bluuebunny/crossref_metadata_2025 huggingface dataset:
Partial imaginary precursor cancelling in DFE for BPSK and GMSK modulations
Authors: Raphaeli Dan , Stark Amit
Published: September 2008

This paper examines the effect of partial imaginary precursor cancelling on the performance of a system which uses a DFE and employs a one dimensional modulation. As opposed to QAM, BPSK or GMSK (after proper manipulations) require minimization of the mean square (MS) of the error real part only. Therefore, a lower MSE is achieved if the DFE coefficients are computed taking this fact into account, by trying to minimize as few imaginary precursors as possible. However, this introduces a delay by the imaginary part of the feedback filter (FBF) which is no longer causal.

In this paper we investigate the influence of the length of imaginary precursor cancelling on system performance, and its effect on carrier phase tracking. The equations for the computation of the DFE coefficients for none or partially cancelled imaginary precursors are derived and performance on GMSK channels is presented.
Speech spectral quantizers for wideband speech coding
Authors: Guibé G. , How H. T. , Hanzo L.
Published: September 2008

In this treatise a rangeof Line Spectrum Frequency(LSF) Vector Quantization (VQ) schemes were studied comparatively, which were designed forwideband speech codecs.Both predictive arrangements and memoryless schemes were investigated. Specifically, both memoryless Split Vector Quantization (SVQ) and Classified Vector Quantization (CVQ) were studied. These techniques exhibit a low complexity and high channel error resilience, but require high bit rates for maintaining high speech quality. By contrast, Predictive Vector Quantizers (PVQ) offer an enhanced Spectral Distortion (SD) performance, although they are sensitive to channel error propagation. It is shown that the family of so—called Safety—Net Vector Quantization (SNVQ) schemes offers a good design compromise, providing an extension to memory—based PVQ, and thereby improving the performances both over noisy and noiseless channels.
A measurement based approach to TCP congestion control
Authors: Ruddle Alan , Allison Colin , Lindsay Peter
Published: September 2008

Congestion is a major Internet problem. TCP has evolved in attemptting to deal with this and now employs congestion control algorithms that effectively limit the bandwidth available to any one connection. However, as TCP is not the only transport protocol used on the Internet, the growth of non‐TCP traffic has lead to the formulation of TCP‐Friendly formulae. It is recommended that all best effort traffic conform to these formulae, and they act as a benchmark against which the success of any congestion control algorithms can be judged. Unfortunately, key assumptions behind the formulation of TCP's algorithms — low bandwidth, and bulk data transfers — are often no longer valid. In this paper it is shown that the combination of increasing bandwidth and short web transfers means that a significant amount of TCP traffic fails to obtain the bandwidth implied by the TCP‐friendly formulae. An alternative, measurement‐based approach that predicts a fairer starting window size for a connection is presented and evaluated. Information about the characteristics of particular network paths is dynamically maintained and used to suggest a fairer starting window size for new connections. Minor modifications to TCP allow these suggestions to be used to set start‐up control variables, and thereby strengthen the negative correlation between the bandwidth used by a connection and the level of congestion on its network path. The measurement‐based approach is realised through the use of a Location Information Server (LIS). The LIS performs centralised passive monitoring of transport headers in order to derive network level path information. Location Information Packets (LIP) are used to communicate suggested start‐up variables to local hosts. The design, implementation and evaluation of an LIS, LIP and participating host's TCP software are presented.
A variable step‐size control of adaptation for acoustic echo cancellation filters
Authors: Pahor Vojko , Carini Alberto
Published: September 2008

This paper presents a low‐complexity variable step‐size control for acoustic echo cancellation filters. The proposed control method is based on two supplementary adaptive filters with low memory length. The first supplementary filter is used to control the double‐talk situations while the second supplementary filter is employed to detect variations of the echo‐path impulse response. The variable step‐size here derived is independent from die echo coupling loss and from the reverberation time and thus it is suitable for both hands‐free and handset terminals.
Frequency‐domain integration of acoustic echo cancellation and a generalized sidelobe canceller with improved robustness
Authors: Herbordt Wolfgang , Kellermann Walter
Published: September 2008

For hands‐free acoustic human/machine interfaces, as required, e.g., for automatic speech recognition, teleconferencing, and other multimedia services, microphone arrays using generalized sidelobe cancellers (GSCs) in conjunction with acoustic echo cancellation (AEC) can be efficiently applied for optimum communication. This contribution first devises a hew structure for combining AEC and GSC in the frequency domain. We show that computational complexity is reduced by more than a factor of ten compared to a time‐domain arrangement. Second, robustness issues of the GSC adaptation mechanism are addressed. We illustrate how robust adaptation is assured in the new structure. Third, we propose alternatives for the fixed beamformer part of the GSC, which allow to specify (a) the width of the target tracking region and (b) improve the suppression of low frequencies. GSC robustness constraints are included using linear optimization with configurable penalty functions.
Performance of microphone array geometries in a reverberant room
Authors: Ward Darren B.
Published: September 2008

