An efficient code-timing estimator for receiver diversity DS-CDMAsystems

We propose an efficient algorithm for estimating the code timing of direct-sequence code-division multiple-access (DS-CDMA) systems that consist of an arbitrary antenna array at the receiver and work in a flat-fading and near-far environment. The algorithm is an asymptotic (for large number of data samples) maximum-likelihood (ML) estimator that is derived by modeling the known training sequence as the desired signal and all other signals including the interfering signals and the additive noise as unknown colored Gaussian noise. The algorithm does not require the search over a parameter space and the code timing is obtained by rooting a second-order polynomial, which is computationally very efficient. Simulation results show that the algorithm is quite robust against the near-far problem and channel fading. It requires a shorter training sequence than the single-antenna-based estimators     

Code timing estimator for DS-CDMA signals in slow fading multipathchannels

A maximum likelihood algorithm for code timing estimation of DS-CDMA signals in slow fading multipath channels is presented. As an extension to a recently presented method, it decomposes a multidimensional minimisation problem into a smaller problem. Simulation results show that the proposed estimator is near-far resistant     

A deterministic multiuser code-timing estimator for long-code band-limited CDMA systems

In this letter, we present a deterministic multiuser code-timing estimator for asynchronous direct-sequence (DS) code-division multiple-access (CDMA) systems with aperiodic long spreading codes and band-limited chip waveforms. A key feature of the proposed estimator is that it captures and capitalizes a deterministic structure of the overall interference, namely multi-access interference (MAI) and intersymbol interference (ISI), in the frequency domain. This allows complete interference elimination in a deterministic manner, which is in general more effective and data-efficient than stochastic approaches. Numerical results show that the proposed estimator can achieve fast acquisition; it is also near-far resistant, providing accurate code acquisition for even overloaded systems (i.e., systems with more users than the processing gain) in multipath fading environments.     

Improved MUSIC algorithm for the code-timing estimation of DS-CDMA multipath-fading channels in multiantenna systems

A multiple-signal-classification (MUSIC) approach of estimating the code timings of a desired user is considered for direct-sequence code-division multiple-access (DS-CDMA) multipath-fading channels when exploiting multiple receive antennas with either spatially uncorrelated or fully correlated fading. The acquisition performance of the conventional MUSIC timing estimator employing a single antenna is not good for the small size of observation windows and low signal-to-noise ratios (SNRs). Multiple antennas allow for rapid acquisition and lowers the range of detectable SNR. An efficient and improved MUSIC algorithm of estimating the multipath timings of a desired user for DS-CDMA systems is presented. In multipath-fading channels, the solution of the proposed algorithm is based on successively optimizing the cost function for increasing numbers of multipath delays, which does not require a multidimensional search for multidelay paths. Furthermore, the estimate of code timing at each path is obtained by finding the zeros of second-order polynomials, which is computationally efficient. The proposed MUSIC algorithm significantly improves the acquisition performance of conventional MUSIC algorithm in the presence of multipath time-varying Rayleigh-fading channels with arbitrary time delays. The acquisition performance of multiple antennas-based MUSIC timing estimators is much better than that of a single-antenna-based timing estimator. The Crame/spl acute/r-Rao bound for the code-timing estimator based on multiple antennas is presented.     

Multiuser ML sequence estimator for convolutionally codedasynchronous DS-CDMA systems

The optimal multiuser sequence estimator is formulated for an asynchronous direct-sequence code-division multiple-access (DS-CDMA) system where each user employs convolutional coding to improve its performance on a nondispersive additive white Gaussian noise (AWGN) channel. It is shown that the decoder may be implemented efficiently using a Viterbi algorithm which operates on a time-varying trellis with a number of states which is exponential in the product of the number of users in the system and the constraint length of the codes used (for the rate 1/2 code case). The asymptotic efficiency of this receiver relative to an uncoded coherent binary phase shift keying (BPSK) receiver (termed asymptotic multiuser coding gain, or AMCG) is then upper and lower bounded. The AMCG parameter unifies the asymptotic coding gain parameter and the asymptotic multiuser efficiency parameter which are traditional figure of merit parameters for single-user coded systems and multiuser uncoded systems, respectively. Finally, some simulations are presented to illustrate the performance of the maximum likelihood sequence estimator (MLSE) at moderate and low bit error rates     

An efficient frequency offset estimator for OFDM systems and itsperformance characteristics

In this paper, we propose a new frequency synchronization algorithm for orthogonal frequency division multiplexing (OFDM) systems requiring only one training symbol, based on a conventional method which requires two training symbols. While the timing synchronization is obtained by using the conventional method, the carrier frequency offset is efficiently estimated by the proposed method. It is shown that the proposed method not only reduces the number of the training symbols but also possesses better performance than the conventional method without increase in complexity     

