time variant channel estimation using discrete prolate spheroidal sequences we propose and analyze a low complexity channel estimator for a multiuser multicarrier code division multiple access mc cdma downlink in a time variant frequency selective channel mc cdma is based on orthogonal frequency division multiplexing ofdm the time variant channel is estimated individually for every flat fading subcarrier assuming small intercarrier interference the temporal variation of every subcarrier over the duration of a data block is upper bounded by the doppler bandwidth determined by the maximum velocity of the users slepian showed that time limited snapshots of bandlimited sequences span a low dimensional subspace this subspace is also spanned by discrete prolate spheroidal dps sequences we expand the time variant subcarrier coefficients in terms of orthogonal dps sequences we call slepian basis expansion this enables a time variant channel description that avoids the frequency leakage effect of the fourier basis expansion the square bias of the slepian basis expansion per subcarrier is three magnitudes smaller than the square bias of the fourier basis expansion we show simulation results for a fully loaded mc cdma downlink with classic linear minimum mean square error multiuser detection the users are moving with 19 4 m s using the slepian basis expansion channel estimator and a pilot ratio of only 2 we achieve a bit error rate performance as with perfect channel knowledge

local preferential attachment model for hierarchical networks abstract in real life networks incomers may only connect to a few others in a local area for their limited information and individuals in a local area are likely to have close relations accordingly we propose a local preferential attachment model here a local area network stands for a node and all its neighbors and the new nodes perform nonlinear preferential attachment k i k i in local areas the stable degree distribution and clustering degree correlations are analytically obtained with the increasing of the clustering coefficient increases while assortativity decreases from positive to negative in addition by adjusting the parameter the model can generate different kinds of degree distribution from exponential to power law the hierarchical organization independent of is the most significant character of this model

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

tight pep lower bound for constellation rotated vector ofdm under carrier frequency offset and fast fading in this paper we study pairwise error probability pep of a system employing constellation rotated vector orthogonal frequency division multiplexing crv ofdm which is an enhancement of vector ofdm v ofdm with enhanced diversity over a frequency selective channel with both carrier frequency offset cfo and doppler frequency spread first we extend the work of rugini and banelli which is known to give a good bit error rate ber approximation for the standard ofdm system only with cfo to pep evaluation for crv ofdm and discuss its limitations then we discuss the conditionally gaussian characteristi2cs of the doppler induced interference and propose a conditionally gaussian pep approximation confirming a good match of the conditionally gaussian pep approximation and with simulation results we propose a new semianalytical pep lower bound the new lower bound is computationally feasible and is shown by simulation to be tight for small to moderate cfo finally a computationally more feasible approximation of the lower bound is also proposed our discussion reveals as a by product why the gaussian assumption for the doppler induced interference works for standard ofdm systems although the assumption is actually incorrect

local world evolving networks with tunable clustering abstract we propose an extended local world evolving network model including a triad formation tf step in the process of network evolution random fluctuation in the number of new edges is involved we derive analytical expressions for degree distribution clustering coefficient and average path length our model can unify the generic properties of real life networks scale free degree distribution high clustering and small inter node separation moreover in our model the clustering coefficient is tunable simply by changing the expected number of tf steps after a single local preferential attachment step

detrended fluctuation analysis of particle condensation on complex networks abstract it has been found that the structure of complex network has significant influence on the condensation of particles and the equilibrium state of particles is dynamical we study here the fluctuation of particles on each individual node at equilibrium status and find that the particle distributions can be normalized to the same framework of gaussian distribution which is independent of the nodes and network structures while their differences are reflected by the asymmetric degree of the distribution by the approach of detrended fluctuation analysis we reveal that the correlation exponents of particle fluctuations may reflect the information of particle condensation and be useful in exploring the network structure

optimum frequency domain partial response encoding in ofdm system time variance of wireless channels destroys the orthogonality among subchannels in orthogonal frequency division multiplexing ofdm systems and causes interchannel interference ici which results in an error floor in this paper we study frequency domain partial response coding prc for reducing the effect of the ici based on the general expression of the ici power for ofdm with prc the optimum weights for prc that minimize the ici power are derived from the numerical and simulation results optimum prc for ofdm can reduce the ici effectively by 4 0 db for two tap prc and 6 2 db for three tap prc respectively

structure properties of collaboration network with tunable clustering abstract a collaboration network model with improved triad formation itf step is proposed and denominated as one mode collaboration network with tunable clustering we systematically study its structural features and find this network model can much more accurately reflect cooperative relationship in real society we acquire expression of nodes degree distribution p k that possesses a power law form with exponent whose range is 1 3 expression of clustering coefficient c k for nodes with degree k is also obtained and composes of a power law section and a constant section we verify clustering coefficient of individual nodes increases monotonically with the increase of parameter p that is the probability of itf step occurring when specific forms of p k and c k are given the model s average clustering coefficient c can be derived approximately finally we analytically testify average path length l n increases with network size n in accordance with logarithmic form when n is fixed average path length of entire network decreases with the increase of p all theoretically predicted results are consistent with simulative experimental results

