on the index for array optimization and the discrete prolate spheroidal functions a class of array optimization problems is considered in which we seek to optimize the array response in a specified angular sector the optimization of array directivity is shown to be a special limiting case of these problems as the width of the specified angular sector approaches zero the optimum array patterns are also shown to be related to the well known prolate spheroidal functions

spatial spectral filtering with linearly constrained minimum variance beamformers in this paper orthogonal low dimension models for broad band sources as observed by an array delay line structure are developed the models are termed broad band source representation spaces and are subspaces of the observation space they are generated from eigenstructures of source sample covariance matrices and represent sources efficiently in the 2nd order statistical sense for a single broad band source propagating in a pure delay environment and observed with an isotropic array an accurate indication of the model dimension is derived which is independent on the array configuration being a function of the observed source time bandwidth product only using these representations procedures are described for controlling the spatial and spectral response of linearly constrained minimum variance broad band array beamformers the procedures employ linear constraints to control complex gain characteristics over selected bands of frequency for multiple points of 3 d regions of source locations and do not require the use of steering delays the constraints are derived from source representation spaces and are termed eigenvector constraints simulations are presented which illustrate the effectiveness of the new linear constraints

wide band nulling performance versus number of pattern constraints for an array antenna the problem of wide band pattern hulling for an array antenna is addressed in particular the relation between the number of constraints imposed and the sidelobe suppression achieved the two constraint methods considered are multiple nulling over a narrow angular sector and hulling of the pattern and its derivatives at a single point to first order the sidelobe cancellation is shown to be independent of the actual pattern type and is determined by only two parameters the number of null constraints m and the number of sidelobes v to be cancelled the latter parameter directly translates into a desired hulling bandwidth delta f a numerical solution to the problem is offered in the form of curves from which the number of nulls required to suppress a jammer over a given bandwidth can be conveniently estimated this number is indicative of how many degrees of freedom a conventional adaptive antenna system must allocate to attain a specific hulling performance

spatial spectrum estimation in a location sector multiple narrowband source localization using arbitrarily configured arrays and spatial spectrum estimation is considered a new eigenspace based approach which uses projections onto a particular vector or vector set in the estimated noise only subspace is described several closest vector estimators are developed by using different measures of closeness first closest is a novel full dimensional element space approach to spatial spectrum estimation which has important performance advantages relative to pertinent established spatial spectrum estimators it incorporates a priori knowledge of the array manifold over a location sector of interest to provide snr spectral resolution thresholds which are lower than those of min norm for some arrays substantially lower second relationships between the closest approach and several established approaches to spatial spectrum estimation are established for a linear equispaced array min norm is shown to be a special case of the closest approach one which is based on projection onto a noise only subspace vector which is close to the array manifold over the entire field of view

iterative map equalization and decoding in wireless mobile coded ofdm orthogonal frequency division multiplexing ofdm system suffers extra performance degradation in fast fading channels due to intercarrier interference ici combining frequency domain equalization and bit interleaved coded modulation bicm the iterative receiver is able to harvest both temporal and frequency diversity realizing that ici channels are intrinsically isi channels this paper proposes a soft in soft out siso maximum a posteriori map equalizer by extending ungerboeck s maximum likelihood sequence estimator mlse formulation to ici channels the siso map equalizer employs bcjr algorithm and computes the bit log likelihood ratios llr for the entire received sequence by efficiently constructing a trellis that takes into account of the ici channel structure a reduced state rs formulation of the siso map equalizer which provides good performance complexity tradeoff is also described utilizing the fact that ici energy is clustered in adjacent subcarriers frequency domain equalization is made localized this paper further proposes two computational efficient linear minimum mean square error lmmse based equalization methods recursive q tap sic lmmse equalizer and recursive sliding window sw sic lmmse equalizer respectively simulations results demonstrate that the iterative siso rs map equalizer achieves the performance of no ici with normalized doppler frequency fdts up to 20 46 in realistic mobile wimax environment

