continuous time equivalents of welch bound equality sequences the theory developed on the sequence optimization for equal power code division multiple access cdma systems is generalized in this paper unlike previous works where discrete time or vector channel models are predominantly employed the overloaded channels are modeled as strictly band limited continuous time multiple access channels corrupted by additive white gaussian noise awgn after being posed as a multiple input single output miso joint transmitter and receiver optimization problem with the total mean squared error mse as the objective functional to be minimized the variational problem is solved by using a frequency domain approach it is shown that there exist continuous time equivalents of welch bound equality cte wbe sequences as the jointly optimum transmit waveforms and that the matched filters are the jointly optimum receivers the user capacity of the band limited channels is characterized by a necessary and sufficient condition for the admissibility of users into the system in terms of the channel load the received signal to noise ratio and the signal to interference plus noise ratio sinr requirement it is also shown that cte wbe sequences achieve the lower bound on the continuous time equivalent of total squared correlation cte tsc

admissibility analysis of discrete linear time varying descriptor systems abstract this paper deals with the problem of admissibility analysis i e regularity causality and exponential stability of discrete time linear descriptor systems with uncertain time varying parameters in the system state space model matrices the parameters enter affinely into all the matrices and both their admissible values and variations are assumed to belong to given intervals first necessary and sufficient admissibility conditions for uncertainty free discrete linear time varying descriptor systems are presented next strict linear matrix inequality conditions based on parameter dependent lyapunov functions are proposed to ensure robust admissibility of uncertain linear descriptor systems both the cases of lyapunov functions with affine and quadratic dependence on the system uncertain parameters are considered the robust admissibility analysis methods incorporate information on available bounds on both the admissible values and variation of the uncertain parameters numerical examples are presented to demonstrate the potentials of the proposed methods

optimum sequence multisets for synchronous code division multiple access channels it is shown that the sum capacity of the symbol synchronous code division multiple access channel with equal average input energy constraints is maximized precisely by those spreading sequence multisets that meet welch s lower bound on total squared correlation it is further shown that the symmetric capacity of the channel determined by these same sequence multisets is equal to the sum capacity

affine parameter dependent lyapunov functions and real parametric uncertainty this paper presents new tests to analyze the robust stability and or performance of linear systems with uncertain real parameters these tests are extensions of the notions of quadratic stability and performance where the fixed quadratic lyapunov function is replaced by a lyapunov function with affine dependence on the uncertain parameters admittedly with some conservatism the construction of such parameter dependent lyapunov functions can be reduced to a linear matrix inequality lmi problem and hence is numerically tractable these lmi based tests are applicable to constant or time varying uncertain parameters and are less conservative than quadratic stability in the case of slow parametric variations they also avoid the frequency sweep needed in real spl mu analysis and numerical experiments indicate that they often compare favorably with spl mu analysis for time invariant parameter uncertainty

an optimal orthogonal overlay for a cyclostationary legacy signal in this paper the design of an overlay system is considered that operates on a channel already occupied by a legacy system it is assumed that the legacy transmitter generates a cyclostationary signal and that the legacy receiver is equipped with a linear filter followed by a uniform sampler under a zero interference constraint at the legacy receiver the meansquared error at the overlay receiver is minimized to find the optimal overlay system that employs linear modulation and demodulation for the trade off between performance and rate of the overlay system the problem is formulated to allow a transmission rate that is a fraction of the fundamental cycle frequency of the legacy signal the notion of a virtual legacy receiver is devised to incorporate cases with non identical frequency bands for the transmission and reception of the overlay signal using the vectorized fourier transform technique the problem is reformulated the solution is derived and a necessary and sufficient condition for the existence of the optimal solution is found all in the frequency domain numerical results show that the proposed scheme significantly improves the spectral efficiency by effectively exploiting the cyclostationarity of the legacy signal

iterative construction of optimum signature sequence sets in synchronous cdma systems optimum signature sequence sets that maximize the capacity of single cell synchronous code division multiple access cdma systems have been identified optimum signature sequences minimize the total squared correlation tsc they form a set of orthogonal sequences if the number of users is less than or equal to the processing gain and a set of welch 1994 bound equality wbe sequences otherwise we present an algorithm where users update their transmitter signature sequences sequentially in a distributed fashion by using available receiver measurements we show that each update decreases the tsc of the set and produces better signature sequence sets progressively we prove that the algorithm converges to a set of orthogonal signature sequences when the number of users is less than or equal to the processing gain we observe and conjecture that the algorithm converges to a wbe set when the number of users is greater than the processing gain at each step the algorithm replaces one signature sequence from the set with the normalized minimum mean squared error mmse receiver corresponding to that signature sequence since the mmse filter can be obtained by a distributed algorithm for each user the proposed algorithm is amenable to distributed implementation