The choice of sensor positions plays a critical role in determining the performance of a nearfield microphone array used for hands‐free speech acquisition. For a single source in a reverberant room, we propose a performance measure based on the expected reverberant power at the array output, and use the proposed measure to choose the microphone locations. One result of applying the proposed method is that it may be better to distribute the microphones throughout the room rather than placing them together in a single array (as conventional farfield array theory would dictate).
Hybrid quality measures for single‐channel speech enhancement algorithms
Authors: Dreiseitel Pia
Published: September 2008

We give an overview of speech quality measures and the most common noise reduction algorithms for the single‐channel approach including a description of their respective shortcomings and apply the quality measures to the special deficiencies of speech enhancement algorithms. This is important for comparing different approaches to noise reduction, and it suggests ways to improve a speech enhancement algorithm. Objective quality measures are then compared to the subjective quality evaluation of human listeners by means of extensive listening tests using the so‐called rank correlation. As a result, we propose a hybrid method for measuring the quality of speech enhancement algorithms.
Adaptive routing algorithms and buffer management in multihop metropolitan area networks
Authors: Giacomazzi Paolo , Melen Riccardo , Voltolini Mirko
Published: September 2008

The present study is devoted to multihop Metropolitan Area Network architectures characterized by some buffer capacity at the nodes and routing algorithm slightly more complex than those already present in the literature. A new adaptive routing algorithm and a new buffer management scheme are presented, and their performance are compared to that of the best algorithms known in the literature. Both the new proposals exhibit significant improvements in throughput and delay with respect to the older ones, for various MAN topologies, sizes and buffer dimensions. These results, which in some cases are very close to the theoretical optimum, are achieved with an implementation complexity well within the limits of present day technology.
Effects of phase noise spectral shape on the performance of dpsk systems for wireless applications
Authors: Corvaja Roberto , Pupolin Silvano
Published: September 2008

Phase noise could be the major source of performance degradation in low cost low bit‐rate digital systems operating in the millimeter wavelength band. A general model considers the phase noise as the sum of several components, namely a white phase noise, a white frequency noise and a flicker frequency noise. All these components are taken into account in the evaluation of error probability in a DPSK receiver and are compared in terms of performance degradation.
Performance of a multicarrier ds/cdma system with successive interference cancellation
Authors: Huh Hoon , Kim Jin Young
Published: September 2008

In this paper, successive interference cancellation (SIC) scheme is proposed for a multicarrier DS/CDMA system and its performance is analyzed in a Rayleigh fading channel. The SIC scheme is employed to mitigate performance degradation due to multiple access interference (MAI) coupled with near‐far problem. Bit error probability of the proposed system is derived for maximal‐ratio and equal‐gain combining schemes of subcarrier signals, respectively. From numerical results, it is Shown that the proposed system achieves significant performance improvement over a conventional multicarrier DS/CDMA system. The proposed system can find application in multicarrier DS/CDMA systems for high‐rate data services.
Optical CDMA codes for use in a lightwave communication network with multiple data rates
Authors: Zhang Jian‐Guo
Published: September 2008

This paper presents a new kind of optical code‐division multiple‐access (CDMA) codes, called unequal‐length optical orthogonal codes (UL‐OOC's), for multirate data communications. The autocorrelation and cross‐correlation functions of UL‐OOC's are dependent only on each individual codeword, in spite of either an aperiodic or a periodic pulse‐sequence pattern at the input of optical CDMA decoders. This property can be thus used to support multirate data communications in a lightwave network, without any violation of the minimum correlation constraint (i.e., “1”) for incoherent optical processing. Theory and simple design of UL‐OOC's are presented. Applications of the proposed codes to multirate optical CDMA is also explained in detail.
Dwell time modeling for wireless communication systems and problems of the velocity distribution
Authors: Boche Holger , Jugl Enrico
Published: September 2008

The distribution of the dwell time, which represents a higher level of abstraction than the use of mobility models, is often required for the performance evaluation of mobile communication systems. This contribution investigates the problem of the dwell time modeling. Using the often employed model for a constant movement, it is shown that a suitable choice of the velocity distribution is very important. Thus, a condition for the probability density function (PDF) of the velocity is derived, which prevents an unbounded mean dwell time. Since an infinite mean dwell time causes some problems for me estimation of the mobility‐related signaling traffic, it is important that the mean dwell time is finite. Furthermore, some basic models for the PDF of the travel length of a subscriber within a radio cell are introduced and the relation wim realistic mobility patterns are shown.
A hidden pilot assisted demodulation technique for channel estimation in m‐qam transmissions
Authors: Mazzenga Franco
Published: September 2008