Adaptive blind retrieval techniques for multiuser DS-CDMA signals

In a multiuser direct sequence code division multiple access system, retrieval of the transmitted signals with a conventional receiver is difficult when there is chip asynchronism, multipath propagation and the associated near-far problem. It is shown that despite these problems, an adaptive linear receiver based on a mixed cross-correlation and constant modulus algorithm has the potential to retrieve all users simultaneously. Moreover, for the assumed transmission model, a necessary condition for global convergence of the proposed algorithm is also provided     

A robust channel estimator for DS-CDMA systems under multipath fading channels

This paper addresses the problem of channel estimation for direct-sequence code-division multiple-access (DS-CDMA) systems with time-varying multipath fading channels. The multipath fading channels are modeled as autoregressive (AR) models. A method is first proposed to convert the time-varying regression model due to the time-varying nature of users' information symbols into a time-invariant one. Then, a polynomial approach is proposed to obtain the minimum mean square error (MMSE) estimator. The uncertainty of the channel model and decision errors of the DS-CDMA detector are taken into consideration in the design of the MMSE estimator. Compared with the Kalman estimator, the computational complexity of the proposed algorithm is much lower. The simulation results show that the proposed estimator provides a comparable estimation performance with the Kalman estimator and is robust for fast-fading channels.     

A space-time channel estimator and single-user receiver for code-reuse DS-CDMA systems

A blind asynchronous single-user code-reuse direct sequence code division multiple access (CDMA) array receiver is proposed for the uplink. By assigning each short PN-code more than once, code reuse allows the number of active users to be increased beyond the spreading gain. The proposed receiver is based on a blind single-code multipath joint space-time channel estimation technique that utilizes the concept of the spatio-temporal array manifold, in conjunction with a novel preprocessor, to deal with the multipath problem. From the estimated space-time channel parameters of a particular active code, the subset of parameters of a specific co-code user is then identified, and a single-user receiving weight vector is finally formed. The proposed approach is a subspace type method, and therefore, it is "near-far" resistant. Furthermore, in contrast to existing receivers such as the space-time decorrelating detector, the proposed receiver weight vector is tolerant to partial channel estimation errors and the incomplete estimation of channel parameters. The theoretical framework is supported by computer simulation studies.     

Angle estimator for signals with known waveforms

A large-sample decoupled maximum likelihood angle estimator for signals with known waveforms is presented based on exploiting the a priori knowledge that the observation noise can be modelled as spatially white. It is shown that the incorporation of this knowledge significantly improves the angle estimation accuracy over existing angle estimators for signals with known waveforms     

Blind adaptive reduced-rank detection for DS-CDMA signals inmultipath channels

Blind adaptive multiuser detection for direct sequence code division multiple access (DS-CDMA) signals over static and time-varying intersymbol interference (ISI) limited channels is considered. Blind adaptive detectors must be robustified for ISI channels, when there is significant mismatch between the received signature vector and the transmitted code (assumed known at the receiver). A new low-complexity detector is presented that improves on some previously proposed methods without explicit estimation of the ISI channel. The key innovation is a reduced-rank detector architecture combined with an efficient subspace tracker that yields direct accurate estimation of the desired user's received signature. Several representative simulation examples of detector output signal-to-noise-and-interference ratio (SINR) for fading channels are provided in support of our claims of improved efficacy of the method     

An efficient algorithm for determination of the optimum base-station assignment in cellular DS-CDMA systems

An algorithm is proposed that finds the optimum assignment of mobile users to base stations, and its associated transmission powers, in a cellular direct-sequence code-division multiple-access network, with a computational complexity that grows polynomially with the number of users and base stations. The algorithm detects infeasible situations and allows the inclusion of power constraints. Its performance is analyzed in terms of complexity and system capacity.     

An efficient VLSI architecture for real-time additive synthesis ofmusical signals

This paper presents a new architecture for the hardware implementation of additive synthesis for high-quality musical sound generation. A marginally stable second-order infinite-impulse-response filter is used to generate each sinusoid, the frequency, amplitude, and phase of which can independently be specified. A chip has been designed with a bit-level systolic array approach. It is capable of performing 1200 sinusoid real-time synthesis. Furthermore, it is possible to connect up to 11 chips, to achieve an outstanding 13 200 sinusoid synthesis. Two completely independent output channels are available as 20-b streams. The system is clocked at 60 MHz when working with a 44.1-kHz sampling rate. The integrated circuit is designed in a 0.5-μm CMOS technology and has a core area of approximately 19 mm²     

Real-time digital median frequency estimator for surfacemyoelectric signals

A prototype instrument is described that uses a digital signal processor to estimate the median frequency of the myoelectric signal in real time. A set of tenth-order digital low-pass filters is used to track the median frequency. An estimate of frequency is obtained once every 256 ms with a resolution of 1 Hz.     

