Decorrelating Detector Research Articles

Performance of Multiuser-Coded CDMA Systems With Transmit Diversity Over Nakagami- $m$ Fading Channels

The performance of a multiple-input-multiple-output (MIMO) code-division multiple-access (CDMA) system, using space-time spreading (STS), is analyzed over a frequency-flat Nakagami- <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">m</i> fading channel. The convolutionally space-time coded system employs a decorrelator detector with <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> = 2 and <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</i> antennas at the user side and base station (BS), respectively. Assuming independent Nakagami fading channels between transmit and receive antennas, we determine the probability density function (pdf) of the signal-to-interference-plus-noise ratio (SINR) at the output of the multiuser detector and after signal combining. Considering binary phase-shift keying (BPSK) transmission, we then evaluate the pairwise error probability and the corresponding bit-error-rate (BER) upper bounds over fast-fading channels. The derived error bounds, when compared to system simulations, are shown to be accurate at all signal-to-noise ratios (SNRs) of interest. Examining the asymptotic performance of the underlying space-time multiuser system, at high SNRs, we evaluate the overall diversity gain as a function of the number of transmit and receive antennas and the minimum free distance of the convolutional code.

Differential decorrelator: a new approach for designing CDMA linear detector

The systems that are designed based on CDMA are suffering from a multiple access interference problem. The decorrelator CDMA detector is a linear detector that can cancel the MAI but with channel noise enhancement. The complexity of the decorrelator is a linear function in the number of the system's users. In this research, a new detector is proposed that can cancel the MAI in the received CDMA signal with a complexity that is independent on the number of the system's users. The new detector does not need to know the users' signature codes. Also, it consists of two matched filter only. No correlation matrix is required. This simple structure reduces the complexity of the proposed CDMA detector if it is compared with the conventional decorrelator detector. The new detector is based on some mathematical operations on the output signals from two different matched filters. The detector idea is based on the symmetry property of the signatures' codes correlation matrix however; it does not need to know this matrix. The algorithm is valid as long as the correlation matrix is symmetry. It can work with synchronous and asynchronous system models.

A linear weighted SIC multiuser detector with optimal weighting factor

In this paper, a linear weighted successive interference cancellation (SIC) detector is introduced to overcome the convergence issues of the conventional linear SIC detector. The latter converges to the decorrelator detector if it converges. The optimal weighting factor is derived using the steepest descent method. Simulation results show that if the optimal weighting factor is used, the convergence speed and thus the BER of the proposed structure can be significantly enhanced.

O3BPSK-based Linear Decorrelating Detector for Asynchronous DS/CDMA Systems over Frequency-Selective Rayleigh Fading Channels

In this paper, a convenient signaling scheme, called orthogonal on---off BPSK (O3BPSK), along with a simple one-shot linear decorrelating detector (LDD) and a whitening Rake bank, is proposed for near---far resistant detection in asynchronous DS/CDMA systems. Based on the maximum multi-path spreading delay, a minimum duration of "off" is suggested, during which the temporally adjacent bits (TABs) that contain multi-user interference (MUI) and inter-symbol interference (ISI) from different users at the receiver are decoupled. The O3BPSK signaling scheme is combined with the whitening Rake receiver to preserve multi-path diversity gain in multi-path fading CDMA channels. The scheme offers low complexity, no detection delay, near---far resistance, and compensation for fading channels.

Performance Analysis of Multiuser DS-CDMA in MIMO Systems Over Rayleigh Fading Channels

The performance of space-time transmit diversity is examined in a multiuser direct-sequence code-division multiple-access (DS-CDMA) system over fast- and slow-fading channels. The underlying space-time system employs transmit antennas and receive antennas at the mobile user and receiver base station, respectively. We consider the performance of the space-time multiuser system when using the linear decorrelator detector to combat the effect of multiuser interference. In our analysis, we derive a closed-form expression for the probability of bit error for both fast- and slow-fading channels. These theoretical results are shown to be very accurate when compared to system simulations. Both simulations and theoretical results prove that, regardless of the system load, the full diversity order of for fast-fading channels and for slow-fading channels is always maintained, and only a signal-to-noise ratio (SNR) loss is incurred. This SNR loss is proved to be a function of only the number of users (i.e., level of interference) and independent of the number of transmit and/or receive antennas. Using our theoretical results, we show that the loss in SNR from the single-user bound can be well approximated by , where represents the level of multiuser interference.

