minimum probability of error for asynchronous gaussian multiple access channels consider a gaussian multiple access channel shared by k users who transmit asynchronously independent data streams by modulating a set of assigned signal waveforms the uncoded probability of error achievable by optimum multiuser detectors is investigated it is shown that the k user maximum likelihood sequence detector consists of a bank of single user matched filters followed by a viterbi algorithm whose complexity per binary decision is o 2 k the upper bound analysis of this detector follows an approach based on the decomposition of error sequences the issues of convergence and tightness of the bounds are examined and it is shown that the minimum multiuser error probability is equivalent in the iow noise region to that of a single user system with reduced power these results show that the proposed multiuser detectors afford important performance gains over conventional single user systems in which the signal constellation carries the entire burden of complexity required to achieve a given performance level

performance of constrained and unconstrained adaptive multiuser detectors for quasi synchronous cdma it has previously been shown that multiuser interference in a quasi synchronous code division multiple access qs cdma system can be effectively rejected using a nonadaptive decorrelator however the qs decorrelator is orthogonal to the inflated subspace spanned by the undesired users with arbitrary delays in the qs uncertainty interval and hence does not perform as well as a decorrelator with a priori knowledge of the delays here we consider a constrained adaptive linear suppression filter which implicitly estimates the delays of the qs undesired signals and thus approaches the ideal decorrelator of lupas and verdu 1990 for qs cdma it is shown through analysis and simulations that the constrained recursive least squares c rls algorithm converges faster than an unconstrained rls adaptive filter and generally outperforms the nonadaptive qs decorrelator a cellular quasi synchronous system is discussed in the present paper

a chip based multiuser detector for the downlink of a ds cdma system using a folded state transition trellis the maximum likelihood sequence estimation mlse multiuser detector has very good performance but at the expense of high computational complexity the viterbi algorithm is employed to implement the mlse in the asynchronous uplink in this scheme the various bit combinations are taken to be the different states in the decoding trellis however the number of states increases exponentially with the number of active users we propose a scheme that exploits the feature of power equality in the downlink to reduce the number of states involved each transmitted chip value can only take a certain number of possible discrete values using these discrete values as the states in the transition trellis diagram and operating the viterbi algorithm at the chip level the number of states is reduced from 4 sup k to k 1 sup l where k is the number of users and l is the number of paths in the multipath channel

blind multiuser detection from moe to subspace methods the minimum output energy moe multiuser receiver has been shown to approach the minimum mean square error mmse receiver at high signal to noise ratio snr however performance degradation is incurred by noise induced channel estimation error in this paper we propose a power of r por technique to significantly improve the performance of the moe receiver it is shown that the new receiver asymptotically converges to the mmse receiver without performance penalty the convergence is established either under high snr with large exponent raised in the power of the covariance matrix or with sufficiently large number of data samples connection between our por method and a widely studied subspace method is investigated from the respective optimization criteria asymptotic equivalence between these two methods is also established extensive simulations based on finite data samples show that the proposed method significantly outperforms the subspace method in systems with medium to heavy loading severe multipath distortion or smaller processing gain moreover adaptive implementation of the proposed method exhibits very robust performance in a dynamic loading environment

