CDMA Receiver Research Articles

A multi‐user CDMA receiver utilizing decorrelating detector with additional dummy pilot response

AbstractA pilot assisted CDMA system which uses extended spreading sequences with guard sequences under a quasi‐synchronous condition is capable of separating the interference components included in the input of a de‐correlating receiver by solving a system of linear equations. The performance of such a system, however, depends on the property of the de‐correlating matrix consisting of the received pilots, which correspond to the respective user spreading sequences and the multi‐path channel conditions. That is, the regularity of the matrix often tends to degrade, and the rank reduces occasionally primarily due to the multi‐paths, resulting in solutions that are vulnerable to AWGN. The present paper proposes an effective technique to solve this problem by introducing a virtual user into a group of real users that are to be served. The simulation results indicate a remarkable improvement in the bit‐error‐rate (BER) performance. In addition, based on the BER performance, the system has a RAKE‐like function that has power‐sum characteristics. Copyright © 2003 John Wiley & Sons, Ltd.

A Decision Aided CDMA Receiver with Partially Adaptive DecorrelatingMulti-User Interference Cancellation

A CDMA receiver with enhanced multiple access interference (MAI) suppression is proposed for a pilot symbols assisted system over multipath channels. The design of the receiver involves the following procedure. First, blind adaptive correlators are constructed at different fingers based on the scheme of generalized sidelobe canceller (GSC) to collect the multipath signals and suppress MAI. A low-complexity partially adaptive (PA) realization of the GSC correlators is proposed which incorporates multi-user information for reduced rank processing. By a judiciously designed decorrelating procedure, a new GSC structure is obtained in which the MAI are decorrelated and suppressed individually. The next step is then a simple coherent combining of the correlator outputs with pilot aided channel estimation. Finally, further performance enhancement is achieved by an iterative scheme in which the signal is reconstructed and subtracted from the GSC correlators input data, leading to faster convergence of the receiver. The proposed low-complexity PA CDMA multi-user receiver is shown to be robust to multipath fading and channel errors, and achieve nearly the same performance of the ideal maximum SINR and MMSE receivers by using a small number of pilot symbols.

A Low Power Correlator for CDMA Wireless Systems

The complex valued matched filter correlators consume maximum power in the DS/SS CDMA receivers. These correlators accumulate 1024 samples lying in the range ?7 to +7. This accumulation needs 3 data bits, 1 sign bit and 10 extra bits for overflow. Hence, the correlator can be implemented as a cascade of 4-bit full adder and a 10-bit incrementer. As a ripple carry adder (RCA) consumes the least power among all the existing adder architectures, we have implemented the 4-bit adder as a RCA. Previous incrementers were implemented as ripple counters. In this paper we propose a novel incrementer which is faster than a ripple counter based incrementer. Hence, it can be operated at a reduced voltage resulting in considerable power reduction. The incrementer is implemented using multiplexers, AND gates and TSPC registers. The ripple-counter correlator and the proposed incrementer correlator were laid out in MAGIC using 0.5 μ CMOS technology followed by power estimation using HSPICE. It is shown that the proposed architecture requires 50% less power than a ripple counter based design.

A low complexity partially adaptive CDMA receiver for downlink mobile satellite communications

AbstractA novel CDMA receiver with enhanced interference suppression is proposed for pilot symbols assisted mobile satellite systems in the presence of frequency offset. The design of the receiver involves the following procedure. First, adaptive correlators are constructed at different fingers, based on the scheme of generalized sidelobe canceller (GSC), to collect the multipath signals and suppress multi‐access interference (MAI). In particular, a partially adaptive (PA) realization of the GSC correlators is proposed based on the Krylov subspace technique, leading to an efficient algorithm without the need of complicated matrix computations. Second, pilot symbols assisted frequency offset estimation, channel estimation and RAKE combining give the estimate of signal symbols. Finally, further performance enhancement is achieved by an iterative scheme in which the signal is reconstructed and subtracted from the GSC correlators input, leading to faster convergence of the receiver. The proposed low complexity PA receiver is suitable or the downlink of mobile satellite CDMA systems, and shown to outperform the conventional fully adaptive MMSE receiver by using a small number of pilot symbols. Copyright © 2003 John Wiley & Sons, Ltd.