A novel frequency estimator for sampled Doppler signals

In multigate pulsed Doppler systems with serial processing the Doppler signals are retained in their sampled form. The time-discrete nature of the Doppler signals affects the procedure to estimate the average frequency of the Doppler signal. Based on computer simulation studies it is demonstrated that a time-discrete frequency estimator based on either the density of zero-crossings or the time average of the instantaneous frequency exhibit a large relative error under poor signal-to-noise conditions or in the case of relatively wide band signals. Especially, the frequency estimator based on the instantaneous frequency functions poorly for average Doppler frequencies close to the Nyquist frequency. However, restricting the detected instantaneous frequency to a specific interval around its average improves the estimator performance considerably, while it allows tracking of center frequencies beyond the Nyquist frequency. A hardware realization of this modified estimator as incorporated in a high-resolution multigate pulsed Doppler system is described. In vitro and in vivo registrations as assessed with this system demonstrate the ability of the system to track frequencies close to and beyond the Nyquist frequency.     

A blind SNR estimator for digital bandpass signals

A subspace-based blind Signal-to-Noise Ratio （SNR） estimation algorithm for digital bandpass signals in Additive White Gaussian Noise （AWGN） channel is discussed. The lower bounds of the mean and variance of the estimation are derived, and simulations are performed for the commonly used digital bandpass signals, such as MPSK （M=2, 4, 8）, MFSK （M=2, 4） and MQAM （M=16, 64, 128, 256） signals. Theoretical analyses and simulation results indicate that the proposed algorithm is effective even when the SNR is below 0dB. Furthermore, the algorithm can provide a blind estimator in that it needs neither the parameters of the received signals, such as the carrier frequency, symbol rate and modulation scheme, nor the synchronization of the system.     

Block modulus precoding for blind multiuser detection of DS-CDMA signals

We propose a blind multiuser detector based on a new data precoding technique for direct-sequence code-division multiple-access signals. The modulus of all users' data is block encoded, using a sequence that is unique for each user. This precoding method, together with the analytical constant modulus algorithm for detection, enables a closed-form, one-shot detection of the desired user's signal in a multipath channel using one or more antennas. The detection process does not involve or require a channel estimation step. The proposed detector is shown to be extremely near-far resistant, and can operate properly in the presence of severe carrier frequency offset.     

An efficient RAKE receiver architecture with pilot signalcancellation for downlink communications in DS-CDMA indoor wirelessnetworks

This paper deals with a RAKE receiver architecture which makes use of a pilot signal broadcast by the base station to obtain channel parameter estimates in a direct-sequence code division multiple access (DS-CDMA) indoor wireless network. A suitable cancellation algorithm, which is different from previous approaches, is used to reduce multipath interference due to the pilot signal. Numerical results are used to demonstrate the performance of the proposed RAKE receiver in terms of the bit error probability under typical multipath fading propagation conditions. Performance comparisons with the ideal case of perfect channel parameter estimation are given in order to highlight the good behavior of the proposed approach     

An efficient blind multiuser detection for improper DS/CDMA signals

This paper proposes a new linear blind multiuser detection based on widely linear (WL) signal processing. The received signal and its complex conjugate are separately filtered and the results are linearly combined. The WL maximum/minimum (max/min) mean-output-energy (MOE) receiver is derived by applying the modified cost function. It is shown that a performance gain can be attained for the improper direct-sequence code division multiple access (DS/CDMA) signal, due to the additional information contained in pseudocovariance matrix of observations. The adaptive implementation with acceptable complexity is also developed. Computer-simulation results show that a significant performance gain is obtained over the other classical methods.     

A simple and efficient estimator for hyperbolic location

An effective technique in locating a source based on intersections of hyperbolic curves defined by the time differences of arrival of a signal received at a number of sensors is proposed. The approach is noniterative and gives an explicit solution. It is an approximate realization of the maximum-likelihood estimator and is shown to attain the Cramer-Rao lower bound near the small error region. Comparisons of performance with existing techniques of beamformer, spherical-interpolation, divide and conquer, and iterative Taylor-series methods are made. The proposed technique performs significantly better than spherical-interpolation, and has a higher noise threshold than divide and conquer before performance breaks away from the Cramer-Rao lower bound. It provides an explicit solution form that is not available in the beamforming and Taylor-series methods. Computational complexity is comparable to spherical-interpolation but substantially less than the Taylor-series method