near optimal limited search detection on isi cdma channels and decoding of long convolutional codes we derive an upper bound on the bit error probability bep in limited search detection over a finite interference channel a unified channel model is presented this includes finite length intersymbol interference channels and multiuser cdma channels as two special cases we show that the bep of the m algorithm ma is bounded from above by the sum of three terms an upper bound on the error probability of the viterbi 1967 algorithm va detection given by forney jr 1972 and upper bounds on the error probabilities of two types of erroneous decision caused by the correct path loss event we prove that error propagation in terms of the mean recovery step number is finite for all finite interference channels the convergence and asymptotic behavior of the upper bounds are studied the results show that if a channel satisfies certain mild conditions all series in the bounds are convergent one of the key results is that for any finite interference channel satisfying certain mild conditions the asymptotic bep of the ma is bounded by the same upper and lower bounds which have the same asymptotical behavior as those for the va if the correct path loss probability is smaller than that of the va furthermore we extend the above results to near optimally decode long convolutional codes in a short packet format about 200 300 bits we present a nonsorting combined m t algorithm and showed that the m t algorithm with m 2 sup d free sup n and t d sub free e sub b n can near optimally decode the code we also propose a hierarchical decoding algorithm hda to further cut down the average decoding complexity numerical results show that the bounds are reasonably tight the hda can achieve a performance within about 0 8 db of the sphere packing lower bound for a packet error rate of 10 sup 4 and a packet length below 200 bits which is the best reported decoding performance so far for block sizes from 100 to 200 bits

optimum asymptotic multiuser efficiency of randomly spread cdma this correspondence analyzes the high signal to noise ratio snr performance of optimum multiuser detectors for synchronous direct sequence spread spectrum with random spreading in an additive white gaussian noise channel under very general conditions on the received powers we show that the optimum asymptotic efficiency of a k user system with spreading gain n converges to 1 almost surely as k spl rarr spl infin and k n is kept equal to an arbitrary nonzero constant therefore the asymptotic behavior of the minimum bit error rate is equivalent to that of a single user system

asymptotic multiuser efficiencies for decision directed multiuser detectors the asymptotic multiuser efficiencies ames are derived for various classes of decision directed multiuser detectors including multistage detectors and decision feedback detectors novel classes of soft decision multistage detectors are proposed and analyzed each class is specified in part by a soft decision nonlinearity such as a symmetric quantizer or a linear clipper closed form expressions for two user ames are derived for soft decision two stage detectors and can be used as a design criterion to optimize the soft decision nonlinearities for a special case of two synchronous users the soft decision two stage detector using an optimized linear clipper with either conventional or decorrelated tentative decisions is shown to achieve optimum ame upper and lower bounds on the ame are obtained for decision feedback detectors using either conventional or decorrelated tentative decisions it is demonstrated that decision directed multiuser detectors with conventional tentative decisions have low near far resistance compared to those with decorrelated tentative decisions

indecomposable error sequences in multiuser detection in this paper we provide a graph based characterization of the set of indecomposable sequences that are useful in the computation of an upper bound to error probability of maximum likelihood ml detection in code division multiple access cdma multiuser systems we apply this characterization to a k user symmetric system and to a two user two rate system it leads to a precise calculation of the bound which is then compared to the performance of a decorrelator

breadth first maximum likelihood detection in multiuser cdma in this letter we derive a recursive additive metric for complexity constrained maximum likelihood detection for multiuser cdma using breadth first detection algorithms the metric requires linear filtering of the matched filtered received signal vector it is shown that a class of filters fulfilling certain requirements lead to identical performance

error probability for direct sequence spread spectrum multiple access communications part i upper and lower bounds upper and lower bounds on the average probability of error are obtained for direct sequence spread spectrum multipleaccess communications systems with additive white gaussian noise channels the bounds which are developed from convexity properties of the error probability function are valid for systems in which the maximum multiple access interference does not exceed the desired signal and the signature sequence period is equal to the duration of the data pulse the tightness of the bounds is examined for systems with a small number of simultaneously active transmitters this is accomplished by comparisons of the upper and lower bounds for several values of the system parameters the bounds are also compared with an approximation based on the signal to noise ratio and with the chernoff upper bound

subspace based channel estimation for code division multiple access communication systems we consider the estimation of channel parameters for code division multiple access cdma communication systems operating over channels with either single or multiple propagation paths the multiuser channel estimation problem is decomposed into a series of single user problems through a subspace based approach by exploiting the eigenstructure of the received signal s sample correlation matrix the observation space can be partitioned into a signal subspace and a noise subspace without prior knowledge of the unknown parameters the channel estimate is formed by projecting a given user s spreading waveform into the estimated noise subspace and then either minimizing the likelihood or minimizing the euclidean norm of this projection both of these approaches yield algorithms which are near far resistant and do not require a preamble