Multi-user multi-antenna CDMA receivers for asynchronous multipath fading channels

In this paper, we investigate joint channel estimation and signal detection approaches for a multi-user CDMA uplink with a base-station antenna array over asynchronous multipath fading channels. Based on the space alternating generalized expectation-maximization (SAGE) algorithm, we develop an iterative multi-user receiver to jointly estimate channel response vectors and bit sequences for all the active users at known fractional time delays, and a BER lower bound is derived. A simplified bit detection method is then proposed for multi-rate CDMA systems. Simulation results show that the new iterative receiver performs equally well for both high- and low-rate users, and the BER performance is close to that of the BER lower bound. Copyright © 2003 AEI.

Multi-user multi-antenna CDMA receivers for asynchronous multipath fading channels

Multi-user multi-antenna CDMA receivers for asynchronous multipath fading channels

Reduced complexity serial interference cancellation for CDMA receivers

A method to significantly reduce the computational complexity of a serial interference cancellation scheme for CDMA receivers, while maintaining the receiver performance, is presented. The reduced complexity is achieved by accelerating the convergence rate of the algorithm with a preconditioning technique.

A comparison between chip fractional and non-fractional sampling for a direct sequence CDMA receiver

This paper examines the effects of chip fractional (CF) and chip non-fractional (CNF) sampling on the performance of a CDMA uplink receiver. The impact of the receiver front end filter, which is sampling rate dependent, is investigated. Models for burst and continuous transmissions are introduced. The discrete-time equivalent channels between the various users and the receiver are assumed to be known. First, the mutual information between the emitted sequences of symbols and the received sequence is investigated. It is analytically shown that the receiver systematically loses information in case of CNF sampling. Second, we have demonstrated that the CF receiver always achieves better performance in terms of minimum mean square error (MMSE) for both linear and decision feedback (DF) structures. A closed-form expression of the gain in performance is provided for the two metrics; under consideration. The importance of the gain due to CF sampling is also illustrated by means of computations for multipath channels. For a typical system setup, a gain of 0.1 bits per emitted symbol is observed for the mutual information. Considering the geometrical mean of symbol SINRs in case of linear and DF joint detection (JD) for a roll-off factor equal to 0.3, a gain of 0.4 dB arises for the CF linear detector, and a gain of 0.2 dB arises for the CF DF detector.

A holey fiber-based nonlinear thresholding device for optical CDMA receiver performance enhancement

We demonstrate the use of an optical thresholder based on a short length of holey fiber to achieve enhanced code recognition quality in a 255-chip 320-Gchip/s superstructured fiber Bragg grating-based optical code-division multiple access code:decode system. The nonlinear thresholder is based on bandpass filtering of spectrally broadened components generated by self-phase modulation and assisted by the Raman effect in an 8.7 m length of highly nonlinear holey fiber. Error free penalty free system performance is obtained with complete recovery of the original input pulse shape.

A beamspace-time interference cancelling CDMA receiver for sectored communications in a multipath environment

A beamspace-time (BT) receiver is proposed for interference suppression and multipath diversity reception in sectored wireless code division multiple access communications. The scheme involves two stages. First, a set of adaptive space-time diversity processors, in the form of beamformer-correlator pairs, is constructed which provides effective suppression of unwanted interference and reception of signals from a prescribed space-time region. Second, the output data obtained by these processors are maximum ratio combined to capture the signal multipath components coherently. The proposed BT receiver is blind in that no training signal is required. The only information required is the signature sequence, timing and a rough estimate of the angle of arrival of the signal for selecting the sector of interest.

Nonlinear Iterative Multiuser Detection and Equalization for CDMA Receivers in the Presence of Interchip Interference

Perreau, S. L., and White L. B., Nonlinear Iterative Multiuser Detection and Equalization for CDMA Receivers in the Presence of Interchip Interference, Digital Signal Processing 11 (2001) 94–109 This paper addresses the problem of (nonlinear) iterative multiuser detection (MUD) for synchronous CDMA. In the literature, there are several related methods for solving this problem. These methods involve a linear estimator combined with nonlinear detection of the transmitted symbols but generally do not apply per-symbol iteration. However, the convergence analysis of iterative schemes has not been addressed yet (to our knowledge). In this paper, we present a new algorithm for MUD and provide a convergence analysis for an approximation to it. We believe this approach yields an understanding not only of the behavior of our proposed scheme but also of other possible iterative algorithms for MUD.