robust stability and stabilization of discrete singular systems an equivalent characterization this note deals with the problems of robust stability and stabilization for uncertain discrete time singular systems the parameter uncertainties are assumed to be time invariant and norm bounded appearing in both the state and input matrices a new necessary and sufficient condition for a discrete time singular system to be regular causal and stable is proposed in terms of a strict linear matrix inequality lmi based on this the concepts of generalized quadratic stability and generalized quadratic stabilization for uncertain discrete time singular systems are introduced necessary and sufficient conditions for generalized quadratic stability and generalized quadratic stabilization are obtained in terms of a strict lmi and a set of matrix inequalities respectively with these conditions the problems of robust stability and robust stabilization are solved an explicit expression of a desired state feedback controller is also given which involves no matrix decomposition finally an illustrative example is provided to demonstrate the applicability of the proposed approach

the maximum squared correlation sum capacity and total asymptotic efficiency of minimum total squared correlation binary signature sets the total squared correlation tsc maximum squared correlation msc sum capacity c sub sum and total asymptotic efficiency tae of underloaded signature sets as well as the tsc and c sub sum of overloaded signature sets are metrics that are optimized simultaneously over the real complex field in this present work closed form expressions are derived for the msc c sub sum and tae of minimum tsc binary signature sets the expressions disprove the general equivalence of these performance metrics over the binary field and establish conditions on the number of signatures and signature length under which simultaneous optimization can or cannot be possible the sum capacity loss of the recently designed minimum tsc binary sets is found to be rather negligible in comparison with minimum tsc real complex valued welch bound equality sets

cdma codeword optimization interference avoidance and convergence via class warfare interference avoidance has been shown to reduce total square correlation tsc for given ensembles of user signature waveforms codewords in a synchronous code division multiple access cdma system in all experiments we have conducted sequential application of interference avoidance produces an optimal codeword set when starting from randomly chosen initial codewords here we provide the first formal proof of convergence to optimal codeword ensembles for greedy interference avoidance algorithms augmented by a technique called class warfare whereby users which reside in more heavily loaded areas of the signal space purposely interfere with attack the reception of users in less crowded areas coordination of deliberate interference by a complete class of aggrieved user is also sometimes necessary such attacks and subsequent codeword adjustment by attacked users are shown to strictly decrease tsc along the way we also show using linear algebra and a variant of stochastic ordering equivalence between minimization of tsc and maximization of sum capacity

a game theoretic framework for interference avoidance various iterative algorithms for interference avoidance ia in networks with co located receivers suitable for distributed implementation have been proposed in the literature in this paper the ia problem is cast in a game theoretic framework and is formulated as a potential game this formulation accommodates previously proposed algorithms and in addition gives us a framework that enables the design of new distributed and convergent algorithms for ia including algorithms with nonidentical utility functions for the users two new convergence results for potential games are then derived the first result establishes the convergence of a class of potential games to the global solution while following best response iterations and when noise is added the second result establishes the convergence of potential games to the nash equilibria of the game while following random better response iterations the first result combined with the potential game formulation allows us to show that for a large class of network scenarios arbitrarily small noise assures the convergence of best response ia algorithms including the eigeniterations to an arbitrarily small neighborhood of the globally optimal signature sequence set the second result enables the design of reduced feedback mechanisms for ia that converge to desirable solutions

bandwidth constrained signature waveforms and walsh signal space receivers for synchronous cdma systems synchronous cdma systems whose transmission bandwidth is quantified through the fractional out of band energy fobe constraint are considered either a conventional matched filter mf receiver or a minimum mean square error mmse receiver is employed for users data detection the total squared correlation tsc and the total mean square error tmse are proposed as the performance parameters for the mf and mmse receivers respectively these parameters need to be minimized in order to maximize the signal to interference ratios sirs at the receivers outputs for a given fobe bandwidth constraint the sets of signature waveforms that minimize either tsc or tmse are obtained from the prolate spheroidal wave functions pswfs furthermore if the number of users is the size of a hadamard matrix then optimal signature waveforms can be obtained to maximize the individual sir for every user due to the complicated nature of the pswfs simplified mf and mmse receivers based on the walsh signal space are developed

optimal sequences and sum capacity of synchronous cdma systems the sum capacity of a multiuser synchronous cdma system is completely characterized in the general case of asymmetric user power constraints this solves the open problem posed by rupf and massey see ibid vol 40 p 1261 6 1994 which had solved the equal power constraint case we identify the signature sequences with real components that achieve sum capacity and indicate a simple recursive algorithm to construct them

ds cdma signature sequences based on pr qmf banks in this paper we propose two classes of multivalued signature sequences for direct sequence code division multiple access ds cdma systems the proposed classes are obtained from the coefficients of the subbands of perfect reconstruction quadrature mirror filterbanks pr qmf the first and second classes are derived from the tree and lattice structure pr qmf banks respectively and they both have perfect correlation properties for synchronous ds cdma systems the second class in addition has generally better correlation properties than other well known sequences in asynchronous ds cdma systems some numerical results for the performance of the asynchronous ds cdma systems with several sequences including the second class of sequences are obtained