indirectly estimated adaptive detectors for code division multiple access signals this paper presents the development of indirectly estimated adaptive idea detectors for code division multiple access cdma signals in wireless communication systems multiuser cdma detection is formulated as an inverse problem in the presence of noise and the estimation of the detector parameters is accomplished using elements of regularization theory the development of autonomous cdma detectors that are capable of adjusting to the channel characteristics with the support of training sequences relies on data dependent procedures designed to reduce the effect of noise and interfering users on the performance of the detector the detector parameters can be computed by adaptive algorithms that require no matrix inversion the proposed idea detector is evaluated and compared with existing multiuser cdma detectors in terms of their robustness to noise resistance to the near far problem and ability to handle multipath signals

on minimal eigenvalues of a class of tridiagonal matrices it is known that the worst case near far resistance of optimum multiuser detectors for asynchronous gaussian multiple access channels can be expressed in terms of a class of block tridiagonal matrices and the minimal eigenvalues of such a class of block tridiagonal matrices serve as a good measure of the worst case near far resistance in this paper we focus on the two user scenario where each block tridiagonal matrix under consideration is a tridiagonal matrix we derive closed form expressions for the minimal eigenvalues of such a class of tridiagonal matrices in terms of the largest real solution of a trigonometric equation in 0 pi we also obtain lower bounds and upper bounds on the minimal eigenvalues which improve on previously known results in the literature

characterization on practical photon counting receiver in optical scattering communication we characterize the practical photon counting receiver in optical scattering communication with a finite sampling rate and electrical noise finite small pulse width incurs dead time effect that may lead to sub poisson distribution on the recorded pulses we analyze the first order and second order moments on the number of recorded pulses with a finite sampling rate at the receiver side under two cases where the sampling period is shorter than or equal to the pulse width as well as longer than the pulse width moreover we adopt the maximum likelihood ml detection in order to simplify the analysis we adopt binomial distribution approximation on the number of recorded pulses in each slot a tractable holding time and decision threshold selection rule is provided to maximize the minimal kullback leibler kl distance between the two distributions the performances of proposed sub poisson distribution and the binomial approximation are verified by the experimental results the equivalent photon arrival rate and pulse holding time predicted by the sub poisson model and the associated proposed binomial distribution are well validated by the simulation results where the finite sampling rate and electrical noise are considered the proposed holding time and decision threshold selection rule performs close to the optimal ml receiver

multiuser detection in single path fading channels the performance analysis of maximum likelihood sequence and decorrelating multiuser detectors is developed for the single path rayleigh fading channel the received signal in this multipoint to point channel is additively composed of white gaussian noise and independently faded waveforms from a finite number of asynchronous users the optimum sequence detector consists of a bank of single user matched filters followed by a dynamic programming algorithm has a demodulation complexity per symbol which is exponential in the number of users and requires the knowledge of the system as well of the fading parameters the decorrelating detector has a complexity which is polynomial in the number of users and does not require knowledge of the signal amplitudes which are assumed to vary slowly relative to the symbol rate the single user error rates are presented for both detectors and compared to conventional detection and isolated transmission and the notion of asymptotic multiuser efficiency is developed for this channel in contrast to conventional detection which suffers from near far effects due to rayleigh fading it is shown that the aforementioned multiuser detectors exhibit performance which rivals that of isolated transmission for sufficiently small thermal noise power the results of the analysis are illustrated with numerical examples and indicate that the near far problem may be alleviated by a more sophisticated receiver design