on channel estimation and detection for multicarrier signals in fast and selective rayleigh fading channels time domain channel estimation and detection techniques are presented for multicarrier signals in a fast and frequency selective rayleigh fading channel as a consequence of the time varying channel the orthogonality between subcarriers is destroyed in conventional frequency domain approaches resulting in interchannel interference which increases an irreducible error floor in proportion to the normalized doppler frequency an important feature of the proposed technique is the ability to exploit the time selective channel as a provider of time diversity this enables us to achieve performance superior to any other structure without increasing bandwidth or incorporating redundancy in order to reduce the complexity of the estimator we apply the theory of optimal low rank approximation to a minimum mean squared error channel estimator and present a theoretical calculation of mean squared error and simulations to confirm that the estimator is robust to changes in channel characteristics

a simple algorithm to achieve desired patterns for arbitrary arrays a simple iterative algorithm which can be used to find array weights that produce array patterns with a given look direction and an arbitrary sidelobe specification is presented the method can be applied to nonuniform array geometries in which the individual elements have arbitrary and differing radiation patterns the method is iterative and uses sequential updating to ensure that peak sidelobe levels in the array meet the specification computation of each successive pattern is based on the solution of a linearly constrained least squares problem the constraints ensure that the magnitude of the sidelobes at the locations of the previous peaks takes on the prespecified values phase values for the sidelobes do not change during this process and problems associated with choosing a specific phase value are therefore avoided experimental evidence suggests that the procedure terminates in remarkably few iterations even for arrays with significant numbers of elements

multiple window based minimum variance spectrum estimation for multidimensional random fields spectrum analysis is viewed as the problem of estimating the power of a process contained within narrow bands this view leads naturally to consideration of filter based methods for estimating spectra multiple window based estimators where the power in a band is estimated as the average of the powers estimated at the outputs of multiple filters are considered the filters are designed using a linearly constrained minimum variance criterion commonly employed in adaptive beamforming this results in filters that automatically adjust their sidelobes to minimize leakage of energy from outside the band of interest expressions for the bias and variance of the power estimates are derived assuming the sample covariance matrix estimate is used to estimate the data covariance matrix and that the data are independent and identically gaussian distributed these expressions lead to the definition of a performance factor that indicates the degree of variance reduction obtained via multiple window processing a technique for obtaining increased suppression of energy leaking through the filter sidelobes at the expense of the response fidelity to energy in the band is presented simulations are presented to illustrate the effectiveness of the technique

joint channel estimation equalization and data detection for ofdm systems in the presence of very high mobility this paper is concerned with the challenging and timely problem of joint channel estimation equalization and data detection for uplink orthogonal frequency division multiplexing ofdm systems in the presence of frequency selective and very rapidly time varying channels the resulting algorithm is based on the space alternating generalized expectation maximization sage technique which is particularly well suited to multicarrier signal formats leading to a receiver structure that also incorporates interchannel interference ici cancelation in order to reduce the computational complexity of the algorithm band limited discrete cosine orthogonal basis functions are employed to represent the rapidly time varying fading channel by the discrete cosine serial expansion coefficients it is shown that depending on the normalized doppler frequency only a small number of expansion coefficients is sufficient to approximate the channel perfectly and there is no need to know the correlation function of the input signal in this way the resulting reduced dimensional channel coefficients are estimated and the data symbols detected iteratively with tractable complexity the proposed sage joint detection algorithm updates the data sequences serially and the channel parameters are updated in parallel leading to a receiver structure that also incorporates ici cancelation computer simulations show that the cosine transformation represents the time varying channel very effectively and the proposed algorithm has excellent symbol error rate and channel estimation performance even with a very small number of channel expansion coefficients employed in the algorithm resulting in substantial reduction of the computational complexity

a new set of linear constraints for broad band time domain element space processors a new set of linear constraints for designing broad band time domain element space antenna array processors is presented the set of linear constraints is used to ensure that a desired look direction response of the processor over a frequency band of interest can be closely approximated the design technique is posed in such a way that three types of presteering can be handled no presteering coarse presteering and exact presteering the elimination of presteering time delays or the possibility to use coarse presteering is an attractive feature in a digital implementation of antenna array processors the relationship that the new processor has to other broad band processors is also established furthermore the approach enables various types of errors and mismatches between signal model and actual scenario to be incorporated in the problem formulation