maximizing the spectral efficiency of coded cdma under successive decoding we investigate the spectral efficiency achievable by random synchronous code division multiple access cdma with quaternary phase shift keying qpsk modulation and binary error control codes in the large system limit where the number of users the spreading factor and the code block length go to infinity for given codes we maximize spectral efficiency assuming a minimum mean square error mmse successive stripping decoder for the cases of equal rate and equal power users in both cases the maximization of spectral efficiency can be formulated as a linear program and admits a simple closed form solution that can be readily interpreted in terms of power and rate control we provide examples of the proposed optimization methods based on off the shelf low density parity check ldpc codes and we investigate by simulation the performance of practical systems with finite code block length

on state feedback h control for discrete time singular systems this technical note deals with the state feedback h control problem for linear time invariant discrete time singular systems relied on the use of auxiliary matrices and a positive scalar a novel necessary and sufficient condition for the bounded real lemma is derived for discrete time singular systems the characterization is reduced to a strict linear matrix inequality lmi when the scalar is fixed and the resulting lmi is non conservative as long as the scalar is chosen sufficiently large moreover the result is further expanded to h controller design and a numerically efficient and reliable design procedure is given since no particular restriction is imposed on the auxiliary matrices the proposed result outperforms the existing methods in the literature numerical examples are included to illustrate the effectiveness of the present result

stability of distributed power and signature sequence control for cdma systems a game theoretic framework the problem of power control and signature sequence adaptation in code division multiple access cdma systems is studied under a game theoretic framework each user tries to maximize his own utility function which may be different from others sufficient conditions for the existence of an equilibrium point in this multi objective optimization problem are identified the methodology for analyzing this class of problems is illustrated by examples

capacity of second order cyclostationary complex gaussian noise channels in this paper we derive the capacity of a continuous time single input single output siso frequency selective band limited linear time invariant lti channel whose output is corrupted by a second order cyclostationary socs complex gaussian noise by using a pair of invertible linear conjugate linear time varying operators called a properizing frequency shift p fresh vectorizer and a p fresh scalarizer it is shown that whether the complex noise is proper or improper the siso channel can always be converted to an equivalent multiple input multiple output mimo lti channel whose output is now corrupted by a proper complex vector wide sense stationary noise a variational problem is then formulated in the frequency domain to find the optimal input distribution that maximizes the throughput of the equivalent mimo channel it turns out that the optimal input to the siso channel obtained through a procedure similar to the water filling is an socs complex gaussian random process with the same cycle period as the noise it is shown that this procedure named cyclic water filling significantly outperforms ordinary water filling by effectively utilizing the spectral correlation of the cyclostationary noise

ensuring convergence of the mmse iteration for interference avoidance to the global optimum viswanath and anantharam 1999 characterize the sum capacity of multiaccess vector channels for a given number of users received powers spreading gain and noise covariance matrix in a code division multiple access cdma system viswanath and anantharam present a combinatorial algorithm to generate a set of signature sequences that achieves the maximum sum capacity these sets also minimize a performance measure called generalized total square correlation tsc sub g ulukus and yates 2001 propose an iterative algorithm suitable for distributed implementation at each step one signature sequence is replaced by its linear minimum mean square error mmse filter this algorithm results in a decrease of tsc sub g at each step the mmse iteration has fixed points not only at the optimal configurations which attain the global minimum tsc sub g but also at other configurations which are suboptimal the authors of claim that simulations show that when starting with random sequences the algorithm converges to optimum sets of sequences but they give no formal proof we show that the tsc sub g function has no local minima in the sense that given any suboptimal set of sequences there exist arbitrarily close sets with lower tsc sub g therefore only the optimal sets are stable fixed points of the mmse iteration we define a noisy version of the mmse iteration as follows after replacing all the signature sequences one at a time by their linear mmse filter we add a bounded random noise to all the sequences using our observation about the tsc sub g function we can prove that if we choose the bound on the noise adequately making it decrease to zero the noisy mmse iteration converges to the set of optimal configurations with probability one for any initial set of sequences

brief paper robust filtering for uncertain linear discrete time descriptor systems this paper is concerned with the problem of robust filtering for uncertain linear discrete time descriptor systems the matrices of the system state space model are uncertain belonging to a given polytope a linear matrix inequality based method is proposed for designing a linear stationary filter that guarantees the asymptotic stability of the estimation error and gives an optimized upper bound on the asymptotic error variance irrespective of the parameter uncertainty the proposed robust filter design is based on a parameter dependent lyapunov function which is shown to outperform parameter independent ones

optimum linear joint transmit receive processing for mimo channels with qos constraints this paper considers vector communications through multiple input multiple output mimo channels with a set of quality of service qos requirements for the simultaneously established substreams linear transmit receive processing also termed linear precoder at the transmitter and linear equalizer at the receiver is designed to satisfy the qos constraints with minimum transmitted power the exact conditions under which the problem becomes unfeasible are given although the original problem is a complicated nonconvex problem with matrix valued variables with the aid of majorization theory we reformulate it as a simple convex optimization problem with scalar variables we then propose a practical and efficient multilevel water filling algorithm to optimally solve the problem for the general case of different qos requirements the optimal transmit receive processing is shown to diagonalize the channel matrix only after a very specific prerotation of the data symbols for situations in which the resulting transmit power is too large we give the precise way to relax the qos constraints in order to reduce the required power based on a perturbation analysis we also propose a robust design under channel estimation errors that has an important interest for practical systems numerical results from simulations are given to support the mathematical development of the problem