the influence of preferential attachment on evolving networks abstract with the consideration of local events and preferential attachment the extended model proposed by albert and barabasi is studied firstly the extended model is built only by local events it is found that without preferential attachment the networks connectivity distribution decays exponentially for large connectivity and the decaying exponent varies with different probabilities of the addition of new links and rewiring secondly the extended model with partial preferential attachment is studied when preferential attachment only presents in the addition of new links or new nodes there is scale free regime in the phase diagram while preferential attachment is brought into the rewiring there coexist two regimes i e the exponential regime and the scale free regime thirdly fitness is introduced to the extended model with complete preferential attachment it is obtained that the network s connectivity distribution is a result affected by many factors including the fitness the local events and preferential attachment

bounds on the interchannel interference of ofdm in time varying impairments in this article tight and universal bounds have been derived for the interchannel interference ici of an orthogonal frequency division multiplexing ofdm signal resulting from doppler spread the universal bound depends only on the product of the maximum doppler frequency and the ofdm symbol duration the tight bound also depends on the variance of the doppler spectrum compared with the exact ici expressions derived by other researchers these bounds are easier to evaluate and can provide useful insight

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

effect of node deleting on network structure abstract the ever increasing knowledge of the structure of various real world networks has uncovered their complex multi mechanism governed evolution processes therefore a better understanding of the structure and evolution of these networked complex systems requires us to describe such processes in a more detailed and realistic manner in this paper we introduce a new type of network growth rule which comprises addition and deletion of nodes and propose an evolving network model to investigate the effect of node deleting on network structure it is found that with the introduction of node deleting network structure is significantly transformed in particular degree distribution of the network undergoes a transition from scale free to exponential forms as the intensity of node deleting increases at the same time nontrivial disassortative degree correlation develops spontaneously as a natural result of network evolution in the model we also demonstrate that node deleting introduced in the model does not destroy the connectedness of a growing network so long as the increasing rate of edges is not excessively small in addition it is found that node deleting will weaken but not eliminate the small world effect of a growing network and generally it will decrease the clustering coefficient in a network

general connectivity distribution functions for growing networks with preferential attachment of fractional power we study the general connectivity distribution functions for growing networks with preferential attachment pa of fractional power i k using simon s method we first show that the heart of the previously known methods of the rate equations for the connectivity distribution functions is nothing but simon s method for word problem secondly we show that for the case of fractional the z transformation of the rate equation provides a fractional differential equation of a new type which coincides with that for pa with linear power when 1 we show that to solve such a fractional differential equation we need to define a transcendental function a b c z that we call upsilon function most of all the previously known results are obtained consistently in the framework of a unified theory

nonlinear barabasi albert network abstract in recent years there has been considerable interest in the structure and dynamics of complex networks one of the most studied networks is the linear barabasi albert model here we investigate the nonlinear barabasi albert growing network in this model a new node connects to a vertex of degree k with a probability proportional to k real each vertex adds m new edges to the network we derive an analytic expression for the degree distribution p k which is valid for all values of m and 1 in the limit the network is homogeneous if 1 there is a gel phase with m super connected nodes it is proposed a formula for the clustering coefficient which is in good agreement with numerical simulations the assortativity coefficient r is determined and it is shown that the nonlinear barabasi albert network is assortative disassortative if 1 and no assortative only when 1 in the limit the assortativity coefficient can be exactly calculated we find r 7 13 when m 2 finally the minimum average shortest path length l min is numerically evaluated increasing the network size l min diverges for 1 and it is equal to 1 when 1

highly clustered scale free networks we propose a model for growing networks based on a finite memory of the nodes the model shows stylized features of real world networks power law distribution of degree linear preferential attachment of new links and a negative correlation between the age of a node and its link attachment rate notably the degree distribution is conserved even though only the most recently grown part of the network is considered as the network grows the clustering reaches an asymptotic value larger than that for regular lattices of the same average connectivity and similar to the one observed in the networks of movie actors coauthorship in science and word synonyms these highly clustered scale free networks indicate that memory effects are crucial for a correct description of the dynamics of growing networks

accelerating hyperaccelerating and decelerating networks many growing networks possess accelerating statistics where the number of links added with each new node is an increasing function of network size so the total number of links increases faster than linearly with network size in particular biological networks can display a quadratic growth in regulator number with genome size even while remaining sparsely connected these features are mutually incompatible in standard treatments of network theory which typically require that every new network node possesses at least one connection to model sparsely connected networks we generalize existing approaches and add each new node with a probabilistic number of links to generate either accelerating hyperaccelerating or even decelerating network statistics in different regimes under preferential attachment for example slowly accelerating networks display stationary scale free statistics relatively independent of network size while more rapidly accelerating networks display a transition from scale free to exponential statistics with network growth such transitions explain for instance the evolutionary record of single celled organisms which display strict size and complexity limits