Multipath effects represent the limiting factor for reliable wireless communications with high efficiency modulation schemes such as M‐QAM or M‐PSK. Pilot assisted modulation techniques have been proposed to overcome multipath fading effects and an equalizer operating on the basis of a received pilot is commonly deployed. The pilot sequence used to train the equalizer is usually time division multiplexed with the information sequence. This solution reduces the bandwidth efficiency and may become unusable when too frequent updating of the communications parameters is needed in the receiver. In this letter it is shown that bandwidth efficiency can be preserved at the expense of an increase in transmit power. The additional transmitted power is used to send a known pilot sequence hidden into the information sequence. The pilot is superimposed onto the information symbols to be transmitted before waveform modulation. Therefore each pilot symbol has the same period of the information symbol so that bandwidth spreading is avoided. The M‐QAM modulation format is considered but the technique can be extended to any digital communication system. The channel estimation problem is analyzed since the equalizer impulse response can be easily derived from the estimated channel impulse response. To prove the effectiveness of the proposed technique simulation results are provided.
Polynomial biasing importance sampling for markov fluid models
Authors: D'Acquisto Giuseppe , Naldi Maurizio
Published: September 2008

Importance Sampling is an established technique for the simulation of rare events in Markov fluid systems, a widely used model in the context of high‐speed multiservice networks. The main issue with Importance Sampling is the definition of the biasing procedure which strongly affects the effectiveness of the simulation technique. In this paper a biasing procedure is proposed which couples the simplicity of application with a performance close to the optimal procedure. The new procedure, named Polynomial Biasing, relies on a single biasing parameter which is used to alter all the elements of the transition rate matrix of the Markov chain with a polynomial sequence. A one‐dimensional minimization algorithm is employed to arrive at the best value of the biasing parameter. Polynomial Biasing provides results identical to the optimal biasing for any superposition of two‐state sources. For the general case of multi‐state sources its performance, analysed through the comparison with the overflow probability obtained through optimal biasing and the evaluation of the optimality indicator in some numerical examples, results close to the optimal one.
An importance sampling algorithm for the simulation of a GPS scheduler
Authors: Giordano Stefano , Pagano Michele , Tartarelli Sandra
Published: September 2008

A key requirement of modern telecommunications networks is the capability of providing a variety of Quality of Service (QoS) guarantees to different classes of users. In this scenario scheduling systems play a major role, thanks to their ability of offering different levels of service, while preventing some “greedy users” from degrading the QoS of other classes. A primary QoS parameter is the Cell Loss Probability (CLP), whose typical values are very small and therefore difficult to estimate through standard Monte Carlo (MC) simulation, since long run times are required to achieve accurate results. Under proper conditions, Importance Sampling (IS) techniques can speed up simulations involving rare events. In this paper we propose an application of IS to the simulation of a Generalized Processor Sharing (GPS) scheduler, fed by Markov Arrival Processes (MAP). The algorithms we propose are based on large deviations principles, which provide asymptotic results on the decay rate of per‐session queue length tail distributions in an idealized GPS discipline. In particular, we first present a scheme for the simulation of a two‐buffer system, then we extend the technique to the general case of a multi‐buffer scheduler. We subsequently employ our algorithms to simulate a packetized realistic system, namely the Packet‐by‐packet Generalized Processor Sharing.
Capacity optimisation in MC‐CDMA systems
Authors: Costa Elena , Haas Harald , Schulz Egon , Filippi Alessio
Published: September 2008

A novel efficient algorithm is proposed for the optimisation of capacity assignment in a MC‐CDMA system with an additional frequency division multiplexing component. Based on an estimate of the frequency‐selective channel, the algorithm optimises the assignment of the sub‐bands to the users with respect to the overall system capacity. Moreover, in the optimisation process the algorithm can efficiently cope with a given prioritisation among classes of user‐services, i.e. user‐groups, identified on the basis of a certain demand, such as throughput, tolerable delay and/or error rate etc. From an initial random solution, the algorithm proceeds progressively towards the optimisation of the target function, yielding, in general, a local optimum. By trying a few different initial solutions a very good approximation of the global optimum can be obtained. Futhermore, the algorithm complexity grows only as the square of the number of carriers. Simulation results show that its application provides a very significant gain in the downlink performance of the overall system or of the prioritised user‐groups.
Joint direction of arrival and delay channel characteristics estimation using circular array antenna for optimal frequency‐space diversity in MC/CDMA receivers
Authors: Fujii Masahiro , Suzuki Shinnosuke , Itami Makoto , Itoh Kohji
Published: September 2008

This paper derives the Maximum Likelihood (ML) algorithm for jointly estimating Direction Of Arrival (DOA) and delay path characteristics of multipath propagation by using array antennas whose elements are placed on a circular circumference. The algorithm is especially made of in a quasi‐synchronous Multi‐Carrier (MC) / Code Division Multiple Access (CDMA) systems in order to achieve the optimal diversity effect. The estimated channel characteristics may be used in Multiple Access Interference (MAI) cancellation or suppression, which remains as the subject of future study. Assuming in the acquisition period, e. g. the preamble period of packet transmission, mutually orthogonal codes are assigned to the set of multiple carrier channels of the CDMA users and the assignments are cyclically shifted from one symbol period to another. Each of the received carrier channel signals at each of the antenna elements is correlated on the time axis with the orthogonal code assigned to each user and in this way the data characterizing the radio wave propagation from each of the users can be separately supplied to the algorithm to follow. The algorithm performs a 2‐dimensional Discrete Fourier Transform (DFT) on the signal sequence thus supplied from the antenna elements and the carrier channels, giving a sufficient statistic by which the DOA and delay path characteristics can be estimated for each of the multiple paths from each of the users. Use of the estimation is demonstrated to obtain the optimal weights for frequency‐space diversity and significant improvement in Bit Error Rate (BER) performance was observed over the conventional way of obtaining the weights.
Array antenna assisted doppler spread compensator for OFDM
Authors: Okada Minoru , Takayanagi Hideaki , Yamamoto Heiichi
Published: September 2008