user capacity of asynchronous cdma systems with matched filter receivers and optimum signature sequences for a symbol asynchronous but chip synchronous single cell code division multiple access cdma system we define a system wide quantity called the total squared asynchronous correlation tsac which for arbitrary signature sets depends on the users delay profile we develop a lower bound for tsac that is independent of the users delays we show that if the signature set achieves this tsac lower bound then the user capacity of the asynchronous cdma system using matched filters becomes the same as that of a single cell synchronous cdma system in this case there is no loss in user capacity due to asynchronism we present iterative signature adaptation algorithms which when executed sequentially by the users appear to converge to these optimum signature sequences however the existence for all user delay profiles of signature sequences achieving this lower bound remains a significant open problem

the pn search algorithm application to tsume shogi abstract this paper proposes a new search algorithm denoted pn for and or tree search the algorithm is based on proof number pn search a best first search algorithm proposed by allis et al artificial intelligence 66 1 1994 91 124 and on korf s rbfs algorithm artificial intelligence 62 1 1993 41 78 pn combines several existing ideas it transforms a best first pn search algorithm into an iterative deepening depth first approach moreover it is enhanced by methods such as recursive iterative deepening dynamic evaluation efficient successor ordering and pruning by dependency relations the resulting algorithm turns out to be highly efficient as witnessed by the experimental results the pn algorithm is implemented in a tsume shogi japanese chess mating problem program and evaluated by testing it on 295 notoriously difficult tsume shogi problems one problem has a depth of search of over 1500 plies the experimental results are compared with those of other programs the pn program shows by far the best results solving all problems but one needless to say it outperforms the best human tsume shogi problem solvers by far

minimum total squared correlation quaternary signature sets new bounds and optimal designs we derive new bounds on the total squared correlation tsc of quaternary quadriphase signature sequence sets for all lengths l and set sizes k then for all k l we design minimum tsc optimal sets that meet the new bounds with equality direct numerical comparison with the tsc value of the recently obtained optimal binary sets shows under what k l realizations gains are materialized by moving from the binary to the quaternary code division multiplexing alphabet on the other hand comparison with the welch tsc value for real complex field sets shows that arguably not much is to be gained by raising the alphabet size above four for any k l the sum capacity as well as the maximum squared correlation and total asymptotic efficiency of minimum tsc quaternary sets is also evaluated in closed form and contrasted against the sum capacity of minimum tsc optimal binary and real complex sets

signature optimization for cdma with limited feedback we study the performance of joint signature receiver optimization for direct sequence code division multiple access ds cdma with limited feedback the receiver for a particular user selects the signature from a signature codebook and relays the corresponding b index bits to the transmitter over a noiseless channel we study the performance of a random vector quantization rvq scheme in which the codebook entries are independent and isotropically distributed assuming the interfering signatures are independent and have independent and identically distributed i i d elements we evaluate the received signal to interference plus noise ratio sinr in the large system limit as the number of users processing gain and feedback bits b all tend to infinity with fixed ratios this sinr is evaluated for both the matched filter and linear minimum mean squared error mmse receivers furthermore we show that this large system sinr is the maximum that can be achieved over any sequence of codebooks numerical results show that with the mmse receiver one feedback bit per signature coefficient achieves close to single user performance we also consider a less complex and suboptimal reduced rank signature optimization scheme in which the user s signature is constrained to lie in a lower dimensional subspace the optimal subspace coefficients are scalar quantized and relayed to the transmitter the large system performance of the quantized reduced rank scheme can be approximated and numerical results show that it performs in the vicinity of the rvq bound finally we extend our analysis to the scenario in which a subset of users optimize their signatures in the presence of random interference

finite step algorithms for constructing optimal cdma signature sequences a description of optimal sequences for direct spread code division multiple access ds cdma is a byproduct of recent characterizations of the sum capacity this paper restates the sequence design problem as an inverse singular value problem and shows that the problem can be solved with finite step algorithms from matrix theory it proposes a new one sided algorithm that is numerically stable and faster than previous methods

mmse interference suppression for direct sequence spread spectrum cdma we consider interference suppression for direct sequence spread spectrum code division multiple access cdma systems using the minimum mean squared error mmse performance criterion the conventional matched filter receiver suffers from the near far problem and requires strict power control typically involving feedback from receiver to transmitter for acceptable performance multiuser detection schemes previously proposed mitigate the near far problem but are complex and require explicit knowledge or estimates of the interference parameters in this paper we present and analyze several new mmse interference suppression schemes which have the advantage of being near far resistant to varying degrees depending on their complexity and can be implemented adaptively when interference parameters are unknown and or time varying numerical results are provided that show that these schemes offer significant performance gains relative to the matched filter receiver we conclude that mmse detectors can alleviate the need for stringent power control in cdma systems and may be a practical alternative to the matched filter receiver

