bluuebunny/crossref_metadata_2025 huggingface dataset:
The Impact of Handover Protocols on the Performance of ABR Flow‐Control Algorithms in a Wireless ATM Network
Authors: Savoris Michael , Wolisz Adam , Krieger Udo R.
Published: September 2008

Considering the high‐speed data transfer by the ABR service class in a wireless ATM network, we investigate the impact of forward and backward hard handover protocols on the performance of the three rate‐based ABR flow‐control schemes Explicit Rate Indication for Congestion Avoidance (ERICA+), Fuzzy Explicit Rate Marking (FERM) and a new promising adaptation of FERM, called FERMA. We evaluatc the performance of these flow‐control protocols by means of an object‐oriented simulation modcl of a basic client‐scrvcr architecture. Considering a standard hard handover protocol in a wircless ATM nctwork. wc show that our proposcd enhanced signaling schemes based on a congestion‐awarencssconceptand their integratibn into thc ABR flow control have a positive influence on the stability and efficiency of the cell transfer by ABR connections. Morcovcr, we point out that the new adaptation of the fuzzy‐control scheme FERM provides a promising altcrnativc lo the ERICA+ algorithm.
On the Use of Sampling Theory for Passively Monitoring QoS in Packet Networks Fed by Different Types of Traffic
Authors: Cozzani Irene , Giordano Stefano
Published: September 2008

Performance measurements are a hot research topic for the design and control of high speed heterogeneous networks, such as the BISDN as well as the Internet. Our research in this field is concemed with a novel measurement system, with real‐time monitoring capabilities, whose working is based on sampling survey. The scenario used to test the accuracy of the proposed methodology is a “Passive QoS 2‐point measurement system” acting on an ATM network, which is fed with different kinds of traffic: synthetic traces (traditional as well as LRD models) and a trace of real ATM traffic captured on a transatlantic UK‐US link. Delay and loss characteristics, evaluated with the proposed sampling technique are compared against those foreseen by the sampling theory for several values of the sampling rate. The final result is a quantitative analysis that trades‐off accuracy and implementation costs of the measurement instruments and is a powerful control tool usable by networks managers to check the quality of service they are providing, to understand network internal dynamics and to design their monitoring systems.
Communication Theory: Letter: Multiuser Detection Analysis in DS‐CDMA Channel with Nakagami Fading
Authors: Novaković Dejan M. , Dukić Miroslav L.
Published: September 2008

In this paper the multiuser detection performance analysis‐ in synchronous direct sequence code division multiple access (DS‐CDMA) system using decorrelating and decision feedback (DFB) receivers, including single path Nakagami fading channel is presented. The lower bounds of the receivers bit error probability (BER) are derived, and the comparison of these receivers' performances to the conventional CDMA receiver is made. The analytical and numerical results show that the DFB receiver performs better in the presence of the high Nakagami fading channels.
Analysis of Ken Effect in IM/DD SCM Systems with Multiple Optical Carriers
Authors: Krishnaswa Rajesh M. , Sircar Pradip
Published: September 2008

The Kerr effect in an intensity modulated direct detection subcarrier multiplexed system with multiple optical carriers is investigated in this paper. An expression for inteference power spectrum is derived taking into consideration the laser phase noise and Kerr effect. The derivation is carried out first for the case of two optical carriers, and then generalised for the lightwave system with N optical camers.
Letter: Blind Adaptive Parameter Estimation for CDMA Systems using Cyclostationary Statistics
Authors: Mazzenga Franco
Published: September 2008

In this letter we present a time domain blind adaptive algorithm to jointly estimate amplitude, phase, and time delay of each user in a code division multiple access (CDMA) signal. It is based on the cyclostationary nature of the CDMA signal and it uses the estimates of normalized cyclic and conjugate cyclic autocorrelation functions of the received signal. Since the latter can be obtained as solution of a minimum mean square error optimization problem adaptive algorithms can be used to obtain an on line implementation of the procedure. Performances are evaluated through simulation.
Letter: Compressed Disparity Information Transmission over Constant Bit Rate ATM Channels
Authors: Papadimatos Dionysis , Antonakopoulos Theodore , Makios Vassilios
Published: September 2008

This letter presents a real‐time lossless compression/decompression unit for disparity map information used in 3D teleconferencing systems. A lossless compression algorithm is used to compress the disparity map data in real‐time, resulting in a variable bit‐rate data stream that has to be transmitted through a constant bit‐rate channel. The system uses a controlled‐data‐loss method for data rate adaptation and for minimizing the loss of information.
An Improved Linear MMSE Detection Technique for Multi‐Carrier CDMA Systems: Comparison and Combination with Interference Cancellation Schemes
Authors: Baudais Jean‐Yves , Hèlard Jean‐François , Citerne Jacques
Published: September 2008