This paper proposes a novel array‐antenna‐assisted Doppler spread compensator for orthogonal frequency division multiplexing (OFDM), which is sensitive to fast time‐variation of the radio propagation channel. In the proposed compensator, a linear array antenna is installed on top of the vehicle. The compensator estimates the received signal at a certain point on the linear array antenna by using space domain interpolation. Because the relative position of the estimated receiving point with respect to the ground does not change during the effective symbol duration of an OFDM signal, the time variation due to the movement of the vehicle can be compensated for. Computer simulation shows that the compensator can compensate for the bit error rate performance degradation due to time‐variation of the channel when the velocity of the vehicle is up to 180km/h and a two‐element array antenna is used at the carrier frequency of 600 MHz. The bit error rate performance can be further improved by using a four‐element array antenna.
Implementation of a sic based MC‐CDMA base station receiver
Authors: McCormick Andrew C. , Thompson John S. , Grant Peter M. , Arslan Tughrul , Erdogan Ahmet T.
Published: September 2008

The implementation of a multi carrier‐code division multiple access (MC‐CDMA) base station receiver incorporating decision statistic ordered successive interference cancellation multi‐user detection in low power CMOS hardware is investigated. Serial and parallel cancellation architectures are compared and it is shown that the parallel one operates in far fewer clock cycles than its serial counterpart. A detailed description of architecture is given including the parallel algoridim used to implement the multi‐user detection. Results show the power consumption and fixed point performance of the circuit.
A comparison of an OFDM system and a single carrier system using frequency domain equalization
Authors: Witschnig Haralo , Koppler Alois , Springer Andreas , Weigel Robert , Huemer Mario
Published: September 2008

In this work we compare an OFDM (Orthogonal Frequency Division Multiplexing) system and a SC/FDE (Single Carrier System with Frequency Domain Equalization) system in terms of their BER performance, channel estimation techniques, bandwidth‐efficiency and other design parameters. In both systems an estimation of the channel transfer function is needed for equalization purposes. Different frequency domain based channel estimation techniques for both systems are presented and compared. BER‐simulations have been performed for channel‐coded data in typical indoor multipath environments. The parameters of both systems are adapted to the Hiperlan/2 and IEEE 802.1 la standards. It will be pointed out that OFDM shows slight advantages when taking the BER performance into account, while the SC/FDE system exhibits advantages in terms of signal processing complexity. Finally a combination of OFDM and SC/FDE is described that exhibits significant advantages.
Transmit/Receive‐antenna diversity techniques for OFDM systems
Authors: Dammann Armin , Kaiser Stefan
Published: September 2008

In this paper, we investigate different antenna diversity concepts, which can be easily applied to orthogonal frequency division multiplexing (OFDM) systems. The focus is on standard compatibility, i.e. these schemes can be implemented to already existing OFDM systems without changing the standards. The introduced diversity techniques are applied exemplarily to the DVB‐T system. Bit error performance investigations were done by simulation for different DVB‐T and diversity parameter sets.
Generalized window advertising for TCP congestion control
Authors: Gerla Mario , Cigno Renato Lo , Mascolo Saverio , Weng Wenjie
Published: September 2008

Congestion in the Internet is a major cause of network performance degradation. The Generalized Window Advertising (GWA) scheme proposed in this paper is a new approach for enhancing the congestion control properties of TCP. GWA requires only minor modifications to the existing protocol stack and is completely backward compatible, allowing GWA‐hosts to interact with non‐GWA hosts without modifications. GWA exploits the notion of end‐host‐network cooperation, with the congestion level notified from the network to end hosts. It is based on solid control theory results mat guarantee performance and stable network operation. GWA is able to avoid window oscillations and the related fluctuations in offered load and network performance. This makes it more robust to sustained network overload due to a large number of connections competing for the same bottleneck, a situation where traditional TCP implementations fail to provide satisfactory performance. GWA‐TCP is compared with traditional TCP, TCP with RED and also ECN using the ns‐2 simulator. Results show that in most cases GWA‐TCP outperforms the traditional schemes. In particular, when compared with ECN, it provides smoother network operation and increased fairness.
Performance evaluation of combined spatial processing and multiple access interference equalization for DS‐CDMA systems
Authors: Cavalcanti Francisco R. P. , Romano Joāo M. T.
Published: September 2008