rms bandwidth constrained signature waveforms that maximize the total capacity of pam synchronous cdma channels optimum time limited signal sets of equal and unequal energies are obtained under root mean square rms bandwidth constraints the total capacity and the total asymptotic efficiency of the pam synchronous gaussian cdma psg cdma channel are considered as the optimality criteria the latter measure is monotonic with the determinant of the correlation matrix r and the former is monotonic with det i spl sigma sup 2 r where spl sigma sup 2 represents the noise level average as well as maximum rms bandwidth constraints are considered in the equal energy case and the energy weighted rms bandwidth constraint is considered for unequal energy signals for the equal energy problem signal sets are found that simultaneously optimize the total asymptotic efficiency under both average and maximum rms bandwidth constraints for the total capacity measure such simultaneously optimal signal sets are also obtained albeit under the restriction that the number of signals n be a hadamard matrix dimension when the hadamard dimension is in particular a power of two we obtain optimum signal sets that are shown to yield equal optimum multiuser detector asymptotic efficiencies for all users of an uncoded psg cdma channel unequal energy signal sets are also found under an energy weighted rms bandwidth constraint for both optimality criteria

performance of linear space search algorithms abstract search algorithms that use space linear in the search depth are widely employed in practice to solve difficult problems optimally such as planning and scheduling in this paper we study the average case performance of linear space search algorithms including depth first branch and bound dfbnb iterative deepening id and recursive best first search rbfs to facilitate our analyses we use a random tree t b d that has mean branching factor b depth d and node costs that are the sum of the costs of the edges from the root to the nodes we prove that the expected number of nodes expanded by dfbnb on a random tree is no more than bd times the expected number of nodes expanded by best first search bfs on the same tree which usually requires space that is exponential in depth d we also show that dfbnb is asymptotically optimal when bfs runs in exponential time and id and rbfs are asymptotically optimal when the edge costs of t b d are integers if bp 0 is the expected number of children of a node whose costs are the same as that of their parent then the expected number of nodes expanded by these three linear space algorithms is exponential when bp o d 4 when bp 0 1 and at most quadratic when bp 0 1 in addition we study the heuristic branching factor of t b d and the effective branching factor of bfs dfbnb id and rbfs on t b d furthermore we use our analytic results to explain a surprising anomaly in the performance of these algorithms and to predict the existence of a complexity transition in the asymmetric traveling salesman problem

a tighter correlation lower bound for quasi complementary sequence sets levenshtein improved the famous welch bound on aperiodic correlation for binary sequences by utilizing some properties of the weighted mean square aperiodic correlation following levenshtein s idea a new correlation lower bound for quasi complementary sequence sets qcsss over the complex roots of unity is proposed in this paper the derived lower bound is shown to be tighter than the welch bound for qcsss when the set size is greater than some value the conditions for meeting the new bound with equality are also investigated

reducing reexpansions in iterative deepening search by controlling cutoff bounds abstract it is known that a best first search algorithm like a 5 6 requires too much space which often renders it unusable and a depth first search strategy does not guarantee an optimal cost solution the iterative deepening algorithm ida 4 achieves both space and cost optimality for a class of tree searching problems however for many other problems it takes too much of computation time due to excessive reexpansion of nodes this paper presents a modification of ida to an admissible iterative depth first branch and bound algorithm ida cr for trees which overcomes this drawback of ida and operates much faster using the same amount of storage algorithm ida cra a bounded suboptimal cost variation of ida cr is also presented in order to reduce the execution time still further results with the 0 1 knapsack problem traveling salesman problem and the flow shop scheduling problem are shown

technical communique robust stabilization for uncertain discrete singular systems this paper considers the problem of robust stabilization for uncertain discrete singular systems subject to time invariant norm bounded unstructured uncertainty in the state matrix the problem we address is the design of state feedback controllers such that the resulting closed loop system is regular causal as well as stable for all admissible uncertainties in this connection the concept of generalized quadratic stabilizability is defined in terms of matrix inequalities a necessary and sufficient condition for generalized quadratic stabilizability is derived under certain assumptions when this condition holds the explicit formula of a state feedback controller solving the problem is also given the results can be viewed as extensions of existing results on robust stabilization of uncertain discrete standard state space systems

sum capacity and tsc bounds in collaborative multibase wireless systems we consider a wireless system with base stations which collaborate and derive bounds on sum capacity and total squared correlation for uniform channels between users and bases the correspondence also investigates structural properties which must be satisfied by user transmit covariance matrices at the optimal sum capacity total squared correlation tsc point and shows that for multibase systems maximizing sum capacity and minimizing tsc are in general not equivalent problems

