an adaptive receiver for the time and frequency selective fading channel an adaptive receiver is presented in this paper for the reception of linearly modulated signals transmitted over a time and frequency selective fading channel the channel is modeled as a truncated power series which represents the dispersive fading channel as a sum of three elementary flat fading channels the proposed receiver consists of a sequence estimator with a parallel channel estimator the channel estimator recovers the instantaneous fading processes associated with each elementary channel and filters them to generate one step predictions of each fading process some implementation difficulties and solutions are also discussed computer simulations using quadrature phase shift keying qpsk and channels with moderate delay spreads and fade rates have been used to evaluate the performance of the receiver the results show that our technique has potential in channels with delay spread of about 20 signal to noise ratio snr greater than 15 db and applications requiring bit error rates ber s less than 10 sup 2

learning equivalence classes of bayesian network structures two bayesian network structures are said to be equivalent if the set of distributions that can be represented with one of those structures is identical to the set of distributions that can be represented with the other many scoring criteria that are used to learn bayesian network structures from data are score equivalent that is these criteria do not distinguish among networks that are equivalent in this paper we consider using a score equivalent criterion in conjunction with a heuristic search algorithm to perform model selection or model averaging we argue that it is often appropriate to search among equivalence classes of network structures as opposed to the more common approach of searching among individual bayesian network structures we describe a convenient graphical representation for an equivalence class of structures and introduce a set of operators that can be applied to that representation by a search algorithm to move among equivalence classes we show that our equivalence class operators can be scored locally and thus share the computational efficiency of traditional operators defined for individual structures we show experimentally that a greedy model selection algorithm using our representation yields slightly higher scoring structures than the traditional approach without any additional time overhead and we argue that more sophisticated search algorithms are likely to benefit much more

continuous phase modulation part i full response signaling the continuous phase modulation cpm signaling scheme has gained interest in recent years because of its attractive spectral properties data symbol pulse shaping has previously been studied with regard to spectra for binary data and modulation index 0 5 in this paper these results have been extended to the m ary case where the pulse shaping is over a one symbol interval the so called full response systems results are given for modulation indexes of practical interest concerning both performance and spectrum comparisons are made with minimum shift keying msk and systems have been found which are significantly better in e b n 0 for a large signal to noise ratio snr without expanded bandwidth schemes with the same bit error probability as msk but with considerably smaller bandwidth have also been found significant improvement in both power and bandwidth are obtained by increasing the number of levels m from 2 to 4

symbol by symbol map demodulation of cpm and psk signals on rayleigh flat fading channels demodulation using the symbol by symbol maximum a posteriori probability map algorithm is presented the algorithm is derived for the case of continuous phase modulation cpm signals transmitted over rayleigh flat fading channels and a corresponding receiver structure is specified it is shown that the map algorithm requires computing for each trellis branch the sum of the products of the weights of all paths through the trellis which pass through that branch and that this generic computational problem can be solved efficiently by an approach that uses a forward and backward recursion through the trellis simulation results are presented which show both the hard and soft decision performance of the map receiver to be robust even in the presence of fade rates of up to 30 of the symbol rate the application of the receiver concept to phase shift keying psk signals is also discussed and then evaluated via simulation the concept of joint demodulation and decoding using iterative processing techniques is introduced it is shown that the map receiver is well suited for iterative processing applications due to its use of a priori symbol probabilities and its production of optimal soft decisions simulation results for the reception of quaternary psk qpsk show that the bit error rate ber performance of the iterative map receiver can approach that of a receiver operating with perfect knowledge of the fading process

a characterization of markov equivalence classes for acyclic digraphs undirected graphs and acyclic digraphs adgs as well as their mutual extension to chain graphs are widely used to describe dependencies among variables in multivariate distributions in particular the likelihood functions of adg models admit convenient recursive factorizations that often allow explicit maximum likelihood estimates and that are well suited to building bayesian networks for expert systems whereas the undirected graph associated with a dependence model is uniquely determined there may however be many adgs that determine the same dependence markov model thus the family of all adgs with a given set of vertices is naturally partitioned into markov equivalence classes each class being associated with a unique statistical model statistical procedures such as model selection or model averaging that fail to take into account these equivalence classes may incur substantial computational or other inefficiencies here it is shown that each markov equivalence class is uniquely determined by a single chain graph the essential graph that is itself simultaneously markov equivalent to all adgs in the equivalence class essential graphs are characterized a polynomial time algorithm for their construction is given and their applications to model selection and other statistical questions are described

