A New Algorithm for Optimum Multiuser Detection in Synchronous CDMA Systems

Optimum multiuser detection for Code Division Multiple Access (CDMA) systems requires the solution of an NP-hard combinatorial optimization problem. It is well known that the computational complexity of the optimum multiuser detector is exponential with the number of active users in the system. In order to reduce the complexity of the optimum multiuser detection, we propose a Reduced Complexity Maximum Likelihood (RCML) algorithm that includes a set of novel certain boundary rules and characteristics. We investigate the performance and complexity tradeoffs for the RCML algorithm by conducting a set of simulations; Maximum Likelihood (ML) detection as a reference for performance comparisons, and relaxation based Semidefinite Programming (SDPB) algorithm as a reference for complexity comparisons. We show that the RCML algorithm is a promising algorithm for its computational savings over relaxation based algorithms in lightly-to-moderately loaded CDMA systems, and for its optimality in highly loaded CDMA systems.     

Experimental Verification of an On-Line Calibration Algorithm for a Circular CDMA Antenna-Array

We present a new on-line internal calibration algorithm for a CDMA (code division multiple access) antenna array. The proposed calibration algorithm gives an accurate calibration vector, and as byproducts, the vector-channel parameters of an arbitrarily selected reference user.     

Time Offsets for Non-orthogonal CDMA Signals

A simple interference reduction method for non-orthogonal downlink CDMA channels is presented in which non-orthogonal CDMA signals are transmitted with different time offsets. An expression of multiple access interference (MAI) is developed and a reduction of MAI power is evaluated by using a numerical method and a computer simulation. The value of the time offset is determined in order to minimize the MAI and the implementation complexity. The performance of the proposed method is evaluated for various channel models.     

Statistical Evaluation of Adaptive Threshold Alignment for a Multi-Carrier CDMA Code Synchronization

This letter presents a statistical analysis of the decision threshold for a multi-carrier (MC) DS/CDMA acquisition system with a reference filter. The probabilities of detection and false alarm are derived, and the mean acquisition time is evaluated as a measure of the system performance. From the results, it is shown that in the performance analysis of the hybrid acquisition system with reference filtering, the statistical evaluation of the decision threshold seems more appropriate than the approximation of the decision threshold adopted in other schemes [7,8].     

Time Delay Estimation by Cross-covariance Maximization of Quadrature Sampled Narrowband Signals

A numerically efficient algorithm for estimating the time delay from observations of a stationary narrowband signal and its delayed version is investigated. Quadrature sampling, a variant of bunched sampling, is applied to estimate samples of the quadrature components of the cross-covariance function of the two signals. The baseband magnitude squared of this function can be maximized for time delay estimation. Because the time delay is unknown, the baseband cross-covariance function cannot be interpolated from the estimated samples. Numerical maximization of the samples' magnitude squared and quadratic interpolation, however, results in a reasonable time delay estimate.     

Spectral Efficiency of Cellular Mobile Radio Systems with Different Traffic Loads

A lower bound on the Shannon capacity of the uplink channel of a narrowband cellular system is evaluated as a function of the reuse distance and the level of shadowing affecting the interfering signals assuming perfect channel state information (CSI) at the receiver. The effect of the first two tiers of interferers is determined, and compared with a simplified evaluation that takes into account only the first tier.Spectral efficiency is evaluated for different traffic loads, to assess the usefulness of traffic control. The performance improvementthat may be obtained by adopting the space diversity, to avoid deep fades in the received signal, is also evaluated. Our results show that traffic control may have a negative effect on the system performance.     

Novel Transimpedance Type First-Order All-Pass Filter Using Single Otra

A novel transimpedance-mode first-order all-pass filter configuration is proposed. It uses a single operational transresistance amplifier (OTRA) and few passive components. The proposed circuit is structurally all-pass that does not impose any component matching condition. The proposed circuit is insensitive to parasitic input capacitances and input resistances due to the internally grounded input terminals of the OTRA. It provides an alternative realization of phase equalizers for analog signal processing applications by providing also current-to-voltage conversion. The theoretical results are verified with PSPICE simulations using a CMOS realization of OTRA.     

