bluuebunny/crossref_metadata_2025 huggingface dataset:
Strategies and Algorithms for Routing both Unicast and Multicast Paths in WDM Networks
Authors: Listanti Marco , Cervelli Andrea , Sabella Roberto
Published: September 2008

This paper introduces the concept of the multicast optical path in WDM networks, and treats the strategies and the algorithms, which allow the routing of plural multicast paths together with unicast paths. This analysis was motivated by the possibility of exploit the optical layer facilities in the transport network, to carry distributive services, such as video channels (e.g. CATV), without the need of dedicated transmission networks. The strategies and algorithms reported in this paper arise from a generalisation of those ones, which have been previously reported in literature. They are wavelength path (WP: which does not make use of wavelength conversion), virtual wavelength path (VWP: which make full use of wavelength conversion), and partial virtual wavelength path (PVWP: making a parsimonious use of wavelength conversion). The analysis shows the characteristics and performance of those strategies and algorithms considering, as a figure of merit, either the number of wavelengths or the system scale (dimensions of the optical nodes), and compare one to each other in order to point out advantages and disadvantages. The main result is that multicast optical paths can favourably be accommodated in a WDM network, and the strategy we propose, making a parsimonious use of wavelength conversion, allows the lowest system scale to be obtained with a very low number of wavelength converters.
Routing and Wavelength Assignment in All‐Optical Networks with Multicast Traffic
Authors: Sahin GÖKhan , Azizǒlu Murat
Published: September 2008

This paper considers multicast routing and wavelength assignment (MCRWA) in a wide area all‐optical network. We present static and dynamic MCRWA algorithms, and evaluate their blocking performance under various fanout splitting policies. Through simulations and comparisons with a lower bound, we demonstrate that dynamic MCRWA can perform significantly better than its static counterparts. We then quantify the throughput gain due to link sharing in multicasting, and the performance penalty that would be suffered by considering each multicast request as multiple unicast connection requests. We also present an analytical model for estimating the blocking performance of the static MCRWA algorithms. The performance improvement with multifiber networks and with wavelength conversion are also investigated.
A Hybrid Interference Cancellation Scheme for Multiuser Systems in Multipath Fading Channels
Authors: Johansson Ann‐Louise , Svensson Arne
Published: September 2008

A new hybrid interference cancellation (IC) scheme is proposed for the uplink of multirate direct‐sequence code division multiple access (DS‐CDMA) systems communicating over mobile radio channels. The hybrid IC scheme includes both non‐decision directed and decision directed IC, together with a modified RAKE combiner to utilize the diversity in a frequency‐selective channel. The performance is evaluated via computer simulations for the two multiple data rate schemes, mixed modulation and multicode, in a multipath environment. Both known channel parameters and channel estimation using pilot symbols are considered. The results show that rnulticode or a combination of multicode and mixed modulation are preferable when handling multirate system and the loss in performance compared to the single‐user bound is small.
Accurate Recovery of Dense Depth Map for 3D Motion Based Coding
Authors: Georgis Nikos , Kittler Josef , Bober Mirek
Published: September 2008

The problem of scene structure recovery from image motion is considered in the context of motion compensated video coding. In order to compensate for full 3D motion of the camera, the scene depth map together with camera egomotion need to be estimated from the image sequence. An important prerequisite of the shape and motion recovery is the establishment of correspondences between the points in two successive frames of a video sequence. In the paper we abandon the traditional feature based approach and develop a correspondence analysis technique based on robust matching of local image intensities under affine transformation. We show that the technique not only establishes true correspondences, but the resulting field of corresponding points is dense, and their displacement is obtained with a subpixel accuracy and free of bias. We demonstrate that these dense high quality matches yield accurate estimates of the imaging geometry which are several orders of magnitude better than estimates obtained with feature based methods. This accuracy is reflected in a superior quality of estimates of the scene structure. The experiments are performed with images of a scene with known geometry, obtained using a calibrated camera. The obtained depth estimates are dense, with error of the order of 1% which contrasts with the sparse depth map and greater than 10% errors offered by a typical feature based approach. The technique is also applied to coding. Preliminary results indicate that significant improvement of the predicted image can be achieved by using accurate dense depth cstimates.
New Channel Assignment Methods for Hot‐Spot Overlaying CDMA Cellular Systems
Authors: Jung‐Shyr Wu
Published: September 2008