plane wave scattering from three dimensional multiple objects using the iterative multiregion technique based on the fdfd method an iterative approach using the finite difference frequency domain method is presented in this paper in order to solve the problem of scattering from large three dimensional electromagnetic scatterers the idea of iterative multiregion technique is introduced to divide one computational domain into smaller subregions and solve each subregion separately then the subregion solutions are combined iteratively to obtain a solution for the complete domain as a result a considerable reduction in the computation time and memory has been achieved

semiblind iterative data detection for ofdm systems with cfo and doubly selective channels data detection for ofdm systems over unknown doubly selective channels dscs and carrier frequency offset cfo is investigated a semiblind iterative detection algorithm is developed based on the expectation maximization em algorithm it iteratively estimates the cfo channel and recovers the unknown data using only limited number of pilot subcarriers in one ofdm symbol in addition efficient initial cfo and channel estimates are also derived based on approximated maximum likelihood ml and minimum mean square error mmse criteria respectively simulation results show that the proposed data detection algorithm converges in a few iterations and moreover its performance is close to the ideal case with perfect cfo and channel state information

low complexity block turbo equalization for ofdm systems in time varying channels we propose low complexity block turbo equalizers for orthogonal frequency division multiplexing ofdm systems in time varying channels the presented work is based on a soft minimum mean squared error mmse block linear equalizer ble that exploits the banded structure of the frequency domain channel matrix as well as a receiver window that enforces this banded structure this equalization approach allows us to implement the proposed designs with a complexity that is only linear in the number of subcarriers three block turbo equalizers are discussed two are based on a biased mmse criterion while the third is based on the unbiased mmse criterion simulation results show that the proposed iterative mmse ble achieves a better bit error rate ber performance than a previously proposed iterative mmse serial linear equalizer sle the proposed equalization algorithms are also tested in the presence of channel estimation errors

low complexity equalization of ofdm in doubly selective channels orthogonal frequency division multiplexing ofdm systems may experience significant inter carrier interference ici when used in time and frequency selective or doubly selective channels in such cases the classical symbol estimation schemes e g minimum mean squared error mmse and zero forcing zf estimation require matrix inversion that is prohibitively complex for large symbol lengths an analysis of the ici generation mechanism leads us to propose a novel two stage equalizer whose complexity apart from the fft is linear in the ofdm symbol length the first stage applies optimal linear preprocessing to restrict ici support and the second stage uses iterative mmse estimation to estimate finite alphabet frequency domain symbols simulation results indicate that our equalizer has significant performance and complexity advantages over the classical linear mmse estimator in doubly selective channels

efficient numerically stabilized rank one eigenstructure updating signal processing rank one eigenvalue decomposition evd updating is well suited to the problem of tracking time varying subspaces previously published rank one evd updating algorithms suffer from a linear buildup of roundoff error which makes them impractical for a large number of recursive updates in the present work the authors develop a numerical stabilization technique which eliminates the error buildup problem in a computationally efficient manner and makes the rank one evd update a practical numerical tool for online computation a simplified eigenvalue iteration is also given it reduces the complexity of the algorithm somewhat as well as the computation time simulations are presented to illustrate numerical performance

analysis of two dimensions sparse multicylinder scattering problem using dd fdtd method an algorithm of two dimensional 2 d domain decomposition finite difference time domain dd fdtd using in sparse multicylinders scattering problem is proposed in this paper the idea of domain decomposition is introduced to divide the sparse problem domain into several subdomains all of subdomains are connected by means of the 2 d time domain green s function as a result a great deal of meshes memory between cylinders is removed especially when the distances between cylinders become large furthermore the coupling between cylinders can be regarded as the equivalent cylindrical wave irradiations the incident signals of the equivalent cylindrical waves are expressed as cylindrical wave input field array cwifa according to huygens principle then the calculation time is significantly reduced the near field to far field transformation is used to obtain the equivalent cylindrical wave as a result the calculation time can be reduced further the new method has been demonstrated in 2 d multicylinders scattering problem numerical results are in good agreement with the results obtained using classical fdtd method and moment of methods mm

low complexity ici isi equalization in doubly dispersive multicarrier systems using a decision feedback lsqr algorithm we propose a low complexity intercarrier interference intersymbol interference ici isi equalizer for multicarrier transmissions over doubly dispersive channels decision feedback or interference cancellation is used with respect to both time and frequency the ici stage employs an extension of the iterative lsqr algorithm using groupwise interference cancellation with reliability based sorting of sets of subcarriers and a band approximation of the frequency domain channel matrix the lsqr algorithm is attractive because of its excellent numerical properties and low complexity optimal pulse design is optionally considered for shaping the ici isi simulation results demonstrate the excellent performance of the proposed ici isi equalizer