Hidden Markov Modeling of Error Patterns and Soft Outputs for Simulation of Wideband CDMA Transmission Systems

In this paper, a Hidden Markov Modeling (HMM) technique for a fast and accurate simulation of bit errors and soft outputs in wireless communication systems is presented. HMMs with continuous probability distributions are considered. Soft outputs and bit errors are combined to error patterns. We focus on binary phase–shift keying (BPSK) modulation for direct–sequence spread spectrum (code–division multiple access, CDMA) transmission as proposed e.g. for the third generation wireless communication system UMTS (uplink for the frequency division duplex mode (FDD)). Comparisons of simulated bit error rates for HMM models and Rake receivers are shown for AWGN, flat fading, and vehicular channel conditions. In order to assess the ability of the HMM to describe the dynamical behaviour of the channel a comparison for transmission with interleaving and convolutional coding is presented. Furthermore calculated autocorrelation functions of the error patterns and error gap distributions corresponding to the Rake receiver and to the HMM, respectively, are presented. Our investigations show a strong dependence of the required HMM order on E_b/N₀ and the channel conditions. The degree of accordance of the HMM outputs and the training data is examined based on calculated statistical scoring indicators.     

Electromagnetic Prospection via Homogeneous and Inhomogeneous Plane Waves: The Case of an Embedded Slab

This paper deals with the linear approximation of the inverse scattering problem for a dielectric slab embedded into a homogeneous half-space. After a proper normalization of the scattered field, the problem at hand is recognized to be related to a Fourier transform inversion with limited data. This permits to resort to the well-known results available in signal processing in order to discuss about the class ofthe retrievable unknowns and the resolution limits achievable by inversion. In particular, the role of both homogeneous and inhomogeneous incident plane waves in carrying out information about the profile under test is investigated and the role played by the dielectric permittivity of the host medium in determining the information content of the scattered field is pointed out.     

Selection of Input Stimulus for Fault Diagnosis of Analog Circuits Using ARMA Model

The paper addresses the problem of fault diagnosis of analog circuits based on dictionary approach. The proposed approach first identifies an adequate set of test frequencies to optimize the process of detection and isolation of simulated fault scenarios. The circuit under test (CUT) is then excited by an input stimulus composed of a set of sinusoidal waveforms with the selected test frequencies. The circuit response, at different fault scenarios, is preprocessed by an autoregressive moving average (ARMA) model to yield a set of features formulating the fault dictionary. Collected features are utilized to train and test a back-propagation (BP) neural network (NN) based classifier. Demonstrative results from soft fault simulation of two active circuit examples prove the excellent effectiveness of the proposed algorithm.     

Deterministic Maximum likelihood Approach for Localization of Near-field Sources

In this paper we proposed deterministic maximum likelihood approach for estimating the direction of arrival and range parameters of the near-field sources. Direct maximum likelihood estimation of near-field source parameters results in complicated multi-parameter optimization problems, we therefore reformulated the estimation problem in terms of actual-datasample, called the incomplete data and a hypothetical dataset, called the complete data and then devised the Expectation/Maximization iterative method for obtaining maximum likelihood estimates. The Expectation/Maximization algorithm decomposes the observed data into its components and then estimates the parameters of each signal component separately providing computationally efficient solution to the resulting optimization problem. The applicability and effectiveness of the proposed algorithm is illustrated by some numerical simulations.     

Model Order Estimation for Joint Demodulation in DS-CDMA Systems

An iterative algorithm for estimating the number of relevant transmission paths characterized by the model order vector L in the uplink of a direct sequence code division multiple access (DS-CDMA) system is presented. Estimates of simplelower bounds on the probability of correct one-shot symbol estimates are formulated as a function of bit-error rate estimates of all users and maximized w.r.t. L. The joint demodulation for a given value Lis accomplished using a combination of a space-alternating generalized expectation-maximization (SAGE) algorithm for channel parameter estimation and a°multistage detector. Simulation results indicate that the modelorder is estimated correctly even in situations with considerably differentsignal power levels.     