gradient estimation for sensitivity analysis and adaptive multiuser interference rejection in code division multiple access systems in this paper we consider a direct sequence code division multiple access ds cdma system in the framework of a discrete event dynamic system deds in order to optimize the system performance based on this formulation we develop infinitesimal perturbation analysis ipa for estimating the sensitivity of the average probability of bit error to factors ranging from near far effects to imperfections in power control the above estimates are shown to be unbiased and this technique is then further incorporated into a stochastic gradient algorithm for achieving adaptive multiuser interference rejection for such systems which is also subject to frequency nonselective slow fading we use an ipa based stochastic training algorithm for developing an adaptive linear detector with the average probability of error being the minimization criterion we also develop a practical implementation of such an adaptive detector where we use a joint estimation detection algorithm for minimizing the average probability of bit error a sequential implementation that does not require a stochastic training sequence or a preamble is also developed

the bit error performance and information transfer rate of spad array optical receivers in this paper the photon counting characteristics the information rate and the bit error performance of single photon avalanche diode spad arrays are investigated it is shown that for sufficiently large arrays the photocount distribution is well approximated by a gaussian distribution with dead time dependent mean and variance because of dead time the spad array channel is subject to counting losses part of which are due to inter slot interference isi distortions consequently this channel has memory the information rate of this channel is assessed two auxiliary discrete memoryless channels dmcs are proposed which provide upper and lower bounds on the spad array information rate it is shown that in sufficiently large arrays isi is negligible and the bounds are tight under such conditions the spad array channel is precisely modelled as a memoryless channel a discrete time gaussian channel with input dependent mean and variance is adopted and the properties of the capacity achieving input distributions are studied using a numerical algorithm the information rate and the capacity achieving input distributions subject to peak and average power constraints are obtained furthermore the bit error performance of a spad based system with on off keying ook is evaluated for various array sizes dead times and background count levels

capacity achieving probability measure for conditionally gaussian channels with bounded inputs a conditionally gaussian channel is a vector channel in which the channel output given the channel input has a gaussian distribution with well behaved input dependent mean and covariance we study the capacity achieving probability measure for conditionally gaussian channels subject to bounded input constraints and average cost constraints many practical communication systems including additive gaussian noise channels certain optical channels fading channels and interference channels fall within this framework subject to bounded input constraint and average cost constraints we show that the channel capacity is achievable and we derive a necessary and sufficient condition for a probability measure to be capacity achieving under certain conditions the capacity achieving measure is proved to be discrete

a block alternating likelihood maximization approach to multiuser detection in this paper we address the maximum likelihood ml multiuser detection problem for asynchronous code division multiple access cdma systems with multiple receiver antennas in frequency selective fading environments multiuser ml detection mld in this case provides attractive symbol error performance but it requires the solution of a large scale combinatorial optimization problem to deal with the computational complexity of this problem we propose an efficient approximation method based on a block alternating likelihood maximization balm principle the idea behind balm is to decompose the large scale mld problem into smaller subproblems assuming binary or quaternary phase shift keying bpsk or qpsk which are often employed in cdma the combinatorial subproblems are then accurately and efficiently approximated by the semidefinite relaxation sdr algorithm an algorithm that has been recently shown to lead to quasi ml performance in synchronous cdma scenarios simulation results indicate that this balm detector provides close to optimal bit error rate ber performance the balm principle is quite flexible and we demonstrate this flexibility by extending balm to multicarrier mc multiuser systems by exploiting the special signal correlation structure of mc systems we develop a variation of balm in which dynamic programming dp is used to solve the subproblems it is shown using simulations that the ber performance of this dp based balm detector is as promising as that of the sdr based balm detector

joint signal detection and parameter estimation in multiuser communications the problem of simultaneously detecting the information bits and estimating signal amplitudes and phases in a k user asynchronous direct sequence spread spectrum multiple access communication system is addressed the joint maximum likelihood ml estimator has a computational complexity that is exponential in the total number of bits transmitted and thus does not represent a practical solution to the problem an estimator that combines a suboptimum tree search algorithm with a recursive least squares estimator of complex signal amplitude is considered the complexity of this estimator is o k sup 2 computations per decoded bit and its performance is very close to that of the joint ml receiver this receiver has the advantage that the transmitted signal powers and phases are extracted from the received signal in an adaptive fashion without using a test sequence