extended mlse diversity receiver for the time and frequency selective channel this paper develops a maximum likelihood sequence estimation mlse diversity receiver for the time and frequency selective channel corrupted by additive gaussian noise when linear constellations m ask m psk m qam are employed the paper extends ungerboeck s derivation of the extended mlse receiver for the purely frequency selective channel to the more general channel although the new receiver structure and metric assume ideal channel state information csi the receiver can be used wherever high quality csi is available such as a comb of pilot tones or time isolated symbols the major contributions of this paper are as follows 1 the derivation of a finite complexity diversity receiver that is maximum likelihood ml for all linear channel models and sources of diversity as long as ideal csi is available 2 a benchmark in that the new receiver s performance is a lower bound on the performance of practical systems which either lack ideal csi or are not ml 3 insight into matched filtering and ml diversity receiver processing for the time and frequency selective channels and 4 bounds on the new receiver s bit error rate ber for ideal csi and pilot tone csi in a fast rayleigh fading channel with multiple independently faded paths the new receiver can seamlessly tolerate square root nyquist pulses without a fading induced isi error floor

gallager bounds for noncoherent decoders in fading channels recently gallager s bounding techniques have been used to derive tight performance bounds for coded systems in fading channels most works in this field have thus far dealt with coherent decoding this paper develops gallager bounds for noncoherent systems in fading channels unlike coherent decoding the exact error probability of a noncoherent decoder detector conditioned on the fading coefficients does not admit a closed form expression this difficulty is overcome in this paper by employing the chernoff technique although it weakens the bounds to some extent the chernoff technique enables the derivations of the limit before average lba bound and gallager bounds in closed form for noncoherent fading channels numerical examples show that the proposed bounds are convergent and are tighter than the conventional union bound

characterizing markov equivalence classes for amp chain graph models chain graphs cg adicyclic graphs use undirected and directed edges to represent both structural and associative dependences like acyclic directed graphs adgs the cg associated with a statistical markov model may not be unique so cgs fall into markov equivalence classes which may be superexponentially large leading to unidentifiability and computational inefficiency in model search and selection it is shown here that under the andersson madigan perlman amp interpretation of a cg each markov equivalence class can be uniquely represented by a single distinguished cg the amp essential graph that is itself simultaneously markov equivalent to all cgs in the amp markov equivalence class a complete characterization of amp essential graphs is obtained like the essential graph previously introduced for adgs the amp essential graph will play a fundamental role for inference and model search and selection for amp cg models

performance evaluation of a discrete time geo x g 1 retrial queue with general retrial times we consider a discrete time geo x g 1 retrial queue with general retrial times the system state distribution as well as the orbit size and the system size distributions are obtained in terms of their generating functions these generating functions yield exact expressions for different performance measures the present model is proved to have a stochastic decomposition law hence a measure of the proximity between the distributions of the system size in the present model and the corresponding one without retrials is derived a set of numerical results is presented with a focus on the effect of batch arrivals and general retrial times on the system performance it appears that it is the mean batch size and not the batch size distribution that has the main effect on the system performance moreover increasing the mean batch size is shown to have a noticeable effect on the size of the stability region finally geometric retrial times are shown to have an overall better performance compared with two other distributions

a graphical representation of equivalence classes of amp chain graphs this paper deals with chain graph models under alternative amp interpretation a new representative of an amp markov equivalence class called the largest deflagged graph is proposed the representative is based on revealed internal structure of the amp markov equivalence class more specifically the amp markov equivalence class decomposes into finer strong equivalence classes and there exists a distinguished strong equivalence class among those forming the amp markov equivalence class the largest deflagged graph is the largest chain graph in that distinguished strong equivalence class a composed graphical procedure to get the largest deflagged graph on the basis of any amp markov equivalent chain graph is presented in general the largest deflagged graph differs from the amp essential graph which is another representative of the amp markov equivalence class