A DS-CDMA tracking mode receiver with joint channel/delay estimation and MMSE detection

A tracking mode receiver for asynchronous direct-sequence CDMA is presented based on the extended Kalman filter (EKF). The EKF jointly estimates the delays and multipath coefficients of the received CDMA waveform, and provides a modified minimum mean-square error (MMSE) estimate of the user data (MMSE-EKF). In order to obtain a practical algorithm, each user signal is tracked individually, with the remaining users modeled as colored Gaussian noise. However, the EKFs are coupled through the multiple access interference (MAI) covariance estimates. In order to obtain meaningful performance measures, approximate worst-case undesired user delays that minimize the desired user SNR and delay estimation Cramer-Rao bound are obtained. It is shown that such worst-case delays can be efficiently computed using the alternating maximization (A-M) algorithm. The resulting bit error rate (BER) performance of the MMSE-EKF tracking receiver is evaluated through a combination of simulation and analysis. The mean-time to lose lock (MTTLL) for a genie-aided EKF delay estimator is also obtained using the A-M computed delays.

Blind asynchronous multiuser CDMA receivers for ISI channels using code-aided CMA

A code-aided constant modulus algorithm (CMA) based approach is presented for blind detection of asynchronous short-code DS-CDMA (direct sequence code division multiple access) signals in intersymbol interference (ISI)/multipath channels. Only the spreading code of the desired user is assumed to be known; its transmission delay may be unknown. A linear equalizer is designed by minimizing the Godard/CMA cost function of the equalizer output with respect to the equalizer coefficients subject to the fact that the equalizer lies in a subspace associated with the desired user's code sequence. Constrained CMA leads to the extraction of the desired user's signal whereas unconstrained minimization leads to the extraction of any one of the active users. The results are further improved by using unconstrained CMA initialized by the results of the code-aided CMA. Identifiability properties of the approach are analyzed. Illustrative simulation examples are provided.

Blind adaptive algorithms for minimum variance CDMA receivers

Constrained optimization methods have received considerable attention as a means to derive blind multiuser receivers with low complexity. The receiver's output variance is minimized subject to appropriate constraints which depend on the multipath structure of the signal of interest. When multipath is present, the constraint equations can be written in parametric form, and the constraint parameters jointly optimized with the linear receiver's parameters. We develop adaptive solutions for this joint, constrained optimization problem. Both stochastic gradient and recursive least-square-type algorithms are developed. The performance of the proposed methods is compared with other blind and trained methods and turns out to be close to the trained minimum mean-square-error receiver.

VLSI Implementation of the Multistage Detector for Next Generation Wideband CDMA Receivers

The multistage detection algorithm has been proposed as an effective interference cancellation scheme for next generation Wideband Code Division Multiple Access (W-CDMA) base stations. In this paper, we propose a real-time VLSI implementation of this detection algorithm in the uplink system, where we have achieved both high performance in interference cancellation and computational efficiency. When interference cancellation converges, the difference of the detection vectors between two consecutive stages is mostly zero. Under the assumption of BPSK modulation, the differences between the bit estimates from consecutive stages are 0 and ±2. Bypassing the zero terms saves computations. Multiplication by ±2 can be easily implemented in hardware as arithmetic shifts. However, the convergence of the algorithm is dependent on the number of users, the interference and the signal to noise ratio and hence, the detection has a variable execution time. By using just two stages of the differencing detector, we achieve predictable execution time with performance equivalent to at least eight stages of the regular multistage detector. A VLSI implementation of the differencing multistage detector is built to demonstrate the computational savings and the real-time performance potential. The detector, handling up to eight users with 12-bit fixed point precision, was fabricated using a 1.2 μm CMOS technology and can process 190 Kbps/user for 8 users.

Iterative interference cancellation for asynchronousCDMAwith reduced-state turbo decoders

An iterative interference cancellation receiver for asynchronous turbo-coded CDMA is proposed. At each stage, iteration is performed using turbo decoders and the derived soft information used to reconstruct multiple-access interference (MAI). The complexity of the receiver is linear with respect to the number of users and can be further simplified via reduced-state implementation of turbo decoders.