In this paper, various detection techniques are studied in the synchronous case of a multiuser MC‐CDMA system operating in frequency selective Rayleigh channel. First of all, an improved linear detection approach named MMSE per user detection technique, which takes into account the despreading process, is presented. Simulation results over a Rayleigh fading channel show that this novel technique offers for non full load systems a significant gain compared to the MMSE equalization per camer method. Then, the performance of various parallel and successive interference cancellation techniques is compared to the performance of the linear MMSE per user scheme. When the power of the interferers is roughly equal to the power of the useful signal, the performance of the linear MMSE per user is really close to the performance of the non linear PIC and SIC schemes.
Spread Spectrum OFDM Modem for HF Voice‐Band Link over Fading Channels
Authors: Zazo Santiago , Bader Faouzi , Páez‐Borrallo José M.
Published: September 2008

This paper deals with the application of spread spectrum techniques to MultiCarrier HF modems. OFDM modems are a special option for the transmission over the high dispersive HF medium in front of single carrier modems. Our present contribution is now focused on the application of Spread Spectrum techniques to improve its fading robustness. The transmission bandwidth is the standard voice band channel in the HF band where the spread spectrum characteristic means that one symbol is transmitted over N times the spectrum in the classical N‐OFDM scheme. The motivation of this idea is the observation of the OFDM modems behaviour over fading channels with deep nulls showing a quite different performance between carriers: those in the vicinity of deep nulls provide a much higher Bit Error Rate (BER) than others far away of the nulls. Our proposal intends to spread different symbols over all the subcarriers simultaneously in order to homogenize performances. This approach, performed by orthogonal codes, shows an evident link with MC‐CDMA techniques. In addition, Multi User Detection (MUD) strategies as Interference Cancellation (IC) with coherent detection are also applied in order to improve its performance.
Flexible Implementation of Multicarrier Systems with Polyphase Filterbanks
Authors: Matheus Kirsten , Kammeyer Karl‐Dirk , Tuisel Ulrich
Published: September 2008

This article will emphasize on how to efficiently implement multicamer (MC‐) systems different from and alternative to OFDM with polyphase filterbanks. It will show that when using polyphase filterbanks this is an incomplex and flexible solution. It will be derived that with polyphase filterbanks an additional MC‐system design parameter exists; not only spacings between adjacent subcarriers in integer multiples of the inverse MC‐symbol duration are possible but that also non‐integer multiples of the inverse MC‐symbol duration are easily realizable.
On the assessment of cochannel interference in wireless systems by the saddle point approximation
Authors: Tomba Luciano
Published: September 2008

Many papers have considered the problem of how to calculate theprobability of outagedue to cochannel interference in cellular radio systems. A general approach based on the characteristic function method has been recently presented in [1,2] and in [3] in the cases of Nakagami fading and Rice fading, respectively. However, closed—form results are available only in a few cases, while numerical integration and/or evaluation of special functions is the solution for other cases. In this paper we follow a similar approach as in the above works, but we evaluate integrals by using thesaddle point approximationmethod which has proven itself to be fast and accurate. Some comparisons between our results and the results published in the above references are reported.
Letter: On the performance of trellis coded M—PSK systems over rician fading channels with phase jitter
Authors: Uysal Mrat , Aygolu Umit , Panayirci Erdal
Published: September 2008

In this paper, the performance of trellis coded M—PSK systems is investigated operating over Rician fading channels with phase jitter. Thus the results reflect not only the degradation due to the effect of fading on the amplitude of the received signal, but also the impairment caused by the noisy carrier reference, which is usually neglected by the assumption of perfect phase tracking at the receiver. Analytical upper bounds on pairwise error probability for Rician fading with phase jitter are derived using Chernoff bounding technique in both cases where channel state information is available or not. The bit error probability performances are evaluated based on the modified transfer function approach. The numerical examples clearly demonstra'e the error performance degradation due to both amplitude fading and phase noise process.
Intact versus fractional switching in SONET/SDH cross—connects
Authors: Park Won Bae , Owen Henry L. , Zegura Ellen Witte
Published: September 2008

Synchronous Optical Network/Synchronous Digital Hierarchy (SONET/SDH) cross connect switching structures may be made non—blocking by providing sufficient hardware resources. For example, in a three—stage Clos switching structure this is accomplished by providing a sufficient number of middle stage switches to allow connections to be made without blocking. Nonblocking SONET/SDH cross—connects are more expensive than architectures which reduce the hardware complexity in their implementations at the expense of increased blocking probability. Under certain circumstances, initially unsuccessful new connections may still be connected if rearrangement of existing connections is possible. Factors which make rearranging connections difficult in a SONET/SDH signal environment is the multirate and multicast nature of the traffic, the requirement that any rearrangements must not interrupt existing sessions, and subnetwork connection protection. Cross—connects with this subnetwork connection protection capability must provide hardware and software functionality that selects between two independent copies of a signal and passes the error—free copy through the matrix. We explain how subnetwork protection introduces a requirement for intact switching of SONET/SDH signals. We also present a comparison of intact and fractional SONET/SDH connection and rearrangement algorithms in the context of subnetwork connection protection requirements. We demonstrate the blocking probability impact of introducing subnetwork protection connections into cross—connects.
Availability and performance of single/multiple site diversity satellite systems under rain fades
Authors: Livieratos Spiros N. , Corns Panayotis G.
Published: September 2008