The CDMA cellular system can provide more capacity than AMPS systems. To deploy microcells overlaying in macrocells to provide more capacity is also attractive. However, system performance and capacity degrade due to the co‐channel interference among microcells and macrocells if a single channel is used. Based on a hot‐spot overlaying environment. we propose three multi‐channel schemes where different channels can be assigned for hot spot and macrocell. Performance measures in terms of capacity, service hole, blocking probability and blocking ratio in uplink and downlink are evaluated. It shows that the schemes with channel selection based on SIR and active users gain more benefits than the scheme with separate channels assigned for macrocells and hot‐spot. Using the proposed multi‐channel access schemes, we can avoid the co‐channel interference among macrocells and hot‐spot and provide more capacities than Baier's suggestion [1]. In addition, the proposed schemes make soft handoff workable and improve the system performance.
Letter: On the Identifiability of a Channel Transfer Function from Cyclic and Conjugate Cyclic Statistics
Authors: Mazzenga Franco
Published: September 2008

The problem of blind identification of digital communication channels from the second order cyclic and conjugate cyclic moments of the received signal is addressed. A novel condition for blind identification is obtained when both second order cyclic and conjugate cyclic moments of the received signal are available. In particular it is shown that the class of the identifable transmission channels is extended with respect to the case where only one of the two statistics is used.
Dynamic Rate Shaping on MPEG‐2 Video Streams for Bandwidth Saving on a Faded Satellite Channel
Authors: Celandroni Nedo , Ferro Erina , Potort Francesco , Chimienti Antonio , Lucenteforte Maurizio
Published: September 2008

In a previous work, we studied how to send a video stream over a noisy satellite channel using the scalability features of the MPEG‐2 standard. The base and the enhancemcnt tlows were sent with different levels of protection (bit and coding rates) in order to deal with the channel degradation. In this paper, we add Dynamic Rate Shaping (DRS), thus obtaining a greater flexibility in the choice of transmission parameters. In fact, it is possible to trim the required bandwidth in continuous mode in the working range, thus saving some bandwidth when keeping the video quality constant during changing fade conditions. The performances of DRS applied to both the non‐scalable and the scalable MPEG‐2 coded video streams are shown separately. An application of thc techniques presented is discussed, and some criteria are given for choosing the most suitable pammcters, according to the service quality requirements.
Channel Allocation with Recovery Strategy in Wireless Networks
Authors: Ma Yue , Han James J. , Trivedx Kishor S.
Published: September 2008

With the increasing penetration of wireless communications systems, customers are expecting the same level of service, reliability and performance from the wireless communication systems as the traditional wire‐line networks. Due to the dynamic environment, such as the roaming of mobile subscribers, maintaining a high radio frequency (RF) availability is one of the most challenging aspects in wireless network management. To date, in wireless network management, a call is dropped when the channel it uses goes down. In order to increase system end‐to‐end availability, an RF channel recovery scheme is proposed in this paper. When an RF channcl Pds, the channcl is replaced by another working channel and the call continues. The methods to replace failed RF channels of ongoing calls and to handle channel failures of handoR and new calls are investigatcd. Markov reward models are developed to compare system availability and performance. Automated generation and solution of Markov reward modcls is facilitatcd by a version of stochastic Petri nets that we call stochastic reward nets. The results show that the rccovcry schcme reduces the dropped calls and the blocked calls significantly under both light and normal trarffics.
A QoS‐Based Handover Initiation Algorithm for Wireless Systems
Authors: Graziosi Fabio , Santucci Fortunato
Published: September 2008

In this paper a novel handover initiation algorithm is proposed for next generation wireless systems, in the presence of a generic packet access protocol. By proper monitoring of the sequence of packets that may be lost due to channel impairments, handover triggering can bc strictly related to the Quality of Service (QoS) parameter of the source, In particular, a threshold length of the sequence of lost packets is used as a handover criterion for delay sensitive sources. In a log‐normal shadowing environmcnt, the algorithm pcrformance is derived analytically. Simulation is also used to validate the approximations introduced in the analysis. From numerical rcsults it is sccn that the novel algorithm can provide better performance than typical signal strcngth nlgorithnls used in second gcncrution mobile radio systems. Moreover, the proposed approach is rather general and may apply 10 a large class of sourecs and acccss protocols.
Efficient Communication Over the Discrete‐Time Memoryless Rayleigh Fading Channel with Turbo Coding/Decoding
Authors: Peleg Michael , Shamai(Shitz) Shlomo
Published: September 2008