Adaptive path selective fuzzy decorrelating detector under impulsive noise for multipath fading CDMA systems

This letter proposes a novel adaptive path selective fuzzy decorrelating (APSFD) detector for direct-sequence code- division multiple-access (DS-CDMA), systems with time-varying multipath fading channels under impulsive noise. The proposed detector combines adaptive path selective decorrelating (APSD) detector with a fuzzy median filter which is based on fuzzy rank ordering of samples to eliminate the effect of impulsive noise. Simulation results clearly show that the proposed detector eliminates the effect of impulsive noise and improves the performance of APSD detector under high impulsive noise.

O [formula omitted]-based linear decorrelating detector for asynchronous UWB systems over multipath fading channels

In this paper, a convenient signaling scheme, termed orthogonal on–off Ultra wideband (UWB) (O 3 UWB), along with a simple one-shot linear decorrelating detector (LDD) and a Rake bank, is proposed. This scheme is useful for conducing multi-user detection because temporally adjacent bits in receiver signals from different users are decoupled using on–off signaling. This mechanism also leads to one-shot detection that overcomes the main disadvantage, long detection delay, of the conventional LDD. In addition, the data rate is maintained, with no increases in the transmission rate, by adopting an orthogonal structure. In this paper, we study the system performance over a flat fading channel. Four types of channel models, including Rayleigh fading, Nakagami- m fading, Rician fading, and Lognormal fading, are considered. Since multipath fading still significantly degrades UWB system performance, the Rake bank is included to recover multipath diversity gain. Numerical analysis and simulation results show that the O 3 UWB LDD receiver still offers a good near-far resistant property over fading channels compared to the conventional UWB receiver.

A Chip-Level BSOR-Based Linear GSIC Multiuser Detector for Long-Code CDMA Systems

We introduce a chip-level linear group-wise successive interference cancellation (GSIC) multiuser structure that is asymptotically equivalent to block successive over-relaxation (BSOR) iteration, which is known to outperform the conventional block Gauss-Seidel iteration by an order of magnitude in terms of convergence speed. The main advantage of the proposed scheme is that it uses directly the spreading codes instead of the cross-correlation matrix and thus does not require the calculation of the cross-correlation matrix (requires floating point operations (flops), where is the processing gain and is the number of users) which reduces significantly the overall computational complexity. Thus it is suitable for long-code CDMA systems such as IS-95 and UMTS where the cross-correlation matrix is changing every symbol. We study the convergence behavior of the proposed scheme using two approaches and prove that it converges to the decorrelator detector if the over-relaxation factor is in the interval ]0, 2[. Simulation results are in excellent agreement with theory.

On the performance of O 3BPSK LDD with diversity combining techniques over fading channels

Zheng and Barton developed a convenient signaling scheme (orthogonal on-off BPSK (O 3BPSK)) with a simple one-shot linear decorrelating detector (LDD) for near–far resistant detection in asynchronous DS/CDMA systems. This paper studies the performance of the O 3BPSK LDD system over a frequency-nonselective and slowly fading channel. Three types of channel model are considered, namely Rayleigh fading, Nakagami- m fading, and Ricean fading. The analytical results show that channel fading causes a significant degradation in the performance of the O 3BPSK LDD receiver. Nevertheless, the receiver still demonstrates a better near–far resistant property than a conventional CDMA receiver over fading channels. This study also analyzes the effect of two well-known diversity combining techniques on the O 3BPSK LDD performance over Rayleigh fading channels. The results indicate that diversity combining techniques significantly improve the performance of the O 3BPSK LDD scheme over fading channels.