the m g 1 retrial queue new descriptors of the customer s behavior we consider queuing systems where customers are not allowed to queue instead of that they make repeated attempts or retrials in order to enter service after some time the performance of telephone systems and communication networks modelled as retrial queues differs from standard waiting lines because typically the retrial group is an invisible queue which cannot be observed as a result the original flow of primary arrivals and the flow of repeated attempts become undistinguished our aim in this paper is to consider some aspects of this problem thus we focus on the main retrial model of m g 1 type and investigate the distribution of the successful and blocked events made by the primary customers and the retrial customers

optimal joint detection estimation in fading channels with polynomial complexity the problem of sequence detection in frequency nonselective time selective fading channels when channel state information csi is not available at the transmitter and receiver is considered in this paper the traditional belief is that exact maximum likelihood sequence detection mlsd of an uncoded sequence over this channel has exponential complexity in the channel coherence time thus for slowly varying channels i e channels having coherence time on the order of the sequence length the complexity appears to be exponential in the sequence length in the first part of this work it is shown that exact mlsd can be computed with only polynomial worst case complexity in the sequence length regardless of the operating signal to noise ratio snr for equal energy signal constellations by establishing a relationship between the aforementioned complexity and the rank of the correlation matrix of the fading process an understanding of how complexity of the optimal mlsd receiver varies as the channel dynamics change is provided in the second part of this paper the problem of decoding turbo like codes in frequency nonselective time selective fading channels without receiver csi is examined using arguments similar to the ones used for the mlsd case it is shown that the exact symbol by symbol soft decision metrics sbssdms implied by the min sum algorithm can be evaluated with polynomial worst case complexity in the sequence length regardless of snr for equal energy signal constellations finally by simplifying some key steps in the polynomial complexity algorithm a family of fast approximate algorithms is derived which yield near optimal performance

decision feedback differential detection based on linear prediction for 16dapsk signals transmitted over flat ricean fading channels in this article prediction based decision feedback differential detection df dd for 16 level differentially encoded amplitude phase shift keying is proposed unlike previously reported df dd schemes this scheme provides a performance gain over conventional differential detection under general ricean fading conditions a further important advantage of the novel scheme is that it is able to compensate a small carrier frequency offset the linear predictor coefficients may be updated using the recursive least squares algorithm which can start blind i e without a priori knowledge about the channel statistics and without a training sequence this makes the scheme attractive for application in mobile communications since the statistics of a nonstationary mobile channel can be tracked

further results on differential space time modulations some novel results on the space time differential modulation schemes described by b m hochwald and w sweldens see ibid vol 48 p 2041 52 2000 and v tarokh and h jafarkhani ieee j select areas commun vol 18 p 1169 74 2000 are derived under the assumption of a quasi static multiple antenna channel first the optimality in the maximum likelihood sense of the one shot detection algorithms of hochwald and sweldens and of tarokh and jafarkhani is proved then the expression of the pairwise error probability for these detectors is derived and as a particular case the bit error rate is computed for the binary phase shift keyed scheme of tarokh and jafarkhani finally a per survivor processing psp detection algorithm for this class of modulations is illustrated numerical results evidence both the superiority of the psp strategy over the one shot space time differential detectors and the accuracy of the above mentioned bit error rate formula

maximum likelihood synchronization equalization and sequence estimation for unknown time varying frequency selective rician channels this paper develops a receiver structure to perform jointly maximum likelihood ml synchronization equalization and detection of a linearly modulated signal transmitted over a time varying frequency selective rician faded channel corrupted by additive white gaussian noise awgn the receiver is particularly suited to a fast fading channel where other receivers that rely on estimating the channel cannot track it quickly enough the signal mean and autocovariance are needed and a scheme is proposed for estimating these quantities adaptively the receiver processes the specular and diffuse components corresponding to the signal mean and autocovariance separately processing the known specular component is the classical detection problem the unknown diffuse component is processed by predictors we show that the predictors can achieve synchronization in a novel manner if synchronization is required a union bound on the receiver s bit error rate ber is derived and it tightly bounds simulated bers in fast fading at high signal to noise ratios snrs