Communication Theory: Letter: Multiuser Detection Analysis in DS-CDMA Channel with Nakagami Fading

In this paper the multiuser detection performance analysis- in synchronous direct sequence code division multiple access (DS-CDMA) system using decorrelating and decision feedback (DFB) receivers, including single path Nakagami fading channel is presented. The lower bounds of the receivers bit error probability (BER) are derived, and the comparison of these receivers' performances to the conventional CDMA receiver is made. The analytical and numerical results show that the DFB receiver performs better in the presence of the high Nakagami fading channels.

Adaptive techniques for multiuser CDMA receivers

A number of CDMA receivers have been proposed that cover the whole spectrum of performance/complexity from the simple matched filter to the optimal Viterbi (1995) processor. Adaptive solutions, in particular, have the potential of providing the anticipated multiuser detection (MD) performance gains with a complexity that would be manageable for third generation systems. Our goal, in this article, is to provide an overview of previous work in MD with an emphasis on adaptive methods. We start with (suboptimal) linear receivers and discuss the data-aided MMSE receiver. Blind (nondata-aided) implementations are also reviewed together with techniques that can mitigate possible multipath effects and channel dispersion. In anticipation of those developments, appropriate discrete-time (chip rate) CDMA models are reviewed, which incorporate asynchronism and channel dispersion. For systems with large spreading factors, the convergence and tracking properties of conventional adaptive filters may be inadequate due to the large number of coefficients which must be estimated. In this context, reduced rank adaptive filtering is discussed. In this approach, the number of parameters is reduced by restricting the receiver tap vector to belong to a carefully chosen subspace. In this way the number of coefficients to be estimated is significantly reduced with minimal performance loss.

Uplink and downlink experimental CDMA overlay ofGSM network in fading environment

A broadband 64 kbit/s DS-CDMA overlay in the same frequency space as a GSM signal has been implemented experimentally in a fading environment. With a single CDMA user, error-free operation of both systems was demonstrated over a 39 and 58 dB dynamic range for the uplink and the downlink, respectively. The work utilised a novel CDMA receiver architecture that enabled a 30 dB digital notch filter to be applied at RF. This represents the first experimental demonstration of a true GSM network overlay in a realistic radio channel.

Blind decentralized projection receiver for asynchronous CDMA in multipath channels

Cet article traite du probleme de l’annulation de brouillage dans un systeme a acces multiple par repartition en code (AMRC) a sequence directe pour des canaux selectifs en frequence et des conditions asynchrones. Le signal etale cyclostationnaire est recu par des antennes multiples ou est sur echantillonne par rapport a la periode des bribes. Il est donc converti en un signal vectoriel sta-tionnaire. Le sur-echantillonnage ou les antennes multiples donnent naissance a un systeme a entrees et sorties multiples (MIMO) disposant de nombreuses connaissances a priori relatives a la forme des sequences d’etalement distinctes pour les signaux d’entree (utilisateurs), En fonction de la longueur de la reponse impulsionnelle finie (RIF) du canal de propagation et du facteur d’etalement, le canal d’un utilisateur determine s’etale sur un certain nombre de symboles, engendrant une interference intersymbole (ISI) qui s’ajoute au brouillage d’acces multiple (MAI) du aux utilisateurs concurrents. L’estimation du canal de l’utilisateur voulu est obtenue par une nouvelle technique aveugle qui utilise la sequence d’etalement de l’utilisateur et la statistique d’ordre deux du signal recu. Le recepteur a forcage de zero minimisant l’erreur quadratique moyenne (MMSE-ZF) s’en deduit. Ce recepteur generalise au cas asynchrone et pour les canaux a trajets multiples le recepteur ancre qui minimise l’energie a la sortie (MDE) [1]. Differents recepteurs lineaires sont obtenus par criteres divers. On montre que le recepteur MMSE-ZF peut etre realise d’une maniere decentralisee en appliquant le critere MOE non biaise, conduisant au recepteur a reponse sans distorsion et a variance minimale (MVDR) pour le signal de l’utilisateur donne. Le recepteur MVDR est ensuite adapte en aveugle en appliquant le principe de Capon. Cela donne, par ailleurs, l’estimation de la reponse impulsionnelle du canal. En etablissant des bornes inferieures sur la longueur du filtre de reception, on obtient une estimation de l’ISI et du MAI tolerable par le recepteur, ainsi que des conditions d’identificabilite de celui-ci.