An analysis is presented for the evaluation of the probability of error achieved by single/multiple site diversity systems taking into account rain attenuation. A systematic method is proposed relating rain attenuation statistics to BER performance masks recommended by ITU for satellite links. The efficiency of site diversity as a countermeasure against severe rain attenuation is investigated for various frequency bands and climatic regions. Single site systems are compared to double and triple site diversity systems.
Broadband capacity upgrade strategies in residential areas: An investment cost analysis
Authors: Ims Leif Aarthun , Myhre Dagfinn , Olsen Borgar Torre
Published: September 2008

The paper presents an analysis of the investment costs associated with broadband' upgrades in various residential areas in which there is an existing twisted pair infrastructure. Upgrade strategies— based on a combination of optical fibre systems and digital subscriber line (DSL) technologies are examined. The analysis highlights the significant differences in investment levels expected in downtown, urban, suburban and rural areas for the ranges of DSL capacities from 2 Mbit/s to 26 Mbit/s. The paper reports work performed in the EURESCOM2 project P614, Implementation Strategies for Advanced Access Networks*. The methodology and tool initially developed by the RACE3 2087/TITAN4 project, and now under further development in the ACTS5 226 OPTIMUM6 project has been applied in the techno—economic analysis.
Performance of a baseband adaptive predistorter in presence of orthogonal multicarrier modulation
Authors: Santella Giovanni , Mazzenga Franco
Published: September 2008

The performance of a baseband adaptive predistortion system to linearize memoryless high power amplifier (HPA) are analyzed in the presence of an Orthogonal Frequency Division Multiplexing (OFDM) signal. The predistortion scheme adopted is based on a digital signal processing microprocessor and a memory look—up table to predistort the baseband drive signal. Stability of the adaptation algorithm, rate of convergence and effect of quantization are assessed through simulation. The curves of Total Degradation versus the Output Back—off have been calculated for QPSK, 16QAM and 64QAM constellations. Starting from these results, the improvement brought by the use of the predistorter and the degradation caused by the quantization has been assessed.
Prioritisation of data partitioned MPEG—4 video over mobile networks
Authors: Worrall Stewart T. , Fabri Simon N. , Sadka Abdul H. , Kondoz Ahmet M.
Published: September 2008

Despite much research in the field of mobile multimedia, delivery of real—time—interactive video over noisy wireless channels remains a challenging problem. Two of the major issues in providing true end—to—end mobile multimedia capability are interoperability between platforms and networks and the poor performance of video compression algorithms in error—prone environments. This paper presents a method for prioritising data partitioned MPEG—4 video in a way suitable for transmission over a mobile network. The effectiveness of the technique is demonstrated by examining the performance when transmitted using the Real—Time Transport protocol over GPRS (General Packet Radio Service) data channels under varying channel conditions.
Hierarchical transmission of MPEG—2 sequences in noisy environments
Authors: Favalli Lorenzo
Published: September 2008

The MPEG—2 standard, describes several options to split the video stream in sub—streams conveying information at different detail level. This fact can be exploited to generate a low quality stream with all what is necessary for a basic service while all other sub—streams carry “detail” information to improve the quality of the reconstructed signal. In a mixed network environment this can be exploited to allow the stream to be decoded by different terminals. Another use is to provide flexibility in tough transmission systems where either traffic or noise can give rise to severe impairments. In this paper signal—to—noise—ratio scalability is used in a radio transmission system targeted as the DVB—S (Digital Video Broadcasting — Satellite) to investigate the eventual gains. It is shown that for a given target rate, differentiated coding allows the received signal to be decoded at a channel Signal to Noise Ratio which is 3 dB lower than the one necessary for a single—stream coded sequence.
Rate—distortion optimal fast thresholding for MPEG—2 image sequence coding
Authors: Tzovaras Dlmitrios , Boulgouris Nlkolaos V. , Argyraki Katerina , Strintzis Michael G.
Published: September 2008

A new Rate—Distortion (R—D) optimized image sequence encoding scheme is introduced, fully compatible with the standard MPEG—2 decoder. R—D optimal DCT coefficient thresholding is used, so that best image reproduction quality is achieved under the constraint of a “local” target bit rate for every image. The proposed technique combines R—D optimal coefficient thresholding and some elements of the MPEG—2 rate control model “Test Model 5” and results in a new video encoding scheme, which offers in most cases a higher representation quality than “Test Model 5”. The performance of the proposed technique is experimentally evaluated for the coding of standard test image sequences.
Error concealment in the transformed domain for DCT—coded picture transmission over Noisy Channels
Authors: Ancis Marcella , Giusto Daniele D. , Perra Cristina
Published: September 2008