nonlinear group blind multiuser detection a nonlinear group blind technique is developed for joint detection of some given users data in a cdma uplink environment with the presence of unknown interference this method performs the so called slowest descent search over a likelihood function of the desired users starting from the estimate closest to the unconstrained maximizer of the likelihood function and along mutually orthogonal directions where this likelihood function drops to the slowest simulation results show that this new nonlinear technique offers substantial performance improvement over the previously proposed linear group blind multiuser detectors with little attendant increase in computational complexity the problem of group blind multiuser detection in the presence of both unknown interference and impulsive ambient noise is also treated under the framework of slowest descent search with the aid of a novel subspace based robust interference cancellation scheme it is seen that this robust group blind method significantly outperforms the robust blind multiuser detection scheme proposed previously

performance bounds for optimum multiuser ds cdma systems in this letter we estimate the bit error probability bep of optimum multiuser detection for synchronous and asynchronous code division multiple access cdma systems on gaussian and fading channels we first compute an upper bound and a lower bound on the bit error probability for a given spreading code then average the bounds over a few thousand sets of spreading codes these bounds are obtained from a partial distance spectrum on gaussian channels the upper bound converges to the lower bound at moderate to large signal to noise ratios however on fading channels the upper bound does not converge hence we present our results for the lower bound only the numerical results show that 1 the bep of a 31 user cdma system with binary random spreading codes of length 31 is only two to four times higher than the bep of the single user system 2 the number of users that can be accommodated in an asynchronous cdma system is larger than the processing gain and 3 optimum multiuser detection outperforms linear detection e g the decorrelating detector by about 2 8 to 5 7 db

near optimum tree search detection schemes for bit synchronous multiuser cdma systems over gaussian and two path rayleigh fading channels simple reduced tree search detection schemes of the breadth first type are applied to suboptimal joint multiuser detection in bit synchronous code division multiple access cdma systems over both gaussian and two path rayleigh fading channels it is pointed out that in contrast to the case of the optimal multiuser detector the choice of the receiver filter severely influences the performance of suboptimal multiuser detectors simulation results supported by analysis show that breadth first tree search algorithms using a decorrelating noise whitening receiver filter perform better than similar receivers which solely use a matched filter mf for virtually all nonsingular spreading code sets studied most of the code sets are randomly generated the m and t algorithm detectors based on decorrelating noise whitening filter wf outputs can achieve near optimum performance at a very low complexity compared to the optimal detector although the proposed detectors are more complex than some known suboptimum detectors furthermore the use of combining techniques is considered for a two path rayleigh fading channel and a semi synchronous cdma structure is proposed it is shown that if maximum ratio combining mrc is employed the decorrelating noise wf still exists the corresponding suboptimal combining detector with a decorrelating noise wf outperforms a similar noncombining detector

periodic and homoclinic solutions of extended fisher kolmogorov equations in this paper we study the existence of periodic solutions of the fourth order equations uiv pu a x u b x u3 0 and uiv pu a x u b x u3 0 where p is a positive constant and a x and b x are continuous positive 2l periodic functions the boundary value problems p1 and p2 for these equations are considered respectively with the boundary conditions u 0 u l u 0 u l 0 existence of nontrivial solutions for p1 is proved using a minimization theorem and a multiplicity result using clark s theorem existence of nontrivial solutions for p2 is proved using the symmetric mountain pass theorem we study also the homoclinic solutions for the fourth order equation uiv pu a x u b x u2 c x u3 0 where p is a constant and a x b x and c x are periodic functions the mountain pass theorem of brezis and nirenberg and concentration compactness arguments are used