on the eigenstructure of the signal only tempo spatial covariance matrix of broad band sources using a circular array the eigenstructure of the signal only part of the data covariance matrix for broadband sources is considered it is well known that unlike the narrowband case all eigenstructure parameters the matrix rank eigenvalues and eigenvectors depend on source spectral and spatial parameters as well as on the relationship between sources it is proved that when a two dimensional equally spaced circular array with flat spectrum broadband sources is used the eigenstructure of the signal only tempo spatial covariance matrix tscm has properties similar to those for the narrowband case most importantly the effective rank of the tscm depends only on the number of sources consequently the authors suggest an alternative approach for resolving frequency location ambiguity in the broadband case that may lead to an effective eigenstructure based parameter estimation algorithm of the music type

an adaptive generalized sidelobe canceller with derivative constraints an adaptive broad band beamforming structure is presented which employs a gradient based weight adjustment algorithm to minimize output variance subject to a set of j linear constraints on broadband directional derivatives in the desired look direction a generalized sidelobe cancelling structure is employed in which a nonadaptive conventional beamformer operates in parallel with an adaptive beamformer the conventional portion has a broad band beampattern which adheres to the specified constraints while the adaptive path is a cascade of a fixed signal blocking matrix and a set of tapped delay line filters blocking is employed to ensure that all incident waveforms which meet the specified constraints do not reach the tapped delay lines as a result an unconstrained least mean square lms power minimization algorithm is employed to adapt the delay line weights it is shown that with the addition of the directional derivative constraints the beamformer quiescent bcampattern becomes a function of the location of the phase center used to specify the constraints a design criterion for choosing this location is suggested and simulation experiments which illustrate the performance of this new adaptive beamformer are presented

channel estimation for ofdm system with two training symbols aided and polynomial fitting a time varying channel estimation method for orthogonal frequency division multiplexing ofdm systems is presented the presented channel estimation method employs two training symbols in combination with polynomial fitting thus obtaining accurate estimation results under a large normalized doppler frequency

a comparison of dct based ofdm and dft based ofdm in frequency offset and fading channels a precise method for calculating the bit error probability bep of a discrete cosine transform dct based orthogonal frequency division multiplexing ofdm system on additive white gaussian noise awgn channels in the presence of frequency offset is derived these accurate results are used to examine and compare the bep performance of a dct ofdm system and the conventional discrete fourier transform dft based ofdm system in an awgn environment several signaling formats such as binary phase shift keying quaternary phase shift keying and 16 ary quadrature amplitude modulation are considered the performance of a dct ofdm with a zero padding guard interval scheme is then compared with a zero padded dft ofdm with the employment of minimum mean square error mmse detection and mmse decision feedback detection with ordering scheme over frequency selective fast rayleigh fading channels analysis and simulation results show that the dct ofdm system outperforms the dft ofdm system in the presence of frequency offset and in frequency selective fast fading environments

a new hybrid method for analysis of scattering from arbitrary configuration of cylindrical objects a new hybrid method for analysis of scattering on configuration of cylindrical objects with arbitrary geometries is presented in this approach each scatterer is treated as an effective circular cylinder represented by its impedance matrix to obtain the impedance matrix representation of the arbitrary geometry post the combination of finite difference frequency domain fdfd and mode matching mm technique is applied in the analysis of an arbitrary configuration of objects the iterative scattering procedure isp is used the error estimation of the method and numerical results are presented and discussed a good agreement of the presented results with analytical approach and commercial finite difference time domain fdtd software is obtained