Analysis and Performance Evaluation of Linear Multiuser Detectors in the Downlink of DS-CDMA Systems Applying Spectral Decomposition

This paper studies the performance of linear multiuser detectors for direct-sequence code division multiple access systems at different loading levels and users' powers, using singular value decomposition (SVD) techniques in the downlink of Rayleigh flat-fading and additive white Gaussian channels. The performance of the matched filter (MF), decorrelator (zero-forcing), and minimum mean-squared error (MMSE) detectors are studied and compared. Analytical and simulation results are also presented in terms of the bit error rate. From this analysis, a simple linear multiuser detector is developed that exploits the structure of the system's spreading codes matrix from the SVD viewpoint. Also, the numerical performance of this proposed detector is compared to that of the conventional detector (MF) as a function of the signal-to-noise ratio. Finally, the performance limits are established in terms of the singular values of the spreading codes matrix. Extensive simulation results validate the analysis presented in the paper for equal or unequal users' powers.

Orthogonal on–off-based near–far-resistant detection with diversity technique in UWB systems

A convenient signalling scheme, called orthogonal on–off (O3), together with a simple one-shot linear decorrelating detector (LDD) and a RAKE bank, is adopted in asynchronous ultra-wideband (UWB) systems. The performance of the proposed system is evaluated over multipath fading channels. This investigation also analyses mathematically the effect of the maximal-ratio combining diversity technique on the O3UWB LDD performance over lognormal fading channels. The analytical and simulation results indicate that the diversity technique significantly improves the performance of the O3UWB LDD scheme over fading channels, and the proposed receiver still demonstrates a better near–far-resistant property than a conventional UWB receiver.

Particle-Swarm-Optimization-Based Multiuser Detector for CDMA Communications

In a code-division multiple-access system, multiuser detection (MUD) can exploit the information of signals from other interfering users to increase system capacity. However, the optimum MUD can be characterized as an NP-hard combinatorial optimization problem such that the computational complexity increases exponentially with the number of users. In this paper, we apply a new evolutionary algorithm, called particle swarm optimization (PSO), to develop a suboptimal MUD strategy. The decorrelating detector (DD) or linear minimum mean square error (LMMSE) detector is used as the first stage to initialize the PSO-based MUD. Then, the PSO algorithm is applied to detect the received data bit by optimizing an objective function incorporating the linear system of the DD or LMMSE detector. Simulation results show that the performance of our proposed decorrelating PSO and LMMSE-PSO MUD are promising and outperform the decorrelating and LMMSE MUD with a slight increase in computation.

On the Performance of CDMA Systems Employing Multiuser Decorrelating Detector and Antenna Array

<para xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> This paper presents analytical expressions to evaluate the capacity of code division multiple access (CDMA) cellular radio systems employing a joint antenna array and a multiuser decorrelating detector. The performance is evaluated for chip-synchronous and asynchronous baseband and bandpass CDMA systems on additive white Gaussian noise and flat-fading Rayleigh channels. Simulation results have shown that the analytical model developed to evaluate the performance of those systems is a very tight approximation. </para>

A New Linear Group-Wise SIC Multiuser Detector

In this work, we introduce a new linear group-wise SIC multi-user detector that can converge to either the decorrelator or the least minimum mean-square error (LMMSE) detector. We study the convergence behavior of the proposed scheme and show that the latter is equivalent to the block Gauss-Seidel iterative method if the group-detection scheme used is the decorrelator detector. Moreover, we prove that the latter is convergent if the group-detection matrix is positive definite. Our simulation results are in excellent agreement with the proposed theory

SSOR preconditioned GPBiCG method for the linear interference cancellation of asynchronous CDMA systems

An ideal computation of the decorrelating or the linear minimum mean-squared-error (LMMSE) detector requires computational complexity of order K 3 when there K is the number of users. To alleviate the computational complexity, iterative decorrelating and LMMSE detectors are proposed for solving a set of linear equations corresponding to linear interference cancellation structures. Iterative conjugate gradient (CG) method has been used for the linear interference cancellation detectors. Its main advantages are to reduce the order of computation complexity and their suitability to highly parallel implementations. In this paper, the symmetric successive overrelaxation (SSOR) preconditioning scheme is applied to the CG method. The performance of the detectors is investigated and it is found that the SSOR preconditioned CG method can provide significantly faster convergence than CG method.