In this paper, the problem of DCT information loss in the transmission of coded pictures over unreliable packet networks is studied. Due to the use of variable length coding, packet losses can make impossible the reconstruction of large areas, causing annoying artifacts in the decoded image. Here, different methods of error concealment directly working in the transformed domain are described and their performance are compared with reference to the results of an extensive testing phase. Our analysis spans from very simple interpolation algorithms, which require only minor computational resources, even if at expense of a limited reconstruction quality, to more complex techniques, based on the use of Artificial Neural Networks, which yield good results even in very detailed areas, but require longer computational time.
Dynamic QoS mapping control for streaming video in relative service differentiation networks
Authors: Shin Jitae , Kim Jin‐Gyeong , Kim Jongwon , Jay Kuo C.‐C.
Published: September 2008

A dynamic quality of service (QoS) mapping control scheme, which includes feedforward and feedback QoS control, is proposed for the differentiated services (DiffServ) networks in this work. To achieve reliable and consistent end—to—end video streaming with relative service differentiation, the proposed solution consists of two parts: (1) relative priority—based indexing and categorization of streaming video content at the sending end—system and (2) dynamic and aggregate QoS mapping control with packet, session, and class—based granularity level for categorized packets at the edge of the DiffServ domain, called the video gateway (VG), based upon the load variation of the network. In particular, we focus on dynamic solutions to handle QoS demand variations of continuous media applications (e. g. varying priorities from aggregated/categorized packets) and QoS supply variations of the DiffServ network (e. g. varying loss/delay due to fluctuating network loads). Thus, with the proposed dynamic QoS mapping control, video streaming with enhanced quality is demonstrated under a pricing model.
Data transfer statistics in internet dial—up sessions
Authors: Naldi Maurizio
Published: September 2008

This letter reports the results of a measurement campaign aimed at characterizing the traffic in Internet dial—up sessions. This is done at the data transfer level, i. e. considering the average data rate and the volume of data transferred during calls to Internet service providers. Major statistics (mean and two percentile values) are provided for both quantities with a hourly time resolution together with the empirical probability density functions relative to the peak hour.
The capacity of the noncoherent channel
Authors: Colavolpe Giulio , Raheli Riccardo
Published: September 2008

The capacity of a random—phase additive white Gaussian noise (AWGN) channel, referred to as noncoherent channel, is investigated in the case of a transmission of N information symbols. The non—Gaussianity of die capacity—achieving distribution is shown and a lower bound on the channel capacity is derived. For increasing values of the number of transmitted symbols N, the capacity of a noncoherent channel is shown to asymptotically approach that of a coherent channel, i. e., a known—phase AWGN channel. The asymptotical Gaussianity of the capacity—achieving distribution is also shown. Based on the derived lower bound, the inherent capacity loss of a noncoherent channel, as compared to a coherent one, may be considered very limited for all but very small values of N. This result may be viewed as the information theoretic counterpart of a similar conclusion derived by many authors with reference to the probability of detection error.
On the capacity of cellular CDMA and TDMA over nondispersive channels
Authors: Sari Hikmet , Steendam Heidi , Moeneclaey Marc
Published: September 2008

We investigate the interuser interference in cellular code—division multiple access (CDMA) and timedivision multiple access (TDMA) systems and compare the two multiple access techniques at different levels of user density per cell. The study includes the interference both on the downstream channel (from base station to users) and on the upstream channel (from users to base station). The purpose is to compare the two multiple access techniques when the total bandwidth occupancy and the maximum number of users per cell are the same. The results indicate that on a nondispersive radio channel with a path loss that is proportional to the fourth power of the distance, the interuser interference is approximately 10 dB higher in CDMA. The implication of this result, which holds for both the downstream and the upstream channels, is that for the same total bandwidth and number of users per cell, TDMA gives substantially superior bit error rate (BER) performance. This is confirmed by our numerical results evaluated for both additive white Gaussian noise (AWGN) channels and memoryless flat Rayleigh fading channels. Alternatively, CDMA needs approximately 10 times more bandwidth to achieve the same signal—to—interference (S/I) ratio, a result which may seem surprising, because the fact that CDMA employs a frequency reuse factor of 1 is often given as an evidence that this technique can support a higher number of users than TDMA in a given geographic area and frequency bandwidth.
Adaptive radio resource management in F/TDMA cellular networks using smart antennas
Authors: Hartmann Christian , Eberspächer Jörg
Published: September 2008

