership may foster performance outcomes for health-care organizations. A frequently neglected area—patient complaints—may be a valid measure to consider when assessing leadership and quality in a health-care context.

Noisy random Boolean formulae:a statistical physics perspective

Properties of computing Boolean circuits composed of noisy logical gates are studied using the statistical physics methodology. A formula-growth model that gives rise to random Boolean functions is mapped onto a spin system, which facilitates the study of their typical behavior in the presence of noise. Bounds on their performance, derived in the information theory literature for specific gates, are straightforwardly retrieved, generalized and identified as the corresponding macroscopic phase transitions. The framework is employed for deriving results on error-rates at various function-depths and function sensitivity, and their dependence on the gate-type and noise model used. These are difficult to obtain via the traditional methods used in this field.

Analysis and optimisation of the performance of nonlinear optical communication systems

We investigate the feasibility of simultaneous suppressing of the amplification noise and nonlinearity, representing the most fundamental limiting factors in modern optical communication. To accomplish this task we developed a general design optimisation technique, based on concepts of noise and nonlinearity management. We demonstrate the immense efficiency of the novel approach by applying it to a design optimisation of transmission lines with periodic dispersion compensation using Raman and hybrid Raman-EDFA amplification. Moreover, we showed, using nonlinearity management considerations, that the optimal performance in high bit-rate dispersion managed fibre systems with hybrid amplification is achieved for a certain amplifier spacing – which is different from commonly known optimal noise performance corresponding to fully distributed amplification. Required for an accurate estimation of the bit error rate, the complete knowledge of signal statistics is crucial for modern transmission links with strong inherent nonlinearity. Therefore, we implemented the advanced multicanonical Monte Carlo (MMC) method, acknowledged for its efficiency in estimating distribution tails. We have accurately computed acknowledged for its efficiency in estimating distribution tails. We have accurately computed marginal probability density functions for soliton parameters, by numerical modelling of Fokker-Plank equation applying the MMC simulation technique. Moreover, applying a powerful MMC method we have studied the BER penalty caused by deviations from the optimal decision level in systems employing in-line 2R optical regeneration. We have demonstrated that in such systems the analytical linear approximation that makes a better fit in the central part of the regenerator nonlinear transfer function produces more accurate approximation of the BER and BER penalty. We present a statistical analysis of RZ-DPSK optical signal at direct detection receiver with Mach-Zehnder interferometer demodulation

Sparsely spread CDMA - A statistical mechanics-based analysis

Sparse code division multiple access (CDMA), a variation on the standard CDMA method in which the spreading (signature) matrix contains only a relatively small number of nonzero elements, is presented and analysed using methods of statistical physics. The analysis provides results on the performance of maximum likelihood decoding for sparse spreading codes in the large system limit. We present results for both cases of regular and irregular spreading matrices for the binary additive white Gaussian noise channel (BIAWGN) with a comparison to the canonical (dense) random spreading code. © 2007 IOP Publishing Ltd.

Statistical physics of error-correcting codes

In this thesis we use statistical physics techniques to study the typical performance of four families of error-correcting codes based on very sparse linear transformations: Sourlas codes, Gallager codes, MacKay-Neal codes and Kanter-Saad codes. We map the decoding problem onto an Ising spin system with many-spins interactions. We then employ the replica method to calculate averages over the quenched disorder represented by the code constructions, the arbitrary messages and the random noise vectors. We find, as the noise level increases, a phase transition between successful decoding and failure phases. This phase transition coincides with upper bounds derived in the information theory literature in most of the cases. We connect the practical decoding algorithm known as probability propagation with the task of finding local minima of the related Bethe free-energy. We show that the practical decoding thresholds correspond to noise levels where suboptimal minima of the free-energy emerge. Simulations of practical decoding scenarios using probability propagation agree with theoretical predictions of the replica symmetric theory. The typical performance predicted by the thermodynamic phase transitions is shown to be attainable in computation times that grow exponentially with the system size. We use the insights obtained to design a method to calculate the performance and optimise parameters of the high performance codes proposed by Kanter and Saad.