optimum multiuser asymptotic efficiency the degradation in bit error rate due to the presence of multiple access interference in a white gaussian channel can be measured by the multiuser asymptotic efficiency defined as the ratio between the snr required to achieve the same uncoded bit error rate in the absence of interfering users and the actual snr in this paper the asymptotic efficiency of the optimum multiuser demodulator a bank of matched filters followed by a viterbi algorithm is investigated and compared to that of the conventional single user matched filter receiver the computation of the optimum asymptotic efficiency of any given user is equivalent to the minimization of the euclidean distance between any pair of multiuser signals which differ in at least one of the symbols of that user it is shown that the optimum multiuser efficiency of asynchronous systems is nonzero with probability 1 and therefore the optimum demodulator does not become multiple access limited in contrast to the single user receiver a class of signal constellations with moderate cross correlation requirements is shown to achieve unit optimum multiuser efficiencies and hence to be equivalent to orthogonal signal sets from the viewpoint of performance of the optimum multiuser detector

reduced complexity time domain equalization for turbo mimo systems a reduced complexity time domain equalization scheme for wideband turbo multiple input multiple output turbo mimo systems is presented this scheme called iterative trellis search equalization is based on a modified version of the m bahl cocke jelinek raviv m bcjr algorithm applied to a suitably chosen trellis representation of the wideband mimo channel process exploiting the properties of quadrature amplitude modulation qam signal constellations with block partitionable labels this modified m bcjr algorithm has complexity per bit that is independent of the constellation size and polynomial in the number of transmit antennas and channel memory results from computer simulations show that the new scheme successfully mitigates intersymbol interference even if only a very small fraction of trellis state transitions is considered it is also demonstrated that asynchronous transmission of the spatially multiplexed symbol streams can result in considerable performance improvement compared to synchronous mimo systems

error probability for direct sequence spread spectrum multiple access communications part ii approximations approximations are obtained for the average probability of error in an asynchronous direct sequence spread spectrum multiple access communications system both binary and quaternary systems are considered and the chip waveforms are allowed to be arbitrary time limited waveforms with time duration equal to the inverse of the chip rate the approximation is based on the integration of the characteristic function of the multiple access interference the amount of computation required to evaluate this approximation grows only linearly with the product of the number of simultaneous transmitters and the number of chips per bit the accuracy of the approximation is extremely good in most cases but it can be improved if necessary by an application of a series expansion numerical results are presented for specific chip waveforms and signature sequences

limited search trellis decoding of convolutional codes the least storage and node computation required by a breadth first tree or trellis decoder that corrects t errors over the binary symmetric channels is calculated breadth first decoders work with code paths of the same length without backtracking the viterbi algorithm is an exhaustive trellis decoder of this type other schemes look at a subset of the tree or trellis paths for random tree codes theorems about the asymptotic number of paths required and their depth are proved for concrete convolutional codes the worst case storage for t error sequences is measured in both cases the optimal decoder storage has the same simple dependence on t the m algorithm and algorithms proposed by g j foschini ibid vol it 23 p 605 9 sept 1977 and by s j simmons phd diss queens univ kingston ont canada are optimal or nearly so they are all far more efficient than the viterbi algorithm

linear multiuser detectors for synchronous code division multiple access channels under the assumptions of symbol synchronous transmissions and white gaussian noise the authors analyze the detection mechanism at the receiver comparing different detectors by their bit error rates in the low background noise region and by their worst case behavior in a near far environment where the received energies of the users are not necessarily similar optimum multiuser detection achieves important performance gains over conventional single user detection at the expense of computational complexity that grows exponentially with the number of users it is shown that in the synchronous case the performance achieved by linear multiuser detectors is similar to that of optimum multiuser detection attention is focused on detectors whose linear memoryless transformation is a generalized inverse of the matrix of signature waveform crosscorrelations and on the optimum linear detector it is shown that the generalized inverse detectors exhibit the same degree of near far resistance as the optimum multiuser detectors the optimum linear detector is obtained