convergence of a ml parameter estimation algorithm for ds ss systems in time varying channels with strong interference an unbiased maximum likelihood ml channel parameter estimation algorithm for direct sequence spread spectrum systems with strong interference is discussed in this paper the algorithm includes correcting terms to the extended kalman filter ekf based on the gradient of the negative log likelihood function of the output of a conventional matched filter by an asymptotic analysis the algorithm is shown to determine the actual parameters a complete implementation of the algorithm is given and its transient behavior is examined by computer simulations results show the ml algorithm albeit optimal in the sense of unbiased parameter estimation is less robust than the modified ekf described in the first reference

the m m c retrial queue with geometric loss and feedback abstract we consider m m c retrial queues with geometric loss and feedback when c 1 2 we find the joint generating function of the number of busy servers and the queue length by solving kummer differential equation for c 1 and by the method of series solution for c 1 2

improving service by informing customers about anticipated delays this paper investigates the effect upon performance in a service system such as a telephone call center of giving waiting customers state information in particular the paper studies two m m s r queueing models with balking and reneging for simplicity it is assumed that each customer is willing to wait a fixed time before beginning service however customers differ so the delay tolerances for successive customers are random in particular it is assumed that the delay tolerance of each customer is zero with probability and is exponentially distributed with mean 1 conditional on the delay tolerance being positive let n be the number of customers found by an arrival in model 1 no state information is provided so that if n s the customer balks with probability if the customer enters the system he reneges after an exponentially distributed time with mean 1 if he has not begun service by that time in model 2 if n s k s then the customer is told the system state k and the remaining service times of all customers in the system so that he balks with probability 1 1 qk where qk p t sk t is exponentially distributed with mean 1 sk is the sum of k 1 independent exponential random variables each with mean s 1 and 1 is the mean service time in model 2 all reneging is replaced by balking the number of customers in the system for model 1 is shown to be larger than that for model 2 in the likelihood ratio stochastic ordering thus customers are more likely to be blocked in model 1 and are more likely to be served without waiting in model 2 algorithms are also developed for computing important performance measures in these and more general birth and death models

mlse for correlated diversity sources and unknown time varying frequency selective rayleigh fading channels yu and pasupathy see ibid vol 43 p 1534 44 1995 derived a maximum likelihood sequence estimation mlse receiver structure for unknown time varying frequency selective rayleigh fading channels and uncorrelated diversity sources this receiver design is extended to the case of correlated diversity sources correlated diversity sources typically arise with space diversity where constraints on antenna volume require that diversity antennae be placed too closely together analytic and simulated bit error rate ber curves are presented for receivers which exploit and ignore the correlation in the former case we find a small ber improvement that reduces with decreasing correlation however for a fixed receiver complexity superior performance is achieved when the correlation is ignored

maximum likelihood sequence estimation of cpm signals transmitted over rayleigh flat fading channels a method for the sequential updating of log likelihood functions for maximum likelihood sequence estimation is presented it is shown that in a general case this method can be implemented using kalman filtering techniques for the special case of rayleigh flat fading and continuous phase modulation cpm signaling this approach is shown to lead to an attractive receiver structure this type of receiver called the linear predictive receiver can be implemented in the form of the viterbi algorithm with the trellis updates being computed using a bank of finite pulse response fir filter and square operations simulation results are presented that demonstrate the superiority of the linear predictive receiver over receivers employing differential detection in the presence of fast fading it is shown that the optimal linear predictive receiver does not possess an irreducible error rate for a class of rayleigh fading channels used to model terrestrial mobile channels

a single server batch arrival queue with returning customers we consider a new class of batch arrival retrial queues by contrast to standard batch arrival retrial queues we assume if a batch of primary customers arrives into the system and the server is free then one of the customers starts to be served and the others join the queue and then are served according to some discipline with the help of lyapunov functions we have obtained a necessary and sufficient condition for ergodicity of embedded markov chain and the joint distribution of the number of customers in the queue and the number of customers in the orbit in steady state we also have suggested an approximate method of analysis based on the corresponding model with losses