comments on symmetric capacity and signal design for l out of k symbol synchronous cdma gaussian channels for original paper see a a alsugair and r s cheng ibid vol 41 p 1072 1082 july 1995 we show that the signature sequences set that maximizes the lower bound on symmetric capacity derived in the above paper also maximizes the sum capacity of the gaussian synchronous code division multiple access cdma channel this is done by relying on the equivalence of the eigenvalues of energy weighted correlation matrices this result establishes the missing link between the above paper and the sum capacity maximization result by viswanath et al 1999

sum rate optimal multicode cdma systems an equivalence result in this paper the sum rate of a multicode code division multiple access cdma system with asymmetric power users is maximized given a processing gain and the power profile of users unlike the sum rate maximization for a single code cdma system the optimization requires the joint optimal distribution of each user s power to its multiple data streams as well as the optimal design of signature sequences the crucial step is to establish an equivalence of the multicode cdma system to restricted frequency division multiple access fdma and time division multiple access tdma systems the cdma system has upper limits on the numbers of multicodes of users whereas the fdma system has upper limits on the bandwidths and the tdma system has upper limits on the duty cycles of users the equivalence facilitates the complete characterization of the maximum sum rate of the multicode cdma system and also provides new insights into single and multicode cdma systems in terms of the parameters of the equivalent fdma and tdma systems

a new robust d stability condition for real convex polytopic uncertainty abstract the problem of robust d stability analysis with respect to real convex polytopic uncertainties is tackled a new lmi based sufficient condition for the existence of parameter dependent lyapunov functions is proposed this condition generalises previously published conditions numerical comparisons with quadratic stability results as well as previous results based on parameter dependent lyapunov functions illustrate the relevance of this new condition finally this result appears to be promising for robust multi objective performance analysis and control synthesis purposes

dilated lmi conditions for time varying polytopic descriptor systems the discrete time case this article addresses the admissibility analysis and state feedback control synthesis problems of discrete time polytopic descriptor systems with possibly time varying parameters first we present a new necessary and sufficient strict linear matrix inequality lmi condition for the admissibility analysis of linear time invariant lti descriptor systems then based on the concept of poly quadratic admissibility introduced in this article we extend this result to the admissibility analysis of possibly time varying parameter dependent descriptor systems this extension which applies to both uncertain and measurable parameters uses parameter dependent lyapunov functions and employs two slack variables the extended conditions are also expressed as strict lmi conditions which are easily tractable numerically compared to the non strict ones often encountered when dealing with descriptor systems note that we have proposed two separate admissibility analysis conditions one directly exploitable for stat

on the ergodic capacity as a function of the correlation properties in systems with multiple transmit antennas without csi at the transmitter in this letter the impact of correlation of the transmit antennas of a multiple input single output miso system with no channel state information csi at the transmitter and perfect csi at the receiver is analyzed we show that the ergodic capacity for the single user miso system is schur concave with respect to the vector with eigenvalues of the channel covariance matrix i e the more correlation that exists between the transmit antennas the less is the achievable capacity furthermore the capacity loss for fully correlated transmit antennas in comparison with the uncorrelated case is derived the results for the ergodic capacity are compared with the impact of correlation on the outage probability the relationship between correlation properties and outage probability is more complicated than the relationship between the correlation properties and the ergodic capacity it is shown that the outage probability is schur convex in the high signal to noise ratio snr regime and schur concave in the low snr regime

depth first iterative deepening an optimal admissible tree search abstract the complexities of various search algorithms are considered in terms of time space and cost of solution path it is known that breadth first search requires too much space and depth first search can use too much time and doesn t always find a cheapest path a depth first iterative deepening algorithm is shown to be asymptotically optimal along all three dimensions for exponential tree searches the algorithm has been used successfully in chess programs has been effectively combined with bi directional search and has been applied to best first heuristic search as well this heuristic depth first iterative deepening algorithm is the only known algorithm that is capable of finding optimal solutions to randomly generated instances of the fifteen puzzle within practical resource limits

agent searching in a tree and the optimality of iterative deepening the agent searching framework models the effort of a search strategy in terms of the distance traversed by an agent while exploring the search space the framework has been found to be useful in modeling search problems where the cost of backtracking and retracing search paths is important in determining search complexity in this paper we show that depth first iterative deepening dfid strategies are optimal for an agent searching in a line in m concurrent rays and in uniform b ary trees in the conventional search model it is known that dfid is asymptotically optimal for uninformed search of uniform b ary trees in this paper we prove the stronger result that for agent searching in uniform b ary trees iterative deepening is optimal up to lower order terms we also discuss the problems involved in optimally performing agent search in a graph