In this paper we discuss different Radio Resource Management (RRM) strategies for smart antenna cellular networks. The focus is on adaptive strategies which combine spatial channel reuse within the cell with dynamic inter—cell channel allocation. In particular, three different SDMA channel allocation strategies are described and compared in terms of complexity as well as traffic capacity. The comparison is done by means of system simulations of a multi—cellular environment. We show that with a growing degree of freedom the RRM strategies yield better results in terms of traffic capacity while the number of handoffs per call increases and the complexity grows.
Adaptive soft—decision decoding for array codes
Authors: Peng Xiao‐Hong , Farrell Paddy G.
Published: September 2008

An adaptive decoding scheme is introduced that achieves efficient soft—decision decoding for row—and—column parity array codes. The special structure of array codes is exploited to make effective use of hard—decision methods to realize soft—decision decoding. This leads to considerable reduction in decoding complexity as the amount of soft—decision computation varies with channel conditions. It is shown that the new decoding algorithm guarantees bounded distance performance. Simulation results indicate that the actual improvement of decoding performance over uncoded systems and previous decoding methods is significant.
Paging area planning in future cellular systems
Authors: Kazantzakis M. , Demestichas P. , Demesticha V. , Tzifa E. , Kazantzakis J.
Published: September 2008

Efficient resource utilisation in future cellular systems is related to the control of the signalling load imposed by the paging operations. This relies on the application of efficient paging schemes, and on the “proper” planning of paging areas. In this paper we define and solve a version of the paging area planning problem. In particular, we provide a low complexity algorithm, under the assumption that the scheme applied falls within the last interaction based paging (LIBP) category.
A large hierarchical network star—star topology design algorithm
Authors: Petrek Jozef , Sledt Volker
Published: September 2008

The hierarchical communication network design includes solving problems like the concentrator quantity problem, the concentrator location problem and the assignment problem. Many algorithms have been proposed to solve these particular problems. The two latter problems are NP—hard and no optimal solution of these problems has been found for large networks. In this paper we present a high complexity algorithm for star—star network topology design which solves all three problems for several hierarchy levels simultaneously and finds a superior solution in a reasonable time even for large networks with a few thousand terminals due to a new and quick assignment algorithm. The computational results showed that our assignment algorithm is superior to “Simulated annealing” and “Tabu search”.
Implications of Interdomain Traffic Characteristics on Traffic Engineering
Authors: Uhlig Steve , Bonaventure Olivier
Published: September 2008

We study the interdomain traffic as seen by a non‐transit ISP, based on a six days trace covering all the interdomain links of this ISP. Our analysis considers the relationships between the interdomain traffic and the interdomain topology. We first discuss the day‐to‐day stability of the interdomain traffic matrix to evaluate the feasibility of interdomain traffic engineering. Then, we study the variability of the interdomain flows for several aggregation levels (prefix, AS and sink tree) and with respect to the interdomain topology seen by BGP. We show that despite the important variability of interdomain flows, it would be useful for a non‐transit ISP to traffic engineer its access traffic by relying on a sink tree aggregation level.
Partitioned residual echo power estimation for frequency‐domain acoustic echo cancellation and postfiltering
Authors: Enzner Gerald , Martin Rainer , Vary Peter , Enzner G. , Martin R. , Vary P.
Published: September 2008

Residual echo arises in hands‐free telephony equipment due to insufficient echo canceler convergence, but can be suppressed using a postfilter. The residual echo power spectral density is the most crucial control parameter for both frequency‐domain acoustic echo cancellation and combined residual echo and noise postfiltering. In this contribution we present and compare residual echo power spectral estimation techniques. We introduce a new partitioned block‐adaptive estimation technique delivering considerably improved residual echo estimates in strongly reverberant and noisy acoustic environments. We show that the adaptation loop of the frequency‐domain adaptive filter (FDAF) can be used simultaneously for residual echo power estimation and tracking of the echo path impulse response. In this way, the FDAF and the postfilter concept supplement each other in a true synergy with low complexity. The resulting echo and noise control system proves to be robust in double talk situations as well.
Most probable paths and performance formulae for buffers with gaussian input traffic
Authors: Addie Ron , Mannersalo Petteri , Norros Ilkka
Published: September 2008

In this paper, performance formulae for a queue serving Gaussian traffic are presented. The main technique employed is motivated by a general form of Schilder's theorem, the large deviation result for Gaussian processes. Most probable paths leading to a given buffer occupancy are identified. Special attention is given to the case where the sample pams of die Gaussian process are smooth. The performance approximations are compared with known analytical results or by means of simulation. The approximations appear to be surprisingly accurate.
Radial basis function receiver for DS‐CDMA
Authors: Tanner Rudolf , Cruickshank David G. M.
Published: September 2008