multiple symbol differential sphere decoding in multiple symbol differential detection msdd for power efficient transmission over rayleigh fading channels without channel state information blocks of n received symbols are jointly processed to decide on n 1 data symbols the search space for the maximum likelihood ml estimate is therefore complex n 1 dimensional and maximum likelihood msdd ml msdd quickly becomes computationally intractable as n grows mackenthun s low complexity msdd algorithm finds the ml estimate only for rayleigh fading channels that are time invariant over an n symbol period for the general time varying fading case however low complexity ml msdd is an unsolved problem in this letter we solve this problem by applying sphere decoding sd to ml msdd for time varying rayleigh fading channels the resulting technique is referred to as multiple symbol differential sphere decoding msdsd

on retrial queueing model with fuzzy parameters abstract this work constructs the membership functions of the system characteristics of a retrial queueing model with fuzzy customer arrival retrial and service rates the cut approach is used to transform a fuzzy retrial queue into a family of conventional crisp retrial queues in this context by means of the membership functions of the system characteristics a set of parametric non linear programs is developed to describe the family of crisp retrial queues a numerical example is solved successfully to illustrate the validity of the proposed approach because the system characteristics are expressed and governed by the membership functions more information is provided for use by management by extending this model to the fuzzy environment fuzzy retrial queue is represented more accurately and analytic results are more useful for system designers and practitioners

adaptive mlse for dpsk in time and frequency selective channels an adaptive equalizer structure based on a state space formulation and maximum likelihood sequence estimation mlse is developed for a time varying frequency selective channel differential phase shift keying dpsk is assumed at the transmitter and effective differential decoding is performed at the receiver the standard models of a time varying linear channel and the karhunen loeve kl expansion underpin the receiver structure analytical and simulation results for the receiver are shown the resulting receiver is a per survivor structure

a robust receiver structure for time varying frequency flat rayleigh fading channels a receiver structure for unknown time varying frequency flat rayleigh fading channels is proposed unlike the maximum likelihood receiver structure this receiver does not need any knowledge of the channel autocovariance or the received signal to noise ratio and yet analytic and simulation results show that its bit error rate performance is close to the optimal structure the receiver is applicable to signals with constant or approximately constant envelopes or to linearly modulated signals using nyquist pulses

analysis of an m dn 1 retrial queue we consider a single server retrial queueing system in which each customer primary or retrial customer has discrete service times taking on value d j with probability p j j 1 2 and j 1 p j 1 an arriving primary customer who finds the server busy tries later moreover each retrial customer has its own orbit and the retrial customers try to enter the service independently of each other we call this retrial queue an m d n 1 retrial queue a necessary and sufficient condition for this system stability is given in the steady state we derive the joint distribution of the state of the server and the number of customers in the retrial orbits the explicit expressions of some performance measures are given in addition the steady state distribution of the waiting time is discussed

on the impact of customer balking impatience and retrials in telecommunication systems this paper deals with a markovian multiserver model where the simultaneous effect of customer balking impatience and retrials is evaluated the study of systems incorporating these features is motivated by the observation of real service systems where these queueing phenomena interact simple approximations based on truncation and generalized truncation are provided these approximations are compared according to different criteria for several selected scenarios finally our results are used to evaluate the optimal ratio between the number of available channels and the number of waiting positions in an application to call center management

generalized polynomial based receiver for the flat fading channel this letter presents a simple polynomial predictor based sequence detector for the rayleigh nonselective fading channel unlike the polynomial predictor based sequence detector proposed by borah and hart the new receiver is not restricted to constant envelope modulation schemes analytical and simulated results are presented in some instances the proposed receiver performs within 6 db of the equivalent maximum likelihood sequence estimation receiver

technical note mean drifts and the non ergodicity of markov chains an important question in stochastic modeling is whether or not a denumerable state markov chain is ergodic we extend a result of kaplan giving a condition for the non ergodicity of a chain based on its drifts we also clarify the condition under which the mean drift in the stationary chain is zero