Adaptive detection in code division multiple access (CDMA) systems aims the reduction of interference in order to allow capacity increase. Spatial processing and multiple access interference (MAI) equalization are two important techniques that have been used separately in adaptive detection for CDMA. The first contribution of this paper is die proposal and performance assessment of a detector that operates simultaneously as a spatial processor and MAI equalizer. Its minimum mean square error performance in a multiple cell CDMA system is addressed and compared with die two existing techniques. A blind adaptive implementation of a similar concept is a second proposition of the paper. Its performance is assessed in terms of wrong capture probability and compared with two other existing techniques, based on constant modulus (CM) or decision‐directed (DD) algorithms. Simulation results show that the first combined scheme performs better than the separated existing techniques, while the proposed blind approach also overcomes limitations of CM and DD‐based solutions.
A new broadband uniform accuracy doa estimator
Authors: Ghavami Mohammad , Kohno Ryuji
Published: September 2008

Conventional estimators of the bearing angle suffer from different accuracies for different angles. In the method discussed in this letter, a set of two‐dimensional digital filters are employed for beamforming. Each filter is called a fan filter and performs broadband characteristics. It will be shown analytically that the accuracy of the bearing estimation is strongly dependent to the angle of arrival in such a way that it is maximum in the broadside and minimum in the endfire of the antenna. We will derive a simple expression for calculating the number of coefficients necessary for beamforming with uniform accuracy. A set of filters is then designed according to this criteria. It will be demonstrated by computer simulations that in the presence of noisy broadband incoming signals, the estimation of different arriving angles is accomplished with almost the same accuracy and hence using this method a complex and separately sectored antenna wilt not be required.
Notes on cost functions and estimators for ‘Bussgang’ adaptive blind equalization
Authors: Fiori Simone
Published: September 2008

The aim of this paper is to present some remarks on the cost functions for blind channel equalization by ‘Bussgang’ algorithms recently discussed in the literature; also, some possible associated non‐Bayesian estimators are considered with details, and the effects of the choice of such estimators in relation to ‘Bussgang’ filtering are investigated.
Infant health production functions: what a difference the data make
Authors: Reichman Nancy E. , Corman Hope , Noonan Kelly , Dave Dhaval
Published: September 2008

We examine the extent to which infant health production functions are sensitive to model specification and measurement error. We focus on the importance of typically unobserved but theoretically important variables (typically unobserved variables, TUVs), other non‐standard covariates (NSCs), input reporting, and characterization of infant health. The TUVs represent wantedness, taste for risky behavior, and maternal health endowment. The NSCs include father characteristics. We estimate the effects of prenatal drug use, prenatal cigarette smoking, and first trimester prenatal care on birth weight, low birth weight, and a measure of abnormal infant health conditions. We compare estimates using self‐reported inputs versus input measures that combine information from medical records and self‐reports. We find that TUVs and NSCs are significantly associated with both inputs and outcomes, but that excluding them from infant health production functions does not appreciably affect the input estimates. However, using self‐reported inputs leads to overestimated effects of inputs, particularly prenatal care, on outcomes, and using a direct measure of infant health does not always yield input estimates similar to those when using birth weight outcomes. The findings have implications for research, data collection, and public health policy. Copyright © 2008 John Wiley & Sons, Ltd.
Prognosis of Primary Hepatocellular Carcinoma
Authors: Okuda Kunio , Obata Hiroshi , Nakajima Yukio , Ohtsuki Toshio , Okazaki Nobuo , Ohnishi Kunihiko
Published: September 2008

Prognosis of 600 consecutive patients with hepatocellular carcinoma was analyzed in relation to treatment. They were divided into three stages based on four parameters of advanced disease: ascites, tumor greater than 50% of the two-dimensional size of the liver, serum albumin below 3 gm per dl, and serum bilirubin above 3 mg per dl. Stage I had none of these signs; Stage II one or two signs, and Stage III three or all signs. Of 600 patients, 98 had resection, 333 had nonsurgical treatment (158 treated by intraarterial chemotherapy, 94 systemic chemotherapy, 77 transcatheter embolization, and 4 others) and 169 no treatment.

The median survival of untreated patients was only 1.6 months from diagnosis, and no untreated Stage III patient lived more than 3 months; there was a median survival of 0.7 month. Surgically treated patients lived significantly longer than nonsurgical patients of comparable stages; median survival was 19.6 months in the former and 2.8 months in the latter. Whereas Stage I patients did fairly well without treatment, chemotherapy significantly prolonged survival of patients of Stages II and III. These results suggest that early diagnosis and hepatic resection improve prognosis in patients with hepatocellular carcinoma in the areas where this cancer frequently emerges unicentrically. In view of the generally poor prognosis, liver transplantation is recommended when resection is not possible or indicated, and before extrahepatic metastasis occurs.
Wilson’s Disease: Indications for Liver Transplants
Authors: Sternlieb Irmin
Published: September 2008

The clinical course of certain patients with Wilson’s disease resembles that of patients with viral or drug-induced fulminant hepatitis lasting only few weeks from recognition of symptoms to severe hepatic insufficiency and death. The disease is complicated by hemolysis and is characterized by hypercupremia. Routine laboratory findings may underestimate the severity of the disease. These patients, as well as patients with decompensated Wilsonian cirrhosis who are not responding to therapy, should be considered as candidates for liver transplants.
The Liver in Crigler-Najjar Syndrome, Protoporphyria, and Other Metabolic Disorders
Authors: Bloomer Joseph R. , Sharp Harvey L.
Published: September 2008