A Blind Adaptive Decorrelating Detector Using Spatial Signature Estimation

The decorrelating detector is one of the detecting methods in a direct sequence code division multiple access systems. We investigate the blind adaptive decorrelating detector (BADD) using only the signature of the desired user (DU) according to the assumption that the algorithm is used in downlink. When the BADD is constructed with an antenna array, both the spatial and temporal signature must be taken into consideration for signal detection. We propose the BADD incorporated with the blind estimation of spatial signature (SS) of the DU only from the received signals. As the estimation procedure of SS, the orthogonal projection approximation and subspace tracking algorithm is adopted. The proposed BADD presented the BER improvement with using antenna array. The BER performance has a lower limit with increasing the number of antenna array elements.

Improved Robust Techniques for Multiuser Detection in Non-Gaussian Channels

In many physical channels where multiuser detection techniques are to be applied, the ambient channel noise is known through experimental measurements to be decidedly non-Gaussian, due largely to impulsive phenomena. This is due to the impulsive nature of man-made electromagnetic interference and a great deal of natural noise. This paper presents a robust multiuser detector for combating multiple access interference and impulsive noise in code division multiple access (CDMA) communication systems. A new M-estimator is proposed for "robustifying" the detector. The approach is corroborated with simulation results to evaluate the performance of the proposed robust multiuser detector compared with that of the linear decorrelating detector, and the Huber and the Hampel M-estimator based detectors. Simulation results show that the proposed detector with significant performance gain outperforms the linear decorrelating detector, and the Huber and the Hampel M-estimator based detectors. This paper also presents an improved robust blind multiuser detection technique based on a subspace approach, which requires only the signature waveform and the timing of the desired user to demodulate that user's signal. Finally, we show that the robust multiuser detection technique and its blind adaptive version can be applied to both synchronous and asynchronous CDMA channels.

Blind adaptive multiuser detector for nonlinearly modulated satellite mobile CDMA systems

This paper presents a novel blind adaptive noncoherent decorrelative multiuser detector for nonlinearly modulated satellite mobile Code Division Multiple Access (CDMA) systems. By using the known signature waveforms of the counterpart earth station in the blind adaptive multiuser detector, the system performance has been improved obviously. The computation results about the convergence properties of the new detector and the previous detectors demonstrate that the proposed multiuser detector has better performance than previous multiuser detectors for nonlinearly modulated CDMA systems.

Performance Analysis of Adaptive Path Selective Decorrelating Detector with Channel Estimation Errors

The system complexity and noise enhancement due to the use of Multipath Decorrelating Detector (MDD) can be reduced by employing adaptive path selection technique. Adaptive Path Selective Decorrelating Detector (APSDD) requires knowledge of the channel coefficients for path selection. Generally, the channel coefficients are assumed to be known at the receiver. However, this is not realistic and the channel coefficients should be estimated. In this paper, we extend the Bit Error Rate (BER) analysis of the path selective receiver to include channel estimation errors.

Semidefinite relaxation for detection of 16-QAM signaling in MIMO channels

We develop a computationally efficient approximation of the maximum likelihood (ML) detector for 16 quadrature amplitude modulation (16-QAM) in multiple-input multiple-output (MIMO) systems. The detector is based on a convex relaxation of the ML problem. The resulting optimization is a semidefinite program that can be solved in polynomial time with respect to the number of inputs in the system. Simulation results in a random MIMO system show that the proposed algorithm outperforms the conventional decorrelator detector by about 2.5 dB at high signal-to-noise ratios.