asynchronous cdma systems with random spreading part i fundamental limits spectral efficiency for asynchronous code division multiple access cdma with random spreading is calculated in the large system limit allowing for arbitrary chip waveforms and frequency flat fading signal to interference and noise ratios sinrs for suboptimal receivers such as the linear minimum mean square error mmse detectors are derived the approach is general and optionally allows even for statistics obtained by undersampling the received signal all performance measures are given as a function of the chip waveform and the delay distribution of the users in the large system limit it turns out that synchronizing users on a chip level impairs performance for all chip waveforms with bandwidth greater than the nyquist bandwidth e g positive roll off factors for example with the pulse shaping demanded in the umts standard user synchronization reduces spectral efficiency up to 12 at 10 db normalized signal to noise ratio the benefits of asynchronism stem from the finding that the excess bandwidth of chip waveforms actually spans additional dimensions in signal space if and only if the users are desynchronized at chip level the analysis of linear mmse detectors shows that the limiting interference effects can be decoupled both in the user domain and in the frequency domain such that the concept of effective interference spectral density arises this generalizes and refines tse and hanly s concept of effective interference in part ii the analysis is extended to any linear detector that admits a representation as multistage detector and guidelines for the design of low complexity multistage detectors with universal weights are provided

a new discrete time robust stability condition a new robust stability condition for uncertain discrete time systems with convex polytopic uncertainty is given it enables to check stability using parameter dependent lyapunov functions which are derived from lmi conditions it is shown that this new condition provides better results than the classical quadratic stability besides the use of a parameter dependent lyapunov function this condition exhibits a kind of decoupling between the lyapunov and the system matrices which may be explored for control synthesis purposes a numerical example illustrates the results

linear space best first search abstract best first search is a general heuristic search algorithm that always expands next a frontier node of lowest cost it includes as special cases breadth first search dijkstra s single source shortest path algorithm and the a algorithm its applicability however is limited by its exponential memory requirement previous approaches to this problem such as iterative deepening do not expand nodes in best first order if the cost function can decrease along a path we present a linear space best first search algorithm rbfs that always explores new nodes in best first order regardless of the cost function and expands fewer nodes than iterative deepening with a nondecreasing cost function on the sliding tile puzzles rbfs with a nonmonotonic weighted evaluation function dramatically reduces computation time with only a small penalty in solution cost in general rbfs reduces the space complexity of best first search from exponential to linear at the cost of only a constant factor in time complexity in our experiments

cyclic wiener filtering theory and method conventional time and space filtering of stationary random signals which amounts to forming linear combinations of time translates and space translates exploits the temporal and spatial coherence of the signals by including frequency translates as well the spectral coherence that is characteristic of cyclostationary signals can also be exploited some of the theoretical concepts underlying this generalized type of filtering called frequency shift fresh filtering are developed the theory of optimum fresh filtering which is a generalization of wiener filtering called cyclic wiener filtering is summarized and the theory is illustrated with specific examples of separating temporally and spectrally overlapping communications signals including am bpsk and qpsk the structures and performances of optimum fresh filters are presented and adaptive adjustment of the weights in these structures is discussed also specific results on the number of digital qam signals that can be separated as a function of excess bandwidth are obtained

an optimal signal design for band limited asynchronous ds cdma communications an optimal signal design for band limited asynchronous direct sequence code division multiple access ds cdma communications with aperiodic random spreading sequences and a conventional matched filter receiver is considered in an additive white gaussian noise awgn channel with bandwidth defined in the strict sense two optimization problems are solved under finite bandwidth and zero interchip interference constraints first a chip waveform optimization is performed given the system bandwidth the data symbol transmission rate and the processing gain a technique to characterize a band limited chip waveform with a finite number of parameters is developed and it is used to derive optimum chip waveforms which minimize the effect of multiple access interference mai for any energy and delay profile of users next a joint optimization of the processing gain and the chip waveform is performed given the system bandwidth and the data symbol transmission rate a sufficient condition for a system to have lower average probability of bit error for any energy profile is found and it is used to derive some design strategies it is shown that the flat spectrum pulse with the processing gain leading to zero excess bandwidth results in the minimum average probability of bit error design examples and numerical results are also provided

multiuser constrained water pouring for continuous time overloaded gaussian multiple access channels in this paper signature waveforms and signal powers are jointly optimized for multiuser communications over strictly bandlimited continuous time overloaded channels corrupted by additive white gaussian noise awgn the total signal power is minimized subject to general asymmetric signal to interference plus noise ratio sinr constraints at the output of linear minimum mean squared error lmmse receivers using vectorized fourier transform vft technique the optimal solutions as well as a necessary and sufficient condition for the existence of a feasible solution are derived in the frequency domain it turns out that every optimal solution performs the same as the optimal frequency division multiple access fdma system a geometric procedure called multiuser constrained water pouring is developed to construct every possible optimal solution of which profiles of signal power and equivalent bandwidth are respectively the same as those of signal power and physical bandwidth of the optimal fdma system it is shown that orthogonal signature waveforms are assigned to oversized users and continuous time equivalents of generalized welch bound equality cte gwbe sequences are assigned to non oversized users

heuristic search in restricted memory research note abstract this paper presents heuristic search algorithms which work within memory constraints these algorithms ma for ordinary graphs and mao for and or graphs guarantee admissible solutions within specified memory limitations above the minimum required the memory versus node expansions tradeoff is analyzed for the worst case in the case of ordinary graphs some experiments using the fifteen puzzle problem are carried out under various pruning conditions these parameterized algorithms are found to encompass a wide class of best first search algorithms