nondata aided joint channel estimation and equalization for ofdm systems in very rapidly varying mobile channels this paper is concerned with the challenging and timely problem of joint channel estimation and equalization for orthogonal frequency division multiplexing ofdm systems in the presence of frequency selective and very rapidly time varying channels the resulting algorithm is based on the space alternating generalized expectation maximization maximum a posteriori probability sage map technique which is particularly well suited to multicarrier signal formats the algorithm is implemented in the time domain which enables one to use the gaussian approximation of the transmitted ofdm samples consequently the averaging process of the nonpilot data symbols becomes analytically possible resulting in a feasible and computationally efficient channel estimation algorithm leading to a receiver structure that yields also an equalized output from which the data symbols are detected with excellent symbol error rate ser performance based on this gaussian approximation the exact bayesian cramer rao lower bound crlb as well as the convergence rate of the algorithm are derived analytically to reduce the computational complexity of the algorithm discrete legendre orthogonal basis functions are employed to represent the rapidly time varying fading channel it is shown that depending on the normalized doppler frequency only a small number of expansion coefficients is sufficient to approximate the channel very well and there is no need to know the correlation function of the input signal the computational complexity of the algorithm is shown to be o nl per detected data symbol and per sage map algorithm cycle where is the number of ofdm subcarriers and is the number of multipath components

blind receiver design for ofdm systems over doubly selective channels we develop blind data detectors for orthogonal frequency division multiplexing ofdm systems over doubly selective channels by exploiting both frequency domain and time domain correlations of the received signal we thus derive two blind data detectors a time domain data detector and a frequency domain data detector we also contribute a reduced complexity suboptimal version of a time domain data detector that performs robustly when the normalized doppler rate is less than 3 our frequency domain data detector and suboptimal time domain data detector both result in integer least squares ls problems we propose the use of the v blast detector and the sphere decoder the time domain data detector is not limited to the doppler rates less than 3 but cannot be posed as an integer ls problem our solution is to develop an iterative algorithm that starts from the suboptimal time domain data detector output we also propose channel estimation and prediction algorithms using a polynomial expansion model and these estimators work with data detectors decision directed mode to reduce the complexity the estimators for the channel statistics and the noise variance are derived using the likelihood function for the data our blind data detectors are fairly robust against the parameter mismatch

eigenstructure based partially adaptive array design a procedure is presented for designing partially adaptive arrays having nearly fully adaptive performance under steady state conditions theory predicts that the required adaptive dimension is less than or equal to the rank of the spatially temporally correlated portion of the interference correlation matrix for arbitrary linearly constrained minimum variance beamformers knowledge of the eigenstructure of the interference correlation matrix is required to implement a beamformer with this adaptive dimension to avoid adaptive estimation of the eigenstructure the eigenstructure of an averaged correlation matrix which spans the interference scenarios of interest is utilized and the adaptive dimension is given by the rank of the averaged correlation matrix construction of a transformation for reducing adaptive dimension is addressed simulations illustrating the utility of the eigenstructure approach are provided

optimum clustered pilot sequence for ofdm systems under rapidly time varying channel as channel time variation increases orthogonality among subcarriers in an orthogonal frequency division multiplexing ofdm symbol can be destroyed because of the relatively long symbol period whereupon intercarrier interference ici appears and hence an irreducible error floor occurs it was shown recently that grouping pilot tones into a number of equally spaced clusters can yield better channel estimation against the doubly selective channel than placing each pilot tone in an equally spaced manner however a random pilot sequence was used in the literature and an optimal sequence has not yet been studied this paper presents how to optimize the clustered pilot sequence first this paper a proves the existence of an optimum pilot sequence and b suggests a guideline for finding an optimum sequence second this paper shows that an optimum sequence is independent of the signal to noise ratio and doppler rate and the sequence is generally optimum for any channels third this paper verifies through analysis and simulation that the coded ofdm system with an optimized cluster pilot sequence can yield a smaller mean square channel estimation error and lower bit error rate than the system with an equidistance pilot or a random cluster pilot under a rapidly time varying channel

time domain iterative multiregion technique for 3 d scattering and radiation problems integration of the finite difference time domain fdtd method into the iterative multiregion imr technique an iterative approach used to solve large scale electromagnetic scattering and radiation problems is presented in this paper the idea of the imr technique is to divide a large problem domain into smaller subregions solve each subregion separately and combine the solutions of subregions after introducing the effect of interaction to obtain solutions at multiple frequencies for the large domain solutions of the subregions using frequency domain solvers has been the preferred approach as such solutions using time domain solvers require computationally expensive bookkeeping of time signals in this paper we present an algorithm that makes it feasible to use the fdtd method a time domain numerical technique in the imr technique to obtain solutions at a prespecified number of frequencies in a single simulation a hybrid method integrated into the imr technique is also presented in this paper this hybrid method combines the desirable features of the fdtd method and the method of moments mom to solve radiation problems more efficiently as a result a considerable reduction in memory storage requirements and computation time is achieved