Communication over the discrete time memoryless Rayleigh fading channel is investigated. Codes achieving bit error rate (BER) lower than 10‐4at bit energy over the noise spectral density ratio (Eb/No) of 1 to 2dB from the channel capacity limit were found with coding rates of 0.05 to 0.36 bits per channel use. Similar codes with rates of up to 0.57 also performed reasonably close to the capacity limit. The codes are serial concatenation of turbo code and a modulation code such as Pulse Position Modulation (PPM). The receiver is based on joint iterative decoding of the turbo code and the modulation code. Information theoretic arguments are used to form guidelines for code design and to estimate performance of the iterative decoder.
Load‐Adaptive MUI/ISI‐Resilient Generalized Multi‐Carrier CDMA with Linear and DF Receivers
Authors: Giannakis Georgios B. , Stamoulis Anastasios , Wang Zhengdao , Anghel Paul A.
Published: September 2008

A plethora of single‐carrier and multi‐carrier (MC) CDMA systems have been proposed recently to mitigate intersymbol interference (ISI) and eliminate multiuser interference (MUI). We present a unifying all‐digital Generalized Multicanier CDMA framework which enables us to describe existing CDMA schemes and to highlight thorny problems associated with them. To improve the bit error rate (BER) performance of existing schemes, we design block FIR transmitters and decision feedback (DF) receivers based on an inner‐code/outer‐code principle, which guarantees MUI/ISI‐elimination regardless of the frequency‐selective physical channel. The flexibility of our framework allows further BER enhancements by taking into account the load in the system (number of active users), while blind channel estimation results in bandwidth savings. Simulations illustrate the superiority of our framework over competing MC CDMA alternatives especially in the presence of uplink multipath channels.
Comparison of Time and Frequency Domain Processing of a Multi‐Carrier Spread‐Spectrum System
Authors: Kühne Jörg , Nahler Achim , Fettweis Gerhard P.
Published: September 2008

Multi‐camer spread‐spectrum (MC‐SS) modulation is a new kind of spread‐spectrum modulation[1],[2]. The spreading code is designed in the frequency domain. Hence, the system is dual to a direct‐sequence spread‐system (DS‐SS) where the spreading code is given in the time domain. There exist two different views: the OFDM view and the DS‐SS view. Many publications deal with the frequency domain processing. Transmitter and receiver structures are similar to OFDM. Therefore all these implementations use a guard interval to avoid ISI. The disadvantage of this guard interval is the higher hardware complexity and the waste of symbol energy. In the following paper the effect of ISI on a MC‐SS system is investigated. Both types of implementation (time or frequency domain) are compared and the dependencies of the performance from the spreading code are demonstrated. It is shown that with proper code selection and a time domain implementation without guard interval (RAKE receiver) it is possible to outperform equivalent frequency domain receivers with guard interval (MRC).
SS‐MC‐MA Systems with Pilot Symbol Aided Channel Estimation in the Asynchronous Uplink
Authors: Kaiser Stefan , Krzymien Witold A. , Fazel Khaled
Published: September 2008

Spread spectrum multi‐carrier multiple access (SS‐MC‐MA) systems exploit in a similar manner as MC‐CDMA systems the advantages of the diversity gain due to the spread spectrum technique and the high bandwidth efficiency due to the orthogonal multi‐carrier modulation. However, the main difference is that SS‐MC‐MA systems use FDMA for the user separation and therefore are less vulnerable to multiple access interference, and thus are especially of interest for the uplink of mobile radio systems. They allow asynchronism between the mobile users. In this paper we investigate the effects of ISIand ICI due to asynchronism in the uplink of an SS‐MC‐MA system on the performance of pilot symbol aided channel estimation. Moreover, we consider the suitability of power boosting of the pilot symbols to improve the link performance.
Non—data—aided ML carrier frequency and phase synchronization in OFDM systems
Authors: Panayirci Erdal , Georghiades Costas N. , Huq Ayesha T.
Published: September 2008