This paper presents a nonlinear sub‐optimum receiver structure for DS‐CDMA. We also draw attention to the nonlinear separation problem in a DS‐CDMA system and suggest me use of ideas known in the field of partem recognition. We visualise the separation problem and the decision boundaries of different receiver structures, which in return give reasons for each receiver's performance. The proposed receiver structure is based upon the radial basis function (RBF) network. Techniques from pattern recognition are exploited to reduce the RBF complexity for mulupath scenarios, for example the use of the Mahalanobis distance measure. The presented RBF receiver has significantly less complexity (in terms of centres or neurons) than the optimum RBF receiver through the use of super centres, and outperforms Verdu's detector.
Adaptive state‐ dependent importance sampling simulation of markovian queueing networks
Authors: Boer Pieter‐Tjerk De , Nicola Victor F.
Published: September 2008

In this paper, a method is presented for the efficient estimation of rare‐event (buffer overflow) probabilities in queueing networks using importance sampling. Unlike previously proposed change of measures, the one used here is not static, i.e., it depends on the buffer contents at each of the network nodes. The ‘optimal’ state‐dependent change of measure is determined adaptively during the simulation, using the cross‐entropy method. The adaptive state‐dependent importance sampling algorithm proposed in this paper yields asymptotically efficient simulation of models for which it is shown (formally or otherwise) that no effective static change of measure exists. Simulation results for queueing models of communication systems are presented to demonstrate the effectiveness of the method.
An adaptive importance sampling approach for the transient analysis of markovian queueing networks
Authors: Buchholz Peter
Published: September 2008

We present a new method for the efficient estimation of rare events and smart probabilities in Markovian queueing networks. The method uses importance sampling to modify the probability distribution of the events to be observed. In contrast to most other importance sampling approaches, transient instead of steady state analysis is considered and the change of the measure is computed adaptively. The whole approach is based on the combination of discrete event simulation and randomization, a technique to transform a continuous time Markov chain into a discrete time chain and an associated Poisson process. By means of randomization it is possible to derive a simple model describing me relation between me occurrence of the rare event and the probability of different transition types. This model can be used to compute adaptively the change of transition probabilities to make the rare event less rare depending on the observed behavior in some replications and the time horizon of transient analysis. If the time horizon increases, the method is extended by introducing regeneration points in the analysis. By means of small examples it is shown that the method yields satisfactory results even for more complex queueing networks for which optimal or approximatively optimal importance sampling parameters are not known from the theory.
Adaptive importance sampling in routing and wavelength assignment problem
Authors: Hyytiä Esa , Virtamo Jorma
Published: September 2008

Dynamic routing and wavelength assignment in all‐optical networks is a well‐known problem for which many reasonably well working heuristic algorithms have been proposed in the literature. The problem can be also approached in the Markov decision process (MDP) framework. Due to the huge size of the state space only heuristic algorithms are viable in practice. However, so called first policy iteration can be used to enhance the performance of any given (standard) policy. In some cases the improvement can be considerable. For the first policy iteration one needs to estimate so called relative values of states, which are defined as the expected difference in future rewards/costs between two states. The relative values can be estimated by short simulations starting the system from a given state. As the blocking probability in network is usually very low this leads to a problem where rare events must be estimated. Also, depending on the system's current state some arrival realisations are more likely to cause blockings than others. An adaptive importance sampling method is proposed to overcome these difficulties.
Analysis of restart simulation: Theoretical basis and sensitivity study
Authors: Villén‐Altamirano Manuel , Villen‐Altamirano Jose
Published: September 2008

RESTART (Repetitive Simulation Trials After Reaching Thresholds) is a widely applicable accelerated simulation technique that allows the evaluation of extremely low probabilities. This paper revisits the theoretical basis of RESTART in a more general and rigorous way. The unbiasedness of the estimator is proved and its variance is derived without need of making any assumption on the simulated system. From this analysis, the gain obtained with RESTART as well as the optimal parameter values that maximise the gain are derived. The sensitivity of the method to the choice of the importance function, threshold values and number of retrials for each threshold is studied, providing simple bounds on me impact on the gain of each of these choices.
Quantum simulation — rare event simulation by means of cloning, thinning and distortion
Authors: Addie R. G.
Published: September 2008

A method of rare event simulation, termed herequantum simulation, and known also (with some variations) aspopulation Monte Carlo, andSequential Markov Chain simulation, is applied in this paper torare event simulationof communication systems. The technique described in this paper generalizesimportance sampling, importance splittingand die Monte Carlo method which uses a collection (population) of particles. The termquantum simulationis used for the rare‐event simulation technique presented in this paper because the entire ensemble of simulations resembles the parallel universes model of quantum mechanics. By using cloning, thinning and distortion it is possible to design simulations with the speed of importance sampling and the flexibility of importance splitting. A particularly difficult system is investigated in this paper by means of quantum simulation, namely a buffer fed by a Poisson‐Pareto Burst Process. The simulations are able to confirm an analytic approximation for this model to a degree not previously achievable. Furthermore, this approximation was originally developed as a consequence of the investigation of this model by means of quantum simulation.
On steady‐state simulation of some weak regenerative networks
Authors: Morozov Evsei , Aminova Irina
Published: September 2008