nyquist 3 pulse shaping in continuous phase modulation continuous phase modulation cpm schemes using nyquist 3 pulse shaping and correlative codes are popular candidates for mobile radio applications this paper presents a detailed study of the effects of nyquist 3 pulse shaping on the spectral characteristics of cpm schemes with a modulation index of 1 2 the influence of pulse parameters such as its shape bandwidth coding polynomial and truncation length on spectral characteristics such as main lobe fractional power bandwidths and out of band power levels is thoroughly investigated error performance is also briefly analyzed among uncoded schemes a new scheme using a pulse satisfying both nyquist 3 and nyquist 2 criteria is pointed out as a promising candidate and as a good compromise between spectral and power efficiencies

analysis of the successful and blocked events in the geo geo c retrial queue this paper considers a discrete time queue of geo geo c type with geometric repeated attempts artalejo et al 2008 18 we investigate the probability distributions the first and second moments and the cross moments of the successful and blocked events made by the external and repeated customers several numerical examples and a cost function illustrate the analysis

predictable processes and wold s decomposition a review the concepts of predictability and band limitedness are reexamined and a simple proof of wold s decomposition is presented in the context of mean square estimation

noncoherent sequence detection of differential space time modulation approximate maximum likelihood noncoherent sequence detection nsd for differential space time modulation dstm in time selective fading channels is proposed the starting point is the optimum multiple symbol differential detection for dstm that is characterized by exponential complexity by truncating the memory of the incremental metric a finite state trellis is obtained so that a viterbi algorithm can be implemented to perform sequence detection compared to existing linear predictive receivers a distinguished feature of nsd is that it can accommodate nondiagonal constellations in continuous fading error analysis demonstrates that significant improvement in performance is achievable over linear prediction receivers by incorporating the reduced state sequence detection techniques performance and complexity tradeoffs can be controlled by the branch memory and trellis size numerical results show that most of the performance gain can be achieved by using an l state trellis where l is the size of the dstm constellation

an adaptive maximum likelihood receiver for correlated rayleigh fading channels the paper develops a receiver structure for random gaussian signals in additive noise based on the classic maximum likelihood m l estimator correlator derivation of kailath 1960 and applies it to differential phase shift keying dpsk on the correlated rayleigh fading channel it is shown to lower the error floor found in the performance of conventional dpsk receivers by orders of magnitude in addition the maximum likelihood procedure is shown to make uncorrelated symbol decisions the performance of both conventional and optimal receivers which require knowledge of the channel statistics is examined analytically a recursive channel adaptive version of the optimal receiver utilizing decision feedback to estimate the channel statistics is developed its simulated performance shows no penalty compared to theoretical calculations which require explicit knowledge of the channel statistics

stochastic decompositions in the m g 1 queue with generalized vacations this paper considers a class of m g 1 queueing models with a server who is unavailable for occasional intervals of time as has been noted by other researchers for several specific models of this type the stationary number of customers present in the system at a random point in time is distributed as the sum of two or more independent random variables one of which is the stationary number of customers present in the standard m g 1 queue i e the server is always available at a random point in time in this paper we demonstrate that this type of decomposition holds in fact for a very general class of m g 1 queueing models the arguments employed are both direct and intuitive in the course of this work moreover we obtain two new results that can lead to remarkable simplifications when solving complex m g 1 queueing models

geo g 1 discrete time retrial queue with bernoulli schedule abstract this paper studies discrete time geo g 1 retrial queues with bernoulli schedule in which the blocked customers either join the infinite waiting space with probability or leave the server and enter the retrial orbit with probability 1 the customers in the retrial orbit will retry their service after a random amount of time first the analytic formula for the generating function of the joint distribution of the numbers of customers in the waiting space and the retrial orbit in steady state is derived it is shown that a stochastic decomposition law holds for the retrial queues under study that is the total number of customers in system is distributed as a sum of two independent random variables second recursive formulas for the marginal steady state probabilities of the numbers of customers in the waiting space and in the retrial orbit was developed since a regular two level priority geo g 1 queue one without retrials with bernoulli schedule and head of line priority discipline is a special case of the studied retrial systems the recursive formulas developed can be used to compute the marginal steady state probabilities of numbers of customers in the priority and non priority groups for this case furthermore a relationship between a continuous time m g 1 retrial queue with bernoulli schedule and its discrete time counterpart is established so that the recursive formulas can also be applied to a continuous time system last several special cases are studied and some numerical examples are presented to demonstrate the use of the recursive formulas