In this paper non—data—aided(NDA), maximum likelihood(ML) algorithms are derived for the carrier frequency and phase offset, separately, for OFDM systems employing M—PSK modulation scheme. NDA ML estimation algorithm for frequency offset estimation exploits the redundant information contained in the cyclic prefix proceeding the OFDM symbols, thus reducing the need for pilots. Its mean—squared performance is obtained analytically and compared with simulation results. It is observed that the resulting algorithm generates very accurate estimation even when the offset is high. It is also shown that the frequency estimator may be used in a tracking mode. The ML algorithm derived for the carrier phase estimation is also a non—data—aided(NDA) and maximizes the lowSNRlimit of the likelihood function averaged over M—PSK signal constellation. It is shown that for sufficiently smallSNRthe ML phase estimator obtained reduces to the familiar M/th order power synchronizer which belongs to the class of NDA feedforward carrier synchronizers introduced earlier in the literature. Its mean—squared performance is obtained analytically and compared with simulation results. We observe that the resulting algorithm generates very accurate estimation even when the phase offset is high, that the self noise is absent and the performance of the algorithm is basically the same as die Cramer—Rao bound for moderate to highSNR.Finally we note mat me error variance derived for the mean—squared performance of this NDA ML synchronizer is an extension of the approximate variance formula appeared in Reference 20,equation(14) for M—PSK.
On the transmission of an analog wideband signal through a narrowband channel
Authors: Wulich Dov , Goldfeld Lev
Published: September 2008

The problem of transmitting a wideband signal through a narrowband channel is considered. A new solution to this problem is proposed in which the wideband signal is divided into two parts: a first part which occupies exactly the spectrum that the channel can carry and a second— the rest of the signal. It is proposed to transmit the first part without any coding (in analog form) while the second part is sampled and quantized and the resulting data stream is transmitted simultaneously with the first part via the same channel. An alternative approach, based on the separation theorem, is to optimally quantize the whole signal using an optimal source encoder at the rate equal to the channel capacity and then to transmit the signal (digitally) using an optimal channel encoder. Such double optimality yields the best achievable MSE performance. It is shown that if the double optimality assumptions are applied to the second part of the signal, in the proposed case, than the MSE performance equals that which follows from the separation theorem. If, however at least one of these optimal encoders can not be achieved, the proposed method has a better MSE performance than that based on the separation theorem when applied to the whole signal.
A markov—based algorithm for the generation of MPEG sequences matching intra— and inter—GoP correlation
Authors: Lombardo Alfio , Palazzo Sergio , Schembra Giovanni , Morabito Glacomo
Published: September 2008

The development of sophisticated multimedia applications requires accurate performance evaluation tools to efficiently design and manage future high bandwidth networks. These tools have to be accurate, in order to closely match the statistical characteristics of network traffic, easy to implement, and efficient. VBR compressed video traffic is expected to represent most of the multimedia traffic loading networks in the immediate future, and the MPEG coding scheme will be the most widely used standard for many types of applications. Although many works in literature present models of video traffic, little effort has been devoted up to now to capturing one of the most important characteristics of MPEG video traffic: intra—GoP correlation. In this paper, after demonstrating that this correlation strongly affects the performance of a network loaded by MPEG video traffic, an accurate, fast and easy generation algorithm is proposed. The algorithm allows a sequence to be generated, imposing the Short Range Dependence (SRD), the Long Range Dependence (LRD) behavior and the probability density function of any given MPEG video sequence.
QoS degradation in cellular systems due to H/O hysteresis
Authors: Hernández Raúl E. Díaz , Rodríguez David Muñoz , Molina Ramírez Carlos A.
Published: September 2008

While hysteresis alleviates the ping—pong effect in a cellullar handoff, it also introduces severe outage degradation. In this letter, the handoff triggering condition as well as the unavailability to handoff completion are studied in terms of mobile location and hysteresis factor. It is shown how hysteresis favors high dropped—call probability zones within the service area. The proposed methodology is applicable to both forward and reverse channel analysis. Numerical results are presented and discussed.
Video Talks: A comprehensive multimedia conferencing system
Authors: Cash Glenn L. , Civanlar M. Reha , Haskell Barry G , Kapilow David A. , Kollarits Richard V. , Paul Baldine B. , Swain Cassandra T.
Published: September 2008