directed accelerated growth application in citation network abstract in many real world networks the number of links increases non linearly with the number of nodes models of such accelerated growth have been considered earlier with deterministic and stochastic number of links here we consider stochastic accelerated growth in a network where links are directed with the number of outgoing links following a power law distribution the results are similar to the undirected case as the accelerated growth is enhanced the degree of distribution becomes independent of the initial attractiveness a parameter which plays a key role in directed networks as an example of a directed model with accelerated growth the citation network is considered in which the distribution of the number of outgoing link has an exponential tail the role of accelerated growth is examined here with two different growth laws

on the design of comb spectrum code for multiple access scheme a new methodology that applies frequency orthogonality to the spreading code design has been presented the main purpose is to reduce the receiver complexity of the cyclic prefix code division multiple access cp cdma over the downlink channel and to enable parallel transmission over the uplink channel the receiver complexity has been found for requiring full band fft and ifft calculations in the channel equalizer to handle this problem we developed the comb spectrum codes organized in frequency orthogonal groups in each of which the codes have time correlation orthogonality for the downlink channel the reduction of complexity in signal processing of a cscdma receiver is due to that the separation of groups is simple and each separated group is in the form of a narrower bandwidth cp cdma thus the full band fft and ifft can be replaced by the partial fft and partial ifft as shown in this proposed approach for the uplink channel we assign the code groups to different users to form a group division multiple access gdma for enabling an independent channel equalization of each user at base station the theoretical analysis and the simulation results agree well with each other and both confirm the effectiveness of this approach

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

generalizations of the projection methods with application to sor theory for hermitian positive semidefinite linear systems ann n complex matrixb is calledparacontracting if b 2 1 and 0 x n i b bx 2 x 2 we show that a productb b k b k 1 b1 ofk paracontracting matrices is semiconvergent and give upper bounds on the subdominant eigenvalue ofb in terms of the subdominant singular values of theb i s and in terms of the angles between certain subspaces our results here extend earlier results due to halperin and due to smith solomon and wagner we also determine necessary and sufficient conditions forn numbers in the interval 0 1 to form the spectrum of a product of two orthogonal projections and hence characterize the subdominant eigenvalue of such a product in the final part of the paper we apply the upper bounds mentioned earlier to provide an estimate on the subdominant eigenvalue of the sor iteration matrix associated with ann n hermitian positive semidefinite matrixa none of whose diagonal entries vanish

further results on paracontracting matrices and correction to optimal semistable control in ad hoc network systems a sequential two stage approach this technical note has twofold aspects the first aspect is to present some new results on characterization of matrix paracontraction and the second aspect is to correct some errors in the author s previous publication by means of these newly developed results for paracontraction specifically new necessary and or sufficient conditions to characterize paracontracting matrices using discrete time semistability singular value decomposition normal matrices and range symmetric matrices are provided using these results one can rectify an error in the author s previous publication

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

on the convergence of asynchronous paracontractions with application to tomographic reconstruction from incomplete data abstract convergence of iterative processes in c k of the form x i r i j i x 1 r i 1 1 p j i x i where j i 1 2 n i 1 2 is analyzed it is shown that if the matrices p 1 p n are paracontracting in the same smooth strictly convex norm and if the sequence j i i 1 has certain regularity properties then the above iterates converge this result implies the convergence of a parallel asynchronous implementation of the algebraic reconstruction technique art algorithm often used in tomographic reconstruction from incomplete data

condensation in a zero range process on weighted scale free networks we study the condensation phenomenon in a zero range process on weighted scale free networks in order to show how the weighted transport influences the particle condensation instead of the approach of grand canonical ensemble which is generally used in a zero range process we introduce an alternate approach of the mean field equations to study the dynamics of particle transport we find that the condensation on the scale free network is easier to occur in the case of weighted transport than in the case of weight free networks in the weighted transport especially a dynamical condensation is even possible for the case of no interaction among particles which is impossible in the case of weight free networks

statistical properties and attack tolerance of growing networks with algebraic preferential attachment we consider growing networks with algebraic preferential attachment and address two questions 1 what is the effect of temporal fluctuations in the number of new links acquired by the network and 2 what is the network tolerance against random failures and intentional attacks we find that the fluctuations generally have little effect on the network properties although they lead to a plateau behavior for small degrees in the connectivity distribution formulas are derived for the evolution and distribution of the network connectivity which are tested by numerical simulations numerical study of the effect of failures and attacks suggests that networks constructed under algebraic preferential attachment are more robust than scale free networks

performance of multicarrier ds ssma over fast rayleigh fading channels with doppler diversity the significant loss in performance of multicarrier direct sequence spread spectrum multiple access systems over fast fading channels is investigated first the channel model for individual subchannels is obtained using a canonical decomposition to a wide sense stationary wss uncorrelated scattering channel next a receiver structure which features both doppler diversity and frequency diversity is presented and analyzed it is found that large doppler spreads cause the average magnitude of the desired signals to diminish dramatically expressions for the bit error rate are derived for both uncoded and coded systems by applying the standard gaussian approximation numerical results which show that doppler diversity is preferable over frequency diversity in fast fading channels are provided for different combinations of the diversity orders numerical results which show that the intersymbol and intersubchannel interference can be effectively suppressed and the multiple access interference tends to degenerate into a wss process with the statistical periodicity destroyed by the fast fading channels are also provided

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