a spectral approach to inter carrier interference mitigation in ofdm systems in this paper we propose a new method for intercarrier interference ici mitigation in orthogonal frequency division multiplexing ofdm systems the proposed approach views the signal reconstruction problem at the receiver end as an integer least squares ils problem and uses a recently developed spectral approach called sequential probabilistic ils spils to solve it the proposed approach outperforms other state of the art approaches while having the same computational complexity in addition we present a novel extension to the spils scheme that allows the generation of soft decisions for communication systems which use soft decision decoding the use of soft decision decoding naturally brings significant improvement in the detection reliability and we show that the proposed method again outperforms other state of the art approaches to better address the tradeoff between performance and complexity we first suggest a novel method to reduce the number of matrix inversions required and hence to reduce the implementation complexity without any degradation in performance we also introduce a novel low complexity scheme termed quick spils qspils in which we lose a little in detection reliability but significantly reduce the implementation complexity index terms digital communications

orthogonal frequency division multiplexing with index modulation in this paper a novel orthogonal frequency division multiplexing ofdm scheme called ofdm with index modulation ofdm im is proposed for operation over frequency selective and rapidly time varying fading channels in this scheme the information is conveyed not only by m ary signal constellations as in classical ofdm but also by the indices of the subcarriers which are activated according to the incoming bit stream different low complexity transceiver structures based on maximum likelihood detection or log likelihood ratio calculation are proposed and a theoretical error performance analysis is provided for the new scheme operating under ideal channel conditions then the proposed scheme is adapted to realistic channel conditions such as imperfect channel state information and very high mobility cases by modifying the receiver structure the approximate pairwise error probability of ofdm im is derived under channel estimation errors for the mobility case several interference unaware aware detection methods are proposed for the new scheme it is shown via computer simulations that the proposed scheme achieves significantly better error performance than classical ofdm due to the information bits carried by the indices of ofdm subcarriers under both ideal and realistic channel conditions

low complexity equalization of time varying channels with precoding this correspondence deals with time varying tv single input multiple output simo channels which are both frequency selective due to high data rate and time selective due to mobility a complex exponential basis expansion model ce bem is used to model the channel we consider a block transmission system where on the transmit side a precoder is employed to enable the maximum available diversity for a ce bem channel after direct decoding on the receive side the resulting channel resembles a finite impulse response fir filter on both block and symbol level we therefore propose an equalizer that bears a structure analogous to the effective channel in comparison with a standard block minimum mean square error decision feedback equalizer bmmse dfe that involves the inversion of a large size matrix the proposed parametric equalizer renders a similar performance but at a lower computational cost if there are multiple outputs present another contribution of this correspondence is a semiblind algorithm to estimate this equalizer when the channel state information is not available the equalizer taps and the information symbol estimates are refined recursively by means of normalized least mean squares nlms adaptation

low complexity equalization for doubly selective channels modeled by a basis expansion we propose a novel equalization method for doubly selective wireless channels whose taps are represented by an arbitrary basis expansion model bem we view such a channel in the time domain as a sum of product convolution operators created from the basis functions and the bem coefficients equivalently a frequency domain channel can be represented as a sum of convolution products the product convolution representation provides a low complexity memory efficient way to apply the channel matrix to a vector we compute a regularized solution of a linear system involving the channel matrix by means of the gmres and the lsqr algorithms which utilize the product convolution structure without ever explicitly creating the channel matrix our method applies to all cyclic prefix transmissions in an ofdm transmission with k subcarriers each iteration of gmres or lsqr requires only o k k flops and o k memory additionally we further accelerate convergence of both gmres and lsqr by using the single tap equalizer as a preconditioner we validate our method with numerical simulations of a wimax like system ieee 802 16e in channels with significant delay and doppler spreads the proposed equalizer achieves bers comparable to those of mmse equalization and noticeably outperforms low complexity equalizers using an approximation by a banded matrix in the frequency domain with preconditioning the lowest bers are obtained within 3 16 iterations our approach does not use any statistical information about the wireless channel