A multimedia conferencing system, dubbed AT&T VideoTalks™, has been designed and constructed at AT&T Labs—Research. It allows presentations generated in one conference room to be received in other conference rooms and at desktop computers. Real—time interaction is provided between the conference rooms. Desktop viewers can submit audio or text based questions. Interconnection is accomplished entirely via the corporate IP intranet. Multimedia presentations, including audio, video and graphics, are also digitally recorded for later re—multicasts or on—demand viewing using indexes based on viewgraphs. Network congestion is prevented by restricting the total number of presentations and playbacks. VideoTalks is a prototype system that can be retrofitted to provide for a variety of Internet services, such as remote learning, narrowcasting to niche audiences, video telephony, video conferencing, low cost video—on—demand, etc.
Interference susceptibility of selected bandwidth—efficient modulation schemes
Authors: Povero Gabriella , Vassallo Enrico , Visintin Monica
Published: September 2008

Butterworth and Square—Root—Raised—Cosine (SRRC) filtered OQPSK, and GMSK with BT = 0. 5 and BT = 1 are compared in the presence of narrow band and wide band interferers, over AWGN and nonlinear channels. The purpose of this work is to help finding out the most suitable modulation scheme for the increasingly crowded frequency bands available for space research missions.
Secure access schemes in mobile database systems
Authors: Hwang Min‐Shiang , Lee Chii‐Hwa
Published: September 2008

Mobile computing is a newly emerging computing paradigm. Whether using the term mobile database system or some other form, the database concept is essential to the mobile computing environment. This work employs a more thorough yet general means of defining the mobile database system, along with several application scenarios. Possible secure access schemes are also presented for databases in a mobile computing environment. The tradeoffs of these schemes are also analyzed. Moreover, the feasibility of applying some possible secure access methods is explored, along with the potential difficulties and unresolved issues. Finally, concluding remarks and recommendations for future works are made.
Using array signal processing to improve rural area coverage for satellite communications services
Authors: Blostein Steven D. , Karimi Joubin
Published: September 2008

We address wide—area coverage for hand—held portables using an on—board signal processing satellite in geostationary (GEO) orbit at minimum payload cost. A GEO link, being power—limited, differs from terrestrial mobile communications systems that use direct radiating antennas by requiring a focal—fed parabolic reflector, which acts as a fixed analog beamformer. We propose to cascade a high—gain parabolic reflector and a digital maximum signal—to—interference—plus—noise ratio (SINR) multi—beamformer for spatial multi—user interference suppression to achieve coverage extension. This involves on—board antenna array signal processing of multiple feeds. It is shown that nearly uniform coverage of an area the size of Canada can be provided using only five parbolic reflector apertures. Compared to conventional focussed multibeam system, link margins are typically increased from a minimum requirement of 5. 25 dB to 7. 5 dB in border areas. The more uniform coverage reduces dynamic power control requirements.
Linear space—time diversity receivers for the downlink of UMTS with WCDMA
Authors: Jarosch Andreas , Dahlhaus Dirk
Published: September 2008

Simple 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

Due 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

Joint 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

We 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

In 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

In 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

In 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

We 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

Multichannel 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

Array 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

In 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

Analyzing 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

This 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

Data 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

Downlink 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

The 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

This 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

Communication 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

The 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.
Chronic Hepatitis and Cirrhosis
Authors: Sherlock Sheila
Published: September 2008

Chronic hepatitis is defined as diffuse chronic liver disease existing for at least 6 months. Cirrhosis is a sequel. It is of multiple etiology. Liver biopsy is essential for diagnosis and prognosis. Hepatitis B-related chronic hepatitis is slowly progressive. Corticosteroid therapy is disappointing. Current antiviral therapy converts the hepatitis B e antigen-positive patient to anti-HBe in about 50%. Non-A, non-B virus hepatitis-related chronic hepatitis suffers from lack of a diagnostic marker. No current therapy is of proven benefit. Autoimmune lupoid chronic active hepatitis presents a very active biochemical and immunological picture. Prednisolone therapy prolongs life but does not prevent the development of cirrhosis. Drug-related liver disease is recognized by its associations. Recovery follows withdrawal of the drug. Deaths often follow continuation of the drug. Indications of progression to a terminal state with likelihood of less than a 6-month survival are detailed. These are helpful in deciding on hepatic transplantation before the patient becomes moribund.