We discuss the possibilities of the so‐called weak regeneration in the steady‐state analysis of multiserver queues and some Jackson‐type and multiclass queueing networks when classical regenerative simulation is ineffective. A few constructions of weak regeneration for open Jackson‐type and multiclass queueing networks are given in an explicit form. Conditions are found when efficiency of weak regeneration (expressed in the simulation time needed for confidence estimation with a given precision) stays very high while the efficiency of classical regeneration is rapidly decreasing. Numerical results are also considered which illustrate an advantage of the weak regeneration in simulation of some tandem queueing networks.
Orthogonal multicarrier CDMA and its detection on frequency‐selective channels
Authors: Sari Hikmet
Published: September 2008

Multicarrier code‐division multiple access (MC‐CDMA) first appeared in the literature in 1993. It combines conventional code‐division multiple access (CDMA) and orthogonal frequency‐division multiplexing (OFDM), and has several variants that depend on how this combination is done. Although there have been a significant number of publications on MC‐CDMA since its introduction, the literature is still very fragmented, and the terminologies used by different authors are inconsistent. In addition, there are also a number of confusions concerning the detection of MC‐CDMA signals on frequency‐selective channels. As an attempt to achieve mis goal, this paper gives a comprehensive review of MC‐CDMA, describes its different variants, and reviews the receiver techniques for orthogonal MC‐CDMA. It also discusses the relative merits of MC‐CDMA and of multicarrier time‐division multiple access (MC‐TDMA) and orthogonal frequency‐division multiple access (OFDMA) which are the two omer multiple access techniques usable in conjunction with multicarrier transmission.
Development and implementation of an advanced airport data link based on multi‐carrier communications
Authors: Haas Erik , Lang Helmuth , Schnell Michael
Published: September 2008

In this paper, both the concept and the demonstrator implementation of an advanced airport data link is described which is used for data exchange between pilots and controllers in future surface movement guidance and control systems on airports. To fulfil future airport capacity requirements, the proposed data link has to be a high capacity, high rate data link. Therefore, the data link design is based on multi‐carrier code‐division multiple‐access which is a flexible and highly efficient multiple‐access transmission technique suitable for mobile communications in fading conditions. Both simulation results with typical aeronautical channel scenarios and bit error rate measurements using the demonstrator hardware show the suitability of multi‐carrier code‐division multiple‐access for aeronautical communications within the airport and its vicinity. Moreover, implementation effects which degrade the performance are identified and countermeasures are proposed and investigated. The results show that a reduction of the implementation loss of up to 2 dB is achievable by applying appropriate countermeasures.
Maximum a posteriori multipath fading channel estimation for ofdm systems
Authors: Panayirci Erdal , Çirpan Hakan A.
Published: September 2008

In this paper, a non‐data‐aided maximum a posteriori (MAP) channel estimation technique for OFDM systems employing M‐PSK modulation scheme is proposed. The technique requires a convenient representation of the discrete multipath fading channel based on the Karhunen‐Loeve orthogonal expansion and estimates the complex channel parameters of each subcarriers iteratively in frequency domain using the Expectation‐Maximization (EM) algorithm. Pilot symbols are employed to choose reliable initial values of the unknown channel parameters. An analytical expression is derived for the exact Cramer‐Rao lower bound of the proposed MAP channel estimator. Moreover, robustness of estimator to changes in channel correlation and signal‐to‐noise ratio is also analyzed. The performance is presented in terms of the mean‐square error and the uncoded symbol error rate for a system employing QPSK signaling. Computer simulations demonstrate that the performance of OFDM systems using coherent demodulation based on our channel estimation can be significantly improved.
Delay analysis of a probabilistic priority discipline
Authors: Jiang Yuming , Tham Chen‐Khong , Ko Chi‐Chung
Published: September 2008

In computer networks, the Strict Priority (SP) discipline is perhaps the most common and simplest method to schedule packets from different classes of applications, each with diverse performance requirements. With this discipline, however, packets at higher priority levels can starve packets at lower priority levels. To resolve this starvation problem, we propose to assign a parameter to each priority queue in the SP discipline. The assigned parameter determines the probability or extent by which its corresponding queue is served when the queue is polled by the server. We thus form a new packet service discipline, referred to as the Probabilistic Priority (PP) discipline. By properly adjusting the assigned parameters, not only is the performance of higher priority classes satisfied, but also the performance of lower priority classes can be improved. This paper analyzes the delay performance of the PP discipline. A decomposition approach is proposed for calculating the average waiting times and their bounds are studied. Two approximation approaches are proposed to estimate the waiting times. Simulation results that validate the numerical analysis are presented and examined. A numerical example which demonstrates the use of the PP discipline to achieve service differentiation is presented. This example also shows how the assigned parameters can be determined from the results of analysis mentioned above.
An analytical expression for the acquisition time and optimal designing graph for a frequency detector of OFDM systems
Authors: Pinchas Monika , Hasson Jaime , Bobrovsky Ben Zion
Published: September 2008