a generalized minimum mean output energy strategy for cdma systems with improper mai it has been shown that in a direct sequence code division multiple access ds cdma system employing binary phase shift keying bpsk modulation the baseband equivalent of the cdma multiplex is under very mild assumptions an improper complex random process i e it has a nonzero pseudoautocorrelation function the problem of linear multiuser detection for asynchronous ds cdma systems with improper multiaccess interference mai is considered a new mean output energy moe cost function is introduced whose constrained minimization leads to two new linear multiuser detectors exploiting the information contained in the pseudoautocorrelation of the observables and which generalize the classical decorrelating and minimum mean square error mmse receivers the problem of blind adaptive receiver implementation based on subspace tracking is also tackled finally the superiority of the new detectors with respect to the classical linear detection structures present in the literature is demonstrated through both theoretical considerations and computer simulations

a new approach to greedy multiuser detection we propose a new suboptimum multiuser detector for synchronous and asynchronous multiuser communications in this approach a greedy strategy is used to maximize the cost function the maximum likelihood ml metric the coefficients of the ml metric are utilized as weights indicating in which order bits can be estimated the complexity of the algorithm is approximately k sup 2 log k per bit where k is the number of users we analyze the performance of the greedy multiuser detection in the additive white gaussian noise channel as well as in the frequency nonselective rayleigh fading channel and compare it with the optimum detector and several suboptimum schemes such as conventional successive interference cancellation decorrelator sequential and multistage detectors the proposed greedy approach considerably outperforms these suboptimum schemes especially for moderate and high loads in low and moderate signal to noise ratio regions the results show that when there is a significant imbalance in the values of the coefficients of the ml metric due to moderate to high noise fading and asynchronous transmission near optimum performance is achieved by the greedy detection

quasi maximum likelihood multiuser detection using semi definite relaxation with application to synchronous cdma the maximum likelihood ml multiuser detector is well known to exhibit better bit error rate ber performance than many other multiuser detectors unfortunately ml detection mld is a nondeterministic polynomial time hard np hard problem for which there is no known algorithm that can find the optimal solution with polynomial time complexity in the number of users in this paper a polynomial time approximation method called semi definite sd relaxation is applied to the mld problem with antipodal data transmission sd relaxation is an accurate approximation method for certain np hard problems the sd relaxation ml sdr ml detector is efficient in that its complexity is of the order of k3 5 where k is the number of users we illustrate the potential of the sdr ml detector by showing that some existing detectors such as the decorrelator and the linear minimum mean square error detector can be interpreted as degenerate forms of the sdr ml detector simulation results indicate that the ber performance of the sdr ml detector is better than that of these existing detectors and is close to that of the true ml detector even when the cross correlations between users are strong or the near far effect is significant

iterative tree search detection for mimo wireless systems this paper presents a reduced complexity soft input soft output detection scheme called iterative tree search detection for multiple input multiple output wireless communication systems employing turbo processing at the receiver in this scheme a reduced search space is selected with the aid of the m algorithm and qam signal constellations with block partitionable labels are used in order to make the detection complexity per bit almost independent of the modulation order as well as asymptotically linear in the number of transmit antennas results from computer simulations are presented which demonstrate the capability of the scheme to approach optimal performance at considerably reduced complexity

differentially coherent decorrelating detector for cdma single path time varying rayleigh fading channels this paper describes a differentially coherent decorrelating detector for a k user reverse link code division multiple access environment that exhibits time varying rayleigh fading the channel is modeled as providing only a single fading path for each user and with no additional means to achieve diversity the design of the detector is based on using fractionally sampled matched filter outputs to simultaneously achieve two goals 1 the novel realization of a one shot decorrelator with lower computational complexity and 2 the forming of the maximum likelihood decision rule on the decorrelated outputs which results in an effective increase of the correlation in the fading process analytical evaluation and simulation of the error probability of the detector demonstrates significant lowering of the error floor in comparison to the decorrelating detector that employs conventional differentially coherent detection