analysis and synthesis of robust control systems via parameter dependent lyapunov functions in this paper the problem of robust stability of systems subject to parametric uncertainties is considered sufficient conditions for the existence of parameter dependent lyapunov functions are given in terms of a criterion which is reminiscent of but less conservative than popov s stability criterion an equivalent frequency domain criterion is demonstrated the relative sharpness of the proposed test and existing stability criteria is then discussed the use of parameter dependent lyapunov functions for robust controller synthesis is then considered it is shown that the search for robustly stabilizing controllers may be limited to controllers with the same order as the original plant a possible synthesis procedure and a numerical example are then discussed

bida an improved perimeter search algorithm abstract in this paper we present a new bidirectional heuristic search algorithm our algorithm can be viewed as a perimeter search algorithm and it uses a new technique for reducing the number of heuristic evaluations we also prove some general results on the behavior of iterative deepening perimeter search algorithms and we discuss some new lazy evaluation techniques for improving their performance the theoretical and experimental results show that perimeter search algorithms outperform the other bidirectional algorithms and we believe it is worthwhile to give them a deep look in subsequent research

power minimization for cdma under colored noise rate constrained power minimization pmin over a code division multiple access cdma channel with correlated noise is studied pmin is shown to be an instance of a separable convex optimization problem subject to linear ascending constraints pmin is further reduced to a dual problem of sumrate maximization rmax the results highlight the underlying unity between pmin rmax and a problem closely related to pmin but with linear receiver constraints subsequently conceptually simple sequence design algorithms are proposed to explicitly identify an assignment of sequences and powers that solve pmin the algorithms yield an upper bound of 2n 1 on the number of distinct sequences where n is the processing gain the sequences generated using the proposed algorithms are in general real valued if a rate splitting and multi dimensional cdma approach is allowed the upper bound reduces to n distinct sequences in which case the sequences can form an orthogonal set and be binary plusmn1 valued

on quasi orthogonal signatures for cdma systems sum capacity optimal signatures in synchronous code division multiple access cdma systems are functions of the codebook length as well as the number of active users a new signature set must be assigned every time the number of active users changes this correspondence considers signature sets that are less sensitive to changes in the number of active users equiangular signature sequences are proven to solve a certain max min signal to interference plus noise problem which results from their interference invariance unions of orthonormal bases have subsets that come close to satisfying the welch bound bounds on the maximum number of bases with minimum maximum correlation are derived and a new construction algorithm is provided connections are made between these signature design problems grassmannian line packing frame theory and algebraic geometry

multirate zero forcing tx rx design for mimo channel under ber constraints this paper is concerned with communicating parallel independent streams of data possibly of different rates through a multiple input multiple output channel each data stream is required to satisfy a bit error rate ber constraint this scenario arises in multimedia communications we find the linear zero forcing transmitter receiver tx rx matrix pair that requires the minimum transmitter power to achieve the ber constraints it is well known that the tx rx pair can be chosen to partition the matrix channel into scalar subchannels corresponding to singular values and different data streams can be transmitted through different subchannels this design is suboptimum in general the optimum design has the following structure the set of input data streams is partitioned into groups called signal groups the set of subchannels is conformably partitioned into subchannel groups each signal group is collectively transmitted through its assigned subchannel group with each signal subchannel group pair is associated a unitary matrix that mixes the signals at the transmitter and unmixes them at the receiver we derive a simple algorithm to determine the optimum signal and subchannel groups we then use a known algorithm from majorization theory to determine the unitary matrices using these we completely specify the optimum tx rx pair

parameter dependent lyapunov functions for discrete time systems with time varying parametric uncertainties abstract in this paper we consider discrete time systems with polytopic time varying uncertainty we look for a class of parameter dependent lyapunov functions which are quadratic on the system state and depend in a polytopic way on the uncertain parameter we show that extending the new discrete time stability condition proposed by de oliveira et al systems control lett 36 1999 135 to the case of time varying uncertainty leads to a necessary and sufficient condition for the computation of such a lyapunov function this allows to check asymptotic stability of the system under study the obtained linear matrix inequalities condition can also be used to cope with the control synthesis problem

high snr performance analysis of blind minimum output energy receivers for large ds cdma systems high signal to noise ratio snr properties of the minimum output energy moe receiver and the capon channel estimation technique are analyzed in the case of large direct sequence code division multiple access ds cdma systems it is shown that if the noise power tends to zero and the system load is less than one then the signal to interference plus noise ratio sinr of the moe receiver tends to infinity and the capon channel estimate converges to a scaled version of the channel vector of the user of interest for the case when the system load is larger than one an asymptotic sinr expression is obtained furthermore expressions for the efficiency and asymptotic efficiency of the moe receiver are derived simple approximations of and upper lower bounds on these expressions are found and the effects of the system physical parameters on the performance of the moe receiver are discussed