A study of performance and regulation in telecommunications in the European Union

This thesis looks at two issues. Firstly, statistical work was undertaken examining profit margins, labour productivity and total factor productivity in telecommunications in ten member states of the EU over a 21-year period (not all member states of the EU could be included due to data inadequacy). Also, three non-members, namely Switzerland, Japan and US, were included for comparison. This research was to provide an understanding of how telecoms in the European Union (EU) have developed. There are two propositions in this part of the thesis: (i) privatisation and market liberalisation improve performance; (ii) countries that liberalised their telecoms sectors first show a better productivity growth than countries that liberalised later. In sum, a mixed picture is revealed. Some countries performed better than others over time, but there is no apparent relationship between productivity performance and the two propositions. Some of the results from this part of the thesis were published in Dabler et al. (2002). Secondly, the remainder of the tests the proposition that the telecoms directives of the European Commission created harmonised regulatory systems in the member states of the EU. By undertaking explanatory research, this thesis not only seeks to establish whether harmonisation has been achieved, but also tries to find an explanation as to why this is so. To accomplish this, as a first stage to questionnaire survey was administered to the fifteen telecoms regulators in the EU. The purpose of the survey was to provide knowledge of methods, rationales and approaches adopted by the regulatory offices across the EU. This allowed for the decision as to whether harmonisation in telecoms regulation has been achieved. Stemming from the results of the questionnaire analysis, follow-up case studies with four telecoms regulators were undertaken, in a second stage of this research. The objective of these case studies was to take into account the country-specific circumstances of telecoms regulation in the EU. To undertake the case studies, several sources of evidence were combined. More specifically, the annual Implementation Reports of the European Commission were reviewed, alongside the findings from the questionnaire. Then, interviews with senior members of staff in the four regulatory authorities were conducted. Finally, the evidence from the questionnaire survey and from the case studies was corroborated to provide an explanation as to why telecoms regulation in the EU has reached or has not reached a state of harmonisation. In addition to testing whether harmonisation has been achieved and why, this research has found evidence of different approaches to control over telecoms regulators and to market intervention administered by telecoms regulators within the EU. Regarding regulatory control, it was found that some member states have adopted mainly a proceduralist model, some have implemented more of a substantive model, and others have adopted a mix between both. Some findings from the second stage of the research were published in Dabler and Parker (2004). Similarly, regarding market intervention by regulatory authorities, different member states treat market intervention differently, namely according to market-driven or non-market-driven models, or a mix between both approaches.

A comparative and longitudinal analysis of the evolving relationship between the environment and the strategy, structure and performance of selected organisations in the British carpet industry 1959-1986:a firms in sector perspective of organisational adaptation

Orthodox contingency theory links effective organisational performance to compatible relationships between the environment and organisation strategy and structure and assumes that organisations have the capacity to adapt as the environment changes. Recent contributions to the literature on organisation theory claim that the key to effective performance is effective adaptation which in turn requires the simultaneous reconciliation of efficiency and innovation which is afforded by an unique environment-organisation configuration. The literature on organisation theory recognises the continuing confusion caused by the fragmented and often conflicting results from cross-sectional studies. Although the case is made for longitudinal studies which comprehensively describe the evolving relationship between the environment and the organisation there is little to suggest how such studies should be executed in practice. Typically the choice is between the approaches of the historicised case study and statistical analysis of large populations which examine the relationship between environment and organisation strategy and/or structure and ignore the product-process relationship. This study combines the historicised case study and the multi-variable and ordinal scale approach of statistical analysis to construct an analytical framework which tracks and exposes the environment-organisation-performance relationship over time. The framework examines changes in the environment, strategy and structure and uniquely includes an assessment of the organisation's product-process relationship and its contribution to organisational efficiency and innovation. The analytical framework is applied to examine the evolving environment-organisation relationship of two organisations in the same industry over the same twenty-five year period to provide a sector perspective of organisational adaptation. The findings demonstrate the significance of the environment-organisation configuration to the scope and frequency of adaptation and suggest that the level of sector homogeneity may be linked to the level of product-process standardisation.