detection by multiple trellises in this paper we present a novel pragmatic approach referred to as detection by multiple trellises to perform trellis based detection over realistic channels more precisely we consider channels with unknown parameters and apply the concept of detection by multiple trellises to forward backward fb algorithms the key idea of our approach consists first of properly quantizing the channel parameters and then considering replication of coherent fb algorithms operating on parallel trellises one per hypothetical quantized value in order to make the receiver robust against a possibly time varying channel parameters the proposed soft output algorithms perform a proper manipulation of the forward and backward metrics computed by the parallel fb algorithms at regularly spaced trellis steps we consider two significant examples of application detection over i phase uncertain channels and ii fading channels the performance of the proposed algorithms is investigated considering differentially encoded de quaternary phase shift keying qpsk and iterative detection schemes based on low density parity check ldpc codes besides having a low complexity the proposed soft output algorithms turn out to be robust flexible blind in the sense that no knowledge of the channel parameter statistics is required and highly parallelizable as it is desirable in high throughput future wireless communication systems

discrete time geo x gh 1 retrial queue with bernoulli feedback hreceive a service of type h h 1 h with probability qh where h l qh 1 we study the markov chain underlying the considered queueing system and the ergodicity condition too we find the generating function of the number of calls in the orbit and in the system we derive a stochastic decomposition law and as an application we give bounds for the proximity between the steady state distributions for our queueing system and its corresponding standard system in the special case of individual arrivals we develop recursive formulae for calculating the steady state distribution of the orbit size besides we prove that the mix gig 1 retrial queue with bernoulli feedback can be approximated by our corresponding discrete time system finally we give numerical examples to illustrate the effect of the parameters on several performance characteristics 2004 elsevier ltd all rights reserved

a blind particle filtering detector of signals transmitted over flat fading channels a new particle filtering detector pfd is proposed for blind signal detection over flat rayleigh fading channels whose model coefficients are unknown the detector employs a hybrid importance function and a mixture kalman filter it also incorporates an auxiliary particle filtering strategy with a smoothing kernel in the resampling step further by considering practical information of communication systems and the physical interpretation of the adopted second order autoregressive ar channel model a fully blind particle filtering implementation is developed the structure of the proposed pfd can be easily adapted to other system requirements simulations are provided that demonstrate the performance of the new pfd

particle filtering for demodulation in fading channels with non gaussian additive noise an efficient particle filtering algorithm is developed to solve the problem of demodulation of m ary modulated signals under conditions of fading channels in the presence of non gaussian additive noise simulations for mdpsk signals are presented the results show that the algorithm outperforms the current methods

diversity combining for ds ss systems with time varying correlated fading branches diversity combining of multiple time varying and correlated fading branches is investigated for direct sequence spread spectrum systems the correlated branches are modeled and estimated jointly as a vector autoregressive var process the joint estimation is shown to provide performance gain over separate estimation of the fading branches the parameter matrices of the var model are estimated via the method of expectation maximization em with two algorithms the first algorithm using results from kalman smoothing provides a closed form solution to the maximization problem in the iterative em procedure however the iterative em kalman algorithm operates repeatedly on a batch of training data and results in large storage requirements and long processing delays to overcome these disadvantages a new algorithm with only forward time recursions is proposed that approximates the iterative em solution and efficiently adapts to slowly changing doppler spreads as a result the new algorithm significantly reduces memory and training sequence requirements through computer simulations a near ideal bit error rate performance is found for both algorithms and the efficacy of the new adaptive algorithm for channels with changing doppler spreads is demonstrated