The liver may be involved in metabolic disorders in two ways, (i) Due to its critical role in several metabolic pathways, the liver may be the major site of expression of a biochemical abnormality. For example, in Type I Crigler-Najjar syndrome, absence of hepatic bilirubin glucuronyl transferase activity causes severe unconjugated hyperbilirubinemia which invariably leads to death from kernicterus. (ii) The liver may also be structurally damaged by the metabolic disorder. Several patients with protoporphyria have died in hepatic failure due to liver damage caused by protoporphyrin deposition. Numerous other metabolic disorders can be sited in which significant liver involvement occurs. In many of these disorders, there is no effective medical therapy.
The Cambridge and King’s College Hospital Experience of Liver Transplantation, 1968-1983
Authors: Rolles Keith , Williams Roger , Neuberger James , Calne Roy
Published: September 2008

The postoperative course of 138 transplants performed in 137 patients in the Cambridge and King’s College Hospital series between May 2, 1968 and April 1, 1983 is presented. During the last 15 years, criteria for selection of transplant candidates has been improved and types of disease categories, both suitable and unsuitable for liver transplantation, have been defined. The acceptance of the concept of brain death and the use of heart-beating donors in the United Kingdom since 1976 has greatly improved the quality of donor organs. Changes in surgical technique, particularly with regard to biliary tract drainage, have reduced morbidity and mortality from biliary tract complications since 1975. The use of partial cardiopulmonary bypass in selected cases and changes in immunosuppressive drug regimens have not significantly improved our overall results. Despite these changes based on our experience, the perioperative mortality in the Cambridge and King’s College Hospital series remains disturbingly high.
Liver Procurement for Orthotopic Transplantation: An Analysis of the Pittsburgh Experience
Authors: Van Thiel David H. , Schade Robert R. , Hakala Thomas R. , Starzl Thomas E. , Denny Donald
Published: September 2008

The incidence of prospective organ donors in the United States and the techniques which are to used to guarantee their optimal use after identification are analyzed. Attitudes of the public and health professionals toward organ donation are discussed.

The organization of the Pittsburgh Organ Procurement Agency and its relationship to other such agencies is described.

Finally, the presently used techniques of liver salvaging and preservation are outlined.
Skills and Resources Needed for Liver Transplantation
Authors: Krom Ruud A. F. , Gips Chris H.
Published: September 2008

Trained specialists in hepatology, liver and biliary surgery, anesthesiology and pathology must form the core of a team for liver transplantation. The hospital must be a center for organ transplantation to provide the essential infrastructure to manage specific aspects of transplantation. A well-organized blood bank is essential to cope with acute requests for large amounts of donor blood. Because of the extremely high costs, liver transplantation has to be approved medically and politically before it can be considered as a generally accepted modality of treatment.
Some Immunological Considerations in Liver Transplantation
Authors: Russell Paul S.
Published: September 2008

Immunological considerations of special interest in liver transplantation are examined. It is concluded that liver transplants between human beings generate typical rejection reactions although they may, overall, be somewhat more mild than those encountered in transplanted kidneys and hearts. This is the case in some animals for reasons which are not yet clear. There is evidence that the transplanted liver can suffer immune attack better than other organs. Whether graft-vs.-host activity is of clinical importance is uncertain. The advent of cyclosporin immunosuppression has improved the outlook for liver recipients but multiple complications of immunosuppression will still occur and demand skillful management.
Medical Aspects of Liver Transplantation
Authors: Van Thiel David H. , Schade Robert R. , Gavaler Judith S. , Shaw Byers W. , Iwatsuki Shunzaburo , Starzl Thomas E.
Published: September 2008

The methods used to screen prospective candidates for orthotopic liver transplantation are described. Both the indication and the contraindications for the procedure are discussed. The timing of the procedure during the course of an individual candidate’s liver disease is also discussed. Additionally, the institutional requirements of a liver transplant center are identified. Finally, the problems experienced by a liver transplant patient and his physician during the postoperative period are identified and discussed.
Epidemiology and Clinical Course of Liver Diseases: Identification of Candidates for Hepatic Transplantation
Authors: Vierling John M.
Published: September 2008

Among a variety of liver diseases that have been treated by hepatic transplantation, several were chosen for evaluation because of their frequency or high rate of fatality. Information regarding the incidence and prevalence in the United States, the natural history with conventional therapy, and prognostic indicators of a terminal phase is reviewed. Based on this information, specific candidates for hepatic transplantation are identified.
Anatomy, Visualization and Sampling of the Biliary Tree in Animals and Man
Authors: Strasberg Steven M. , Harvey Robert P.C. , Gallinger Steven
Published: September 2008

Difficulties in obtaining proper bile samples are due to inaccessibility of the biliary tree and to distortions induced by sampling methods. One must be cognizant of the effects of diet on bile secretion and gallbladder motility. Experimental methods which interrupt the enterohepatic circulation or alter the intestinal migrating myoelectric complex induce spurious changes in bile flow and composition. Biliary tract pressure-flow relationships must be maintained or the gallbladder will be made functionless. Dead space errors lead to distortions unless studies are performed in the steady state, or dead space is measured and corrections are applied. Surgery has major effects on some parameters of interest, and animals should be allowed to recover when these are studied. The effect of the mixing of bile with other secretions in the duodenum must be considered when using bile-rich duodenal fluid. For some parameters of bile secretion, mixing is unimportant but for others, special precautions for handling bile and interpreting results are required.
Bilirubin Chemistry, Ionization and Solubilization by Bile Salts
Authors: OSTROW Donald J. , Celic Lillian
Published: September 2008

Bilirubin is a linear tetrapyrrole whose conformation is affected by internal hydrogen bonds formed between the carboxyl side chains and dipyrromethenone rings. Structural variations include: constitutional isomerism of the vinyl or carboxyethyl side chains, geometric isomerism of the methene bridges, tautomerism of the lactam groups, conformational rotations about the central methylene bridge and ionization of one or both carboxyl groups. Aggregation of the dianion into dimers and multimers may occur. The pKa’ values of the two carboxyl groups are affected greatly by the environment and may differ widely in micellar solutions like bile. Solubility of bilirubin in water is less than 1 nM at pH = 7 and about 0.1 μM at pH = 8. Nonetheless, it dissolves poorly in most lipid solvents, except for asymmetrical chloroalkanes. Hydrogen bond-breaking solvents, especially dimethyl sulfoxide, are most effective in solubilizing bilirubin. In bile salt solutions, solubility of bilirubin is well above the concentrations of unconjugated bilirubin found in normal human gallbladder bile, and is impaired by lecithin but unaffected by cholesterol. At physiological pH in bile salt solutions, bilirubin is predominantly in its monoanion form that binds readily to the micelles. In such solutions, addition of physiological concentrations of calcium precipitates calcium bilirubinate, leaving residual bilirubin concentrations of up to 15 μM in 50 mM taurocholate or close to the maximum bilirubin concentrations in normal bile. Studies in which disodium bilirubinate is dissolved in bile salt solutions and pH is adjusted to the physiological range reveal that metastable supersaturation with bilirubin may occur and that a mesophase may also form in the presence of lecithin, akin to that seen with cholesterol. These results form a basis for understanding the precipitation of calcium bilirubinate from bile during pigment gallstone formation.
Role of Gallbladder Mucin in Pathophysiology of Gallstones
Authors: Lamont Thomas J. , Smith Bernard F. , Moore James R. L.
Published: September 2008

A critical step in the formation of cholesterol gallstones is nucleation (i.e., the formation of cholesterol monohydrate crystals from supersaturated bile). The rate of nucleation of cholesterol depends upon a critical balance between pronucleating and antinucleating factors in bile. Mucin, a high molecular weight glycoprotein secreted by the gallbladder and biliary duct epithelium, is a pronucleating agent in experimental and human gallstone disease. Gallbladder mucin shares with other epithelial mucins the ability to bind lipids and bile pigment. The hydrophobic binding sites in the polypeptide core of mucin may provide a favorable environment for nucleation of cholesterol monohydrate from supersaturated bile. In nearly all animal models of cholelithiasis, mucin hypersecretion is prominent. The stimulus for gallbladder mucin hypersecretion appears to be a component of lithogenic bile. Prostaglandins regulate mucin release in gallbladder epithelium in vitro and probably in vivo. In the cholesterol-fed prairie dog, blockage of mucin release with aspirin inhibits gallstone formation. These findings suggest that inhibition of mucin release may prevent cholesterol stone formation during high-risk periods or after dissolution therapy with bile salts.
Bile Acids and Bile Salts: Ionization and Solubility Properties
Authors: Carey Martin C.
Published: September 2008

The number, position and orientation of nuclear OH substituents profoundly influence the equilibrium solubilities of undissociated bile acids in water. Estimates from several studies range from 5 × 10″8 for lithocholic acid to 1.6 × 10−3 Af for ursocholic acid at 37°C. Fully dissociated sodium bile salts are extremely soluble in water, attaining values as high as 1 to 2 M. However, ionized unconjugated bile salts are appreciably less soluble than their glycine and taurine conjugates. In contrast to aqueous solutions of typical soaps and detergents, aqueous bile salt systems in concentrations beyond the micellar phase limit exhibit no liquid-crystalline phases. The critical micellar temperatures of the common di- and trihydroxy bile salts lie below 0°C, hence most bile salts form micelles at all ambient temperatures. With many monohydroxy and certain uncommon dihydroxy bile salts, the critical micellar temperatures lie above body temperature, and a micellar phase is observed only at temperatures in excess of 50°C. The pKa’ values of aqueous micellar concentrations of bile salts are influenced by the nuclear substituents, the state of conjugation and bile salt concentration. In contrast, the intrinsic thermodynamic pK’a values of all monocarboxy-lated bile salts in monomeric solution are similar, are not influenced by nuclear substituents and are equivalent to that expected for propionic acid (pKa’ ˜4.8 to 5.0). These values can be deduced by extrapolation from acidimetric titrations of bile salts in aqueous methanol mixtures or from acidometric aqueous titration of the carboxylate groups of pansulfated bile salts which remain water soluble when the ionic group of the side chain is protonated. The precipitation pH values of the common protonated bile acids are influenced by bile salt concentration, the state of conjugation and the solubilizing capacity of bile salt micelles for the otherwise sparingly soluble undissociated bile acid species.
Effect of Nuclear Hydroxy Substituents on Aqueous Solubility and Acidic Strength of Bile Acids
Authors: Roda Aldo , Fini Adamo
Published: September 2008

The aqueous solubility and thermodynamic dissociation constants of a representative series of bile acids with varying numbers and configuration of nuclear hydroxyl substituents were determined. The pKa values were calculated by extrapolating pKa’ values determined in solutions of aqueous methanol of different mole fractions at 25o C. All bile acids had the same acidic strength in water, indicating that the pattern of hydroxyl nuclear substituents does not affect the acidic strength probably because of the distance between the hydroxyl and ionizing groups. In contrast, aqueous solubility was dependent on the number, position and orientation of the hydroxyl groups. The solubility values ranged from 5 x 10−8
M (0.05 μM) for 3αr-hydroxy-5β-cholanoic acid (lithocholic acid) to 1.67 x 10−3
M (1,670 μM) for 3α,7β,12α-trihydroxy-5β-cholanoic acid (urso-cholic acid). The hydroxy groups notably affect solubility by forming hydrogen bonds with water but also by reducing, according to their orientation, hydrophobicity of the steroid nucleus.
Size of Bile Salt Micelles: Techniques, Problems and Results
Authors: Kratohvil Josip P.
Published: September 2008

The current state of the knowledge of the size and the aggregation numbers of micelles formed in solutions of bile salts was evaluated. The experimental techniques considered include static light scattering, sedimentation equilibrium, membrane osmometry, sedimentation velocity and transla-tional diffusion. The theoretical background of each method is briefly discussed, the working equations are summarized, and the limitations of each technique are enumerated. Various interaction parameters and their effects on the measured micellar size are discussed. A brief survey of the recent experimental results from the author’s laboratory is presented.
Bile Salt Micelles: Nuclear Magnetic Resonance Spectroscopy and Crystallographic Studies
Authors: Barnes Stephen
Published: September 2008

Studies of bile salt structure using nuclear magnetic resonance (NMR) and X-ray crystallography have suggested that micellar association of bile salts is a function of side chain length and of packing of the side chain and the steroid nucleus. Lanthanide NMR probe analysis has been used to determine the structure of what is probably the monomer of the bile salt glycocholate. These approaches to the structural chemistry of bile salt intra- and intermolecular interactions are providing the basis for the design of artificial bile salt-like compounds for use in the study and treatment of human gallstone disease.
Micelles and Microemulsions in Ionic Surfactant and Bile Salt Systems Studied by Self-Diffusion
Authors: Lindman BjÖRn
Published: September 2008

Multicomponent self-diffusion studies provide a general picture of amphiphile self-association. Both micelles and microemulsions based on bile salts show a lower degree of association coopera-tivity, lower aggregate charge densities and more extensive hydration than corresponding ionic surfactant systems. Effectively bicontinuous isotropic solutions of the microemulsion type are more easily formed by bile salts than by surfactants.
Molecular Organization in Phases of Lecithin- Cholate-Water As Studied by Nuclear Magnetic Resonance
Authors: Lindblom GÖRan , Eriksson Per-Olof , Arvidson GÖSta
Published: September 2008

The molecular organization in the liquid crystalline phases and the micellar solution phase has been investigated using numerous nuclear magnetic resonance techniques. A brief review of previous studies on the lamellar, hexagonal and cubic liquid crystalline phases is given. Mixed micelles were studied by measurements of 2H T1 and T2 nuclear magnetic resonance relaxation times of 2H-labeled phosphatidylcholine. Using simple and rough models, the size and shape of the micellar aggregate were estimated.
Dynamic Micelle Laser Light Scattering Studies of the to Vesicle Transition in Model and Native Bile
Authors: Schurtenberger Peter , Mazer Norman A. , Känzig Werner
Published: September 2008

Using quasielastic light scattering, we investigated the formation of vesicles produced spontaneously by diluting a mixed micellar solution of bile salt and lecithin beyond the micellar phase boundary. The vesicles were nearly monodisperse, and their size varied between 120 to 500 A depending upon the dilution conditions. A systematic study of the dependence of the vesicle size upon the bile salt and lecithin concentration was made and interpreted in terms of a simple equilibrium model. Likewise, the particle size distribution in native bile obtained from the dog was studied. A comparison of the dilution effect on native bile and on model bile of identical biliary Iipid composition provides unambiguous evidence for micellar aggregates and the occurrence of a micelle to vesicle transition in native bile.