In this paper we introduce an analytical expression for the acquisition time of a frequency detector used in OFDM systems. We present a useful “designing graph” which eliminates the need to simulate the whole system in order to obtain the optimal number of guard interval samples, that should be used by the frequency detector.
Study of an european all optical transport transparent network based on the 16×2.5 Gbit/S transmission
Authors: Joindot Michel , Matera Francesco , Settembre Marina , Weinert Carl
Published: September 2008

In this work we report a transmission study on the possibility of implementing an all optical transparent network based on WDM 16×2.5 Gbit/s systems, which connects the main cities of Europe. We show that severe transmission problems are present and that they are due to the impact of the fibre nonlinear Kerr effect in terms of cross phase modulation. Numerical results shows that such an impairment can be strongly limited by the use of the dispersion management with fibre gratings and good performance can be achieved for the longest paths of the order of 2000 km.
Performance comparison of square root raised‐cosine and lerner filters for the MDFT‐TMUX filter bank
Authors: Madhu Rao A. Sudana , Prabhu K. M. M.
Published: September 2008

This paper deals with prototype filters, namely, square root raised — cosine (SRC) and Lerner filters in the modified discrete Fourier transform (MDFT) transmultiplexer (TMUX) filter bank. The error obtained in the reconstruction of the original signal is compared for various filter orders. These filters can be employed in multicarrier modulation system‐ overlapped discrete multitone (overlapped DMT) or discrete wavelet multitone (DWMT) system. However, with the SRC filter, a large number of computations are required. By employing Lerner filter, we can reduce this by atleast 3‐ folds. We have carried out simulation studies for an 8‐channel MDFT‐ TMUX filter bank and the results obtained are presented.
Increasing learning among developmental education students
Authors: Moore William
Published: September 2008

The learning of marginally prepared students can be increased‐but increases require intensive effort, especially on the part of the faculty, and changes in administrative and faculty attitudes toward these students.
A Bayesian model averaging approach for cost‐effectiveness analyses
Authors: Conigliani Caterina , Tancredi Andrea
Published: September 2008

We consider the problem of assessing new and existing technologies for their cost‐effectiveness in the case where data on both costs and effects are available from a clinical trial, and we address it by means of the cost‐effectiveness acceptability curve. The main difficulty in these analyses is that cost data usually exhibit highly skew and heavy‐tailed distributions so that it can be extremely difficult to produce realistic probabilistic models for the underlying population distribution, and in particular to model accurately the tail of the distribution, which is highly influential in estimating the population mean. Here, in order to integrate the uncertainty about the model into the analysis of cost data and into cost‐effectiveness analyses, we consider an approach based on Bayesian model averaging: instead of choosing a single parametric model, we specify a set of plausible models for costs and estimate the mean cost with a weighted mean of its posterior expectations under each model, with weights given by the posterior model probabilities. The results are compared with those obtained with a semi‐parametric approach that does not require any assumption about the distribution of costs. Copyright © 2008 John Wiley & Sons, Ltd.
Physical inactivity and its impact on healthcare utilization
Authors: Sari Nazmi
Published: September 2008

Physically inactive people are expected to use more healthcare services than active people. This inactivity imposes costs on the collectively funded health insurance programs. In this paper, excess utilization of healthcare services due to physical inactivity is examined using count data models and the Canadian Community Health Survey. The aim of the paper is to estimate utilization of healthcare services associated with inactivity and to estimate its impact on the Canadian healthcare system. The results suggest that physical inactivity increases hospital stays, and use of physician and nurse services. On average, an inactive person spends 38% more days in hospital than an active person. S/he also uses 5.5% more family physician visits, 13% more specialist services, and 12% more nurse visits than an active individual. The subsequent social cost of inactivity for the healthcare system is substantial. Copyright © 2008 John Wiley & Sons, Ltd.
Structural Aspects of Bile Salt-Lecithin Mixed Micelles
Authors: Müller Karl
Published: September 2008

Mixed micelles formed by the major constituents of native bile, i.e., bile salts and lecithin, were studied by X-ray scattering differential scanning calorimetry and electron spin resonance spectroscopy. The aim of this study was to find differences in micellar structure and thermodynamic properties which might explain the different abilities of various biles to keep cholesterol in solution at comparable degrees of oversaturation. The data indicate not only that different types of micelles exist but also that different types of complex bile salt/phospholipid arrangements exist within the micelles. It is speculated that the stabilizing and destabilizing factors found by other authors influence the balance between the different micellar structures and thereby the cholesterol solubilization power of bile.