Blind multiuser channel estimation in asynchronous CDMA systems

In asynchronous code division multiple access (A-CDMA) systems transmitting over multipath channels, both intersymbol interference (ISI) as a result of interchip interference (ICI) and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users' signature waveform. Recently, Liu and Xu (see Proc. 29th Asilomar Conf. Signals, Systems and Computers, 1996) introduced a blind estimation algorithm for synchronous CDMA (S-CDMA) systems to estimate the multiuser channels. However, this algorithm cannot be directly applied to an asynchronous CDMA (A-CDMA) system. In this paper, we study a similar blind estimation scheme that provides estimates of the multiuser channels by exploiting the structure information of the data output and the users' delays. In particular, we show that the subspace of the data matrix contains sufficient information for unique determination of channels and, hence, the signature waveforms. By utilizing antenna arrays, we extended our approach to overloaded systems, where the number of users may exceed the spreading factor     

Performance Analysis of Subspace Based Downlink Channel Estimation for W-CDMA Systems Using Chaotic Codes

This paper presents blind channel estimation for downlink W-CDMA system that employs chaotic codes and Walsh codes for spreading information bits of the multiple users. In a W-CDMA system, while transmitting over multipath channels, both intersymbol interference (ISI) as a result of Inter Chip Interference and multiple access interference (MAI) cannot be easily eliminated. Although it is possible to design multiuser detectors that suppress MAI and ISI, these detectors often require explicit knowledge of at least the desired users’ signature waveform. Earlier work focused on a subspace based channel estimation algorithm for asynchronous CDMA systems