em based joint data detection and channel estimation of ds cdma signals we present two efficient iterative receiver structures of tractable complexity for joint multiuser detection and multichannel estimation jde of direct sequence code division multiple access signals the schemes result from an application of the expectation maximization em and the space alternating generalized expectation maximization sage algorithms respectively the em jde receiver updates the data bit sequences in parallel while the sage jde receiver reestimates them successively the channel parameters are updated in parallel in both schemes the em algorithm provides a set of free parameters called weight coefficients which can be selected to optimize its performance two optimality criteria are defined and analytical expressions for the corresponding optimized weight coefficients are given monte carlo simulations of a synchronous scenario show that the proposed jde receivers have excellent multiuser efficiency and are robust against errors in the estimation of the channel parameters moreover very short training sequences are required for the jde schemes to converge simulation results further demonstrate that the sage jde receiver exhibits a better performance when the users bit sequences are updated in the order of increasing signal strength i e the bit sequence of the user with the weakest signal strength is updated first at each stage

a near ideal noise whitening filter for an asynchronous time varying cdma system a near ideal noise whitening filter for a time varying code division multiple access cdma system is examined first the structure of the ideal noise whitening filter is determined the ideal noise whitening filter for a time varying cdma system is dependent on unknown future system parameters and is therefore impractical a near ideal realizable whitening filter is introduced as a solution the convergence of the factorization method for a time varying cdma system is considered the truncation of the number of taps of the ideal noise whitening filter is studied and a metric function based on the near ideal noise whitening filter for a tree search detection algorithm is formulated simulation results are obtained for five and ten user time varying cdma systems with binary random signature sequences of length 10 and a rectangular chip waveform the results show that the near ideal noise whitening filter can accurately approximate the ideal noise whitening filter at a low complexity level the performance degradation of a suboptimal tree search detector for a time varying asynchronous cdma system using a typical near ideal noise whitening filter is minimal compared to an identical system using the ideal noise whitening filter

periodic solutions of nonlocal semilinear fourth order differential equations abstract in this paper we study the existence of periodic solutions with prescribed wavelength for two classes of nonlocal fourth order nonautonomous differential equations existence of nontrivial solutions for the first equation is proved using clark s theorem existence of nontrivial solutions for the second equation is proved using the symmetric mountain pass theorem

existence and numerical approximations of periodic solutions of semilinear fourth order differential equations abstract a multiplicity result of existence of periodic solutions with prescribed wavelength for a class of fourth order nonautonomous differential equations related either to the extended fisher kolmogorov or to the swift hohenberg equation is proved variational approach is used some numerical solutions are calculated via the finite element method

applicability of the fisher equation to bacterial population dynamics the applicability of the fisher equation which combines diffusion with logistic nonlinearity to population dynamics of bacterial colonies is studied with the help of explicit analytic solutions for the spatial distribution of a stationary bacterial population under a static mask the mask protects bacteria from ultraviolet light the solution which is in terms of jacobian elliptic functions is used to provide a practical prescription to extract fisher equation parameters from observations and to decide on the validity of the fisher equation

semidefinite relaxation based multiuser detection for m ary psk multiuser systems because of the powerful symbol error performance of multiuser maximum likelihood ml detection recently there has been much interest in seeking effective ways of approximating multiuser ml detection mld with affordable computational costs it has been illustrated that for the synchronous code division multiple access cdma scenario the so called semidefinite relaxation sdr algorithm can accurately and efficiently approximate multiuser mld this sdr mld algorithm however can only handle binary and quadratic phase shift keying psk symbol constellations in this sequel we propose an extended sdr algorithm for mld with m ary psk mpsk constellations for the synchronous cdma scenario the proposed sdr algorithm provides an attractive polynomial time complexity order of k sup 3 5 where k is the number of users simulation results indicate that the proposed detector provides improved symbol error performance compared with several commonly used multiuser detectors