A New State-space-based Algorithm to Assess the Stability of the Finite-difference Time-domain Method for 3D Finite Inhomogeneous Problems

The finite-difference time-domain (FDTD) method is an explicit time discretization scheme for Maxwell's equations. In this context it is well-known that explicit time discretization schemes have a stability induced time step restriction. In this paper, we recast the spatial discretization of Maxwell's equations, initially without time discretization, into a more convenient format, called the FDTD state-space system. This in turn allows us to derive a new algorithm in order to determine the stability limit of FDTD for lossy, inhomogeneous finite problems. It is shown that a crucial parameter is the spectral norm of the matrix resulting from the spatial discretization of the curl operator. In a rectangular simulation domain the time step upper bound can be calculated in closed form and results in a time step limit less stringent than the Courant condition. Finally, the validity of the technique is illustrated by means of some pertinent numerical examples.     

Low-Voltage Low-Power CMOS RF Four-Quadrant Multiplier

This paper describes a CMOS four-quadrant multiplier intended for use in the front-end receiver by utilizing the square-law characteristic of the MOS transistor in the saturation region. The circuit was simulated in standard 0.5 μm CMOS level 3 MOSIS (BSIM3 SPICE-based). The mixer has a third-order inter modulation (IM3) of 34.7 dBmV, a third-order intercept point (IP3) of -5.7 dBm, 1-dB compression (P-1dB) of -10.4 dBm and the power consumption is 1.18 mW from a single 1.5 V power supply. One of the features of the proposed design is using two MOS transistors limitation to reduce the supply voltage, which leads to reduce the power consumption.     

A Wave Approach to Signal and Noise Modeling of Dual-Gate MESFET

A new procedure for signal and noise modeling of dual-gate MESFET is described in this paper. The small-signal model is based on two cascoded single-gate MESFET intrinsic equivalent circuits embedded in a network representing device parasitics. The wave interpretation of noise is used for defining the noise parameters of each single gate MESFET. Applying this approach, a CAD oriented procedure for extracting the dual-gate MESFETmodel parameters as well as the noise wave temperatures is developed. Modeled scattering and noise parameter characteristics are comparedto the measured ones and quite a good agreement is observed.     

Low Voltage Digitally Controlled CMOS Current Conveyor

In this paper, a digitally controlled current conveyo(DCCC) is presented. The proposed DCCC is based on rail-to-rail folded cascode implementation with a current division network (CDN). The CDN is used to provide control on the current gain of the DCCC. The CDN uses a novel current division technique based on differential pairs. The proposed DCCC can operate from ±1.5 V supply voltages. Applications of the proposed DCCC such as variable gain amplifiers (VGA) and digitally tunedfilters have been investigated. PSpice simulations based on the AMI 1.2 µm N-well level 3 parameters are in agreement with the presented work.     

Application of Fuzzy Genetic Algorithm to the Problem of Synthesizing Circular Microstrip Antenna Elements with Thick Substrates

Fuzzy genetic algorithm (FGA) is applied to the problem of synthesizing a probe-fed circular microstrip antenna element with thick substrate taking resonant frequency, gain, bandwidth and input impedance of the antenna element into consideration simultaneously. The results obtained by FGA are compared with the results obtained by classical genetic algorithm (CGA).     

Transparent Optical Intersatellite Link Using Double-Sideband Modulation and Homodyne Reception

A coherent, transparent optical communication system has been developed for free-space lasercomm terminals. The laser synchronisation scheme has been chosen such that the transmission of either an ensemble of subcarrier-modulated signals or of a binary baseband signal is equally possible. By using double-sideband modulation and an average-power-limited booster amplifier, the link capacity for subcarrier-multiplexed transmission approaches that of a homodyne PSK transmission system to within 0.5 dB.     

Network Management Information Models

In the first part of this paper, a basic introduction to network management is given and the demand for standards and models of management information is addressed. Existing and new network management standards are then classified, and an overview on the Common Information Model CIM and the Web-Based Enterprise Management WBEM is given. Furthermore, classes for a Network Security Service and finally a Firewall Policy Model are proposed. The proposals are CIM extensions for security management and are being discussed with the Distributed Management Task Force DMTF to become a standard.     

Optimal Equalization of Distortions due to Group Delay Ripples of Chirped Fiber Bragg Gratings (CFBG)

We present a closed solution for an optimal equalizer which compensates for signal distortions caused by group delay ripples in chirped fiber Bragg gratings (CFBG) employed for dispersion compensation in optical communication systems. The theory is verified with the help of group delay measurement results of a dispersion compensating CFBG and the improvements achieved by the equalizer are presented on the basis of simulation results.