bluuebunny/crossref_metadata_2025_split huggingface dataset:
Linear space—time diversity receivers for the downlink of UMTS with WCDMA
Authors: Jarosch Andreas , Dahlhaus Dirk 
Published: September 2008

AbstractSimple linear demodulation schemes exploiting the space and time diversity of the mobile radio channel are derived for the downlink of the Univeral Mobile Telecommunication System (UMTS) employing wideband codedivision multiple access (WCDMA). Using different stochastic models of the signals at the M × 1 receiver antennas and different objective functions including minimum—mean square error (MMSE) and maximum—likelihood approaches, we obtain time—invariant and time—variant detection schemes which make use of channel and receiver parameter estimates being calculated upon observation of a single slot. It turns out that the bit—error rate (BER) of the conventional receiver carrying out a maximum—ratio combining w. r. t. time and space can be improved considerably, where the achievable gain depends critically on the accuracy of the aforementioned estimates. Efficient ways for implementation are proposed and quantified in terms of the computational complexity. Analytical approximations and simulations of the BER for different system parameters reveal that the schemes represent means to get robust against both channel fading and multiple access interference caused by multipath propagation.
Smart antennas for UTRA TDD
Authors: Haardt Martin , Mecklenbräuker Christoph F. , Vollmer Marius , Slanina Peter 
Published: September 2008

AbstractDue to the increasing number of users in mobile communications and the demand for mobile multimedia services with high data rates, third generation mobile radio systems are currently one of the key communication technologies in research, development, and international standardization bodies. Smart antennas can be used to increase the capacity of wireless communication systems even further. TDD systems are particularly attractive for the employment of smart antennas, since the channel information (e. g., in terms of spatial covariance matrices) estimated on the uplink can be used directly for downlink beamforming. In this paper, we discuss efficient uplink and downlink processing techniques with smart antennas for UTRA TDD, the TDD mode of third generation mobile radio systems that is based on TD—CDMA. On the uplink, joint space—time processing is used to eliminate co—channel interference created by other users that transmit on the same frequency and in the same time slot. These joint detection techniques are efficiently implemented in the frequency domain. On the downlink, the data is only transmitted in the direction of the desired mobile user while interference in the direction of co—channel users is avoided. Moreover, short—term spatial channel estimates can be improved by projecting them into the dominant subspace of the long—term spatial covariance matrix. System level simulations have been performed to evaluate the gain in spectral efficiency (available bit rate per bandwidth per cell) if simple downlink beamforming techniques are used at the base stations. There is a two— to three—fold spectral efficiency gain if antenna arrays of M = 8 elements are deployed in a sectorized macro—cellular environment.
Performance assessment of joint transmission (JT) multi—user downlinks with multi—element transmit antennas
Authors: Tröger Hendrik , Weber Tobias , Meurer Michael , Baier Paul Walter 
Published: September 2008

AbstractJoint transmission (JT), a novel downlink transmission scheme, was introduced and described in recent papers. Questions still open concern the performance assessment of JT and the comparison of JT with conventional downlink transmission schemes. In order to answer these questions, quantities termed SNR degradation and transmission efficiency are presented in this paper. Based on these quantities, exemplary system assessments and comparisons are performed, which clearly show the suitability of these measures and also the advantages of the novel downlink transmission scheme JT.
Theoretical and experimental comparison of optimisation criteria for downlink beamforming
Authors: Boche Holger , Schubert Martin 
Published: September 2008

AbstractWe address the problem of joint downlink beamforming and power control in wireless communication systems. The goal is to minimise the overall transmission power radiated from the base station while satisfying individual signal—to—interference levels at the mobiles. Several iterative algorithms have been proposed for this non—linear optimisation task. One important problem in this context is that for certain scenarios the problem has no feasible solution. One of the main contributions of this paper is to derive a necessary and sufficient condition for the existence of a solution. This not only gives new insight into the analytical structure of the problem, but also provides a unifying framework by which several optimisation criteria known from literature can be compared. Finally, simulations are carried out using a deterministic multi—path channel model.
Multistage multiuser detector for multipath fading channel
Authors: Novaković Dejan M. , Dukić Miroslav L. 
Published: September 2008

AbstractIn this paper, the multistage multiuser detector for communication over multipath fading channel is considered. The detector consists of a matched filters bank and a linear equalizer whose outputs are diversity combined to produce initial data estimates. Using these data estimates multiuser interference is reconstructed and removed from the output signals of the matched filters bank. Afterward, diversity combining is applied again and it produces final decision statistics. Simulation results indicate that this detector outperforms conventional RAKE receiver and the linear equalizer with diversity combining. Also, the proposed detector is near—far resistant.
Turbo equalization applied to multiuser detection in a convolutionally coded asynchronous flat rayleigh fading DS—CDMA System
Authors: Cances J. P. , Hècart E. J. , Meghdadi V. 
Published: September 2008

AbstractIn this paper we propose a structure of a multiuser receiver which reuses the basic principles of turbo—equalization for ISI canceling. The receiver is mainly made of a centralized MUI (Multi—User Interference) canceler which detects the coded data by means of a Viterbi algorithm. The metrics are computed using Verdu's studies and the decisions are weighted using the log—MAP algorithm. This weighting operation followed by an interleaver block which scatters error—bursts enables the channel decoder to work with analog inputs. The channel decoder uses the log—MAP algorithm to weight the decisions that are fed back, in a closeii loop manner, to the centralized MUI canceler. This scheme is clearly close to turbo—decoding or turbo—equalization principles. Simulation results are presented over both additive white Gaussian noise channel and Rayleigh flat fading channel showing each time that the upper bound represented by the single—user detection scheme case can be approximately reached by such a system.
An efficient network architecture and protocols for WDM networks
Authors: Pountourakis Ioannis E. 
Published: September 2008

AbstractIn this paper we study a single—hop WDM network for a passive star topology, based on a new network architecture that uses several wavelengths as control channels with an appropriate Network Interface Unit at each station for coordination of packet transmissions on the data channels referred to as Multichannel Control Architecture (MCA). With MCA, control information is distributed over the total control channels reducing the electronic processing bottleneck at one of end station. We propose a multiple access protocol which operates asynchronously, i. e data channels are not slotted. The proposed scheme adopts the ALOHA protocol for the access to MCA and data channels. Analytic model is developed and analyzed for performance measures evaluation based on Poisson approximations statistics of both the infinite and Finite population. Numerical results are discussed for various number of control and data channels. The effect of receiver collision is analyzed and estimated by the average rejection probabilities at destination.
Internet expansion, refinement and churn
Authors: Broido Andre , Nemeth Evi , Claffy Kc 
Published: September 2008

AbstractWe analyze the evolution of the global Internet interdomain routing system on AS, prefix and IP address level granularities, using snapshots of Route Views BGP tables from 1997 to 2001. We introduce the notion ofsemiglob‐ally routed prefixes, those present in the majority of backbone tables, and classify them intostandalone ‐those which have no subsets, no supersets;root— have subsets, but no supersets; andsubset, or more specific, which are subsets of other blocks. Using these distinctions we find that from 1999 to 2001 many measures of routing system complexity demonstrated stability in the form of slow growth, dynamic equilibrium, and occasional contraction. We find that many net change measures reflect contributions of opposite sign, and that true measure of variation, or chum, is the sum of their absolute magnitudes rather than the difference. Appearance and disappearance of prefixes, ASes and Route Views peers, as well as status changes (an AS changing from transit to non‐transit, or a prefix shifting from a standalone prefix to a root prefix) are instances of routing systemchurn.One advantage of using our notion of semiglobal prefixes is that they exhibit less chum than global prefixes (those prefixes common to all backbone tables) and as such allow for derivation of more robust macroscopic statistics about the routing system. We study route prefix instability at a medium time granularity for late 2001 using 2‐hour snapshots of BGP tables, and find that half of all prefix reannouncements(flips)are contributed by 1% of all ASes, with government networks, telecoms in developing countries and major backbone ISPs at the top of the list of instability contributors. Small ASes (those who originate only a few prefixes into the global routing system) do not contribute more than their fair share of either route entries or churn to the global routing system. We conclude that during 1999‐2001 many Internet metrics were stable, and that the routing system's growth and instability are mostly caused by large and medium‐sized ISPs.
A multichannel acoustic echo canceler double‐talk detector based on a normalized cross‐correlation matrix
Authors: Benesty Jacob , Gänsler Tomas 
Published: September 2008

AbstractMultichannel acoustic echo cancellation is basically composed of two parts. One part is a multichannel system identification problem which is nontrivial to solve. The other part, which is also nontrivial, is the so‐calleddouble‐talkdetection problem. Near‐end speech detection is based on a test statistic. Recently, a new double‐talk test statistic based on the normalized cross‐correlation vector was proposed For the single‐channel case. Obviously, in the multichannel case, there are several solutions, but what should be the optimal one is not yet known. Then, a fundamental question arises: how do we deal optimally with multiple channels? In this paper, we generalize the idea of normalized cross‐correlation vector (single‐channel) to the matrix case (multichannel), derive a frequency‐domain version, and show how to combine both the multichannel frequency‐domain adaptive filter and the multichannel double‐talk detector.
Aspects of first‐order differential microphone arrays in the presence of sensor imperfections
Authors: Buck Markus 
Published: September 2008

AbstractArray processing for broadband signals like speech is not a trivial task. Most sensor arrangements and the related beamforming algorithms show strong frequency dependent characteristics. Differential microphone arrays promise frequency independent high directional gain with compact arrangements. However, this type of microphone array has also the problem that even small deviations in microphone properties can cause severe degradation of the array's performance. Especially at low frequencies the achievable gain is limited by the influence of sensor mismatch and sensor noise.In this contribution the performance of first‐order differential arrays is examined and important parameters such as array response and directivity index are thoroughly studied. The frequency dependence of these parameters is investigated in order to derive limits for a favorable frequency range. First, the performance of the array is examined for ideal sensors. Then, two different models for sensor mismatch are introduced to describe the degradation of the array's performance. In addition, solutions for sensor calibration and experimental results with real microphones are presented.
A multi‐channel subband generalized singular value decomposition approach to speech enhancement
Authors: Spriet Ann , Moonen Marc , Wouters Jan 
Published: September 2008

AbstractIn speech communication applications, the presence of noise (and reverberation) in the environment may render the captured speech unintelligible. Noise reduction algorithms are thus needed to improve the intelligibility. Recently, a multi‐channel noise reduction technique, based on a Generalized Singular Value Decomposition (GSVD) has been proposed. The GSVD based filter minimizes the Mean Square Error (MSE) between the desired signal portion in the received signals and the sum of the filtered received microphone signals. In this paper, we propose a subband implementation of the GSVD based filter. It is shown that — in the case of coloured signals — the subband implementation improves intelligibility more than the fullband approach. In addition, a significant complexity reduction is achieved, allowing for low cost, real‐time applications.
A new method for evaluating the cell loss probability in an atm multiplexer
Authors: Hong Xinwei , Huang Zailu , Chan Edward 
Published: September 2008

AbstractAnalyzing and estimating the cell loss probability in an ATM multiplexer is one of the most important problems concerning traffic control and resource management in an ATM‐based B‐ISDN. In this paper, an extension of the discrete fluid‐flow technique is proposed to calculate the cell loss probability of an ATM multiplexer fed by a 2‐state MMDP source. This technique is also used to analyze and estimate the cell loss performance of an ATM multiplexer loaded with homogeneous On‐Off sources. Numerical calculation and simulation results indicate that the method is both accurate and computationally efficient.
A simple and illustrative analytical model for the performance analysis of queuing systems loaded with mpeg streams
Authors: Blefari‐Melazzi Nicola 
Published: September 2008

AbstractThis paper consists of three parts. In the first one, we present a statistical analysis of experimental MPEG video sources and we discuss some modeling issues relevant to the performance evaluation of a queuing system loaded with such bit streams. In the second one, we propose a modeling approach (basic model) that considers the particular structure of MPEG streams and gives accurate results in a well‐defined set of the queuing system parameters. In the third part, we introduce an extension of the basic model to take into account Long Range Dependences (LRD). This extension enlarges the range of applicability of the basic model. Our results show that the impact on the performance of the particular structure of MPEG sequences and the relevant bit rate distribution is always significant, while the impact of long term correlations is important only for large buffer sizes and high loads. These cases do not correspond often to realistic scenarios. The proposed model is validated with simulation results produced by using many experimental video sequences (both MPEG1 and MPEG2).
Traffic generation algorithm for sdh digital cross‐connects
Authors: Johnston Bob , Owen Henry L. 
Published: September 2008

AbstractData in Synchronous Digital Hierarchy (SDH) networks may be routed, managed, and regulated bydigital cross‐connect systems(DCSs). The adequacy and performance of current and proposed DCSs are determined through simulations, which require realistic connection requests ortraffic.This work entailed the creation and analysis of a traffic generation algorithm for SDH DCSs. The algorithm accepts any valid configuration of desired traffic characteristics, and it produces connection and disconnection requests according to the provided configuration. The algorithm incorporates a probabilistic approach that allows the statistics of the configuration to determine the distributions of the types of connections and disconnections that are attempted. Special considerations are made to maintain the requested output load, to apportion the payloads, and to create opportunities for bidirectional connections. This paper includes an analysis of factors that may affect traffic generation.
Time and frequency synchronisation in tdma‐ofdm based high speed wireless local area networks
Authors: Piazzo Lorenzo , Falaschi Alessandro , Manoarini Paolo , Keller Thomas 
Published: September 2008

AbstractDownlink synchronisation error estimators suited for OFDM‐TDMA wireless LAN are presented. The focus is on the timing error estimator, which is of the Early‐Late type and has a low complexity, making it attractive for high speed systems. It also allows a large sharing of the computational burden with the frequency error estimator. The estimator is presented and analysed. Simulations are performed to asses the performance of the synchronisation system over indoor channels.
An extended maximum entropy method for estimation of rare event probabilities
Authors: Lee Tae‐Eog , Lee Young‐Doo 
Published: September 2008

AbstractThe maximum entropy model that maximizes the entropy function under a set of constraints derived from the simulation samples has been used for estimating probability distributions. We propose the use of the maximum entropy model for estimating rare event probabilities from the simulation samples. To improve estimation of the far tail part, the entropy maximization problem is generalized by relaxing the head part constraints and focusing on the tail part sample information. The generalized maximum entropy model is formulated as a convex optimization problem in a normed linear vector space. Global optimality of die Lagrangian solution and die asymptotic consistency of the solution sequence for the increased sample sizes are proved. We discuss implementation issues such as parameter estimation methods, solution procedures, and model selection techniques based on accelerated simulation.
Spreading sequences for uplink and downlink MC‐CDMA systems: PAPR and MAI minimization
Authors: Nobilet Stéphane , Hélard Jean‐François , Mottier David 
Published: September 2008

AbstractThis paper deals with spreading sequences selection for downlink and uplink Multi‐Carrier Code Division Multiple Access (MC‐CDMA) systems with the aim of minimizing the dynamic range of the transmitted multicarrier signal envelope and the multiple access interference. The crest factor of orthogonal and non‐orthogonal sequences are compared analytically and by simulation for downlink and uplink phase shift keying MC‐CDMA transmissions. Then, in order to minimize the multiple access interference produced by frequency selective channels, an optimized spreading sequence allocation procedure is presented. Finally, a selection of the spreading codes which jointly reduces the multiple access interference and the crest factor is proposed for downlink MC‐CDMA systems.
A study on the application of blind turbo channel estimators to ofdm related systems
Authors: Kammeyer Karl‐Dirk , Petermann Thorsten 
Published: September 2008

AbstractCommunication standards based on coherent data demodulation require an accurate estimation of the channel in order to ensure a reliable transmission. This is a difficult task especially for fast fading channels caused by high‐speed mobile units. In this paper, we compare different nonblind and blind channel estimation approaches for OFDM related systems. Since most blind channel estimators are known to be extremely complex or sensitive towards certain system parameters, we present a novel algorithm which delivers sufficient estimates with comparatively little effort. Furthermore, an iterative channel estimation scheme is shown that leads to significant performance improvements particularly for high speed propagations.
Performance of Turbo‐TCM over a noise burst channel
Authors: Goldman Haim 
Published: September 2008

AbstractThe paper analyses the performance of the Turbo‐TCM coding scheme operating over a channel subject to bursts of noise. Two modes of operation are discussed: A mode in which Channel State Information (CSI) on the occurrence of noise‐bursts is available and a mode with no CSI. The performance is analysed as function of the frequency and duration of the noise bursts and of the SNRs during the “noise burst state” and me “benign state”. It is shown that there is a rather low threshold value of the noise‐burst SNR above which it is preferable to ignore the noise bursts. It is demonstrated that the Turbo‐TCM scheme operating over a channel subject to noise bursts can achieve good performance even with no estimation of the parameters of the noise‐burst process. Even if the noise bursts are very strong, all that is required is a simple burst‐detector and application of the rule: Erase very strong noise bursts, ignore other bursts.
Extrahepatic Biliary Atresia
Authors: Alagille Daniel 
Published: September 2008

Extrahepatic biliary atresia is defined as partial or total absence of permeable bile duct between porta hepatis and the duodenum. The incidence varies from 1:8,000 to 1:10,000. Cholestasis is total and permanent. 131I Rose Bengal test and needle liver biopsy allow correct identification of 95% of cases before surgery Before the surgical procedure described by Kasai in 1959, all patients died between 1 and 2 years of age. Through the use of different types of Kasai’s procedures by experienced groups, 30 to 35% of patients are successfully operated on. In our group, 248 infants underwent surgery, 121 before 1977: 44 (36.6%) are alive at least 5 years after surgery. Growth is within normal limits in all children still alive, even in those who presented with numerous and severe episodes of cholangitis during the first two postoperative years. Portal hypertension was progressive in 23 children with splenomegaly and esophageal varices developing between 1 and 2 years of age. Five patients bled profusely between 2 and 4 years of age: portal systemic shunts were performed in 4. Surgical liver biopsies were performed in 20 children, 5 to 8 years old: cirrhosis was present in all and was micronodular in 13 and macronodular in 7. The most impressive histologic change was absence of biliary ducts or ductules. This histologic finding can be related with percutaneous transhepatic cholangiographic pictures.
Despite the abnormal appearance of intrahepatic bile ducts and the constant presence of cirrhosis with portal hypertension, all 44 patients who are alive at least 5 years after surgery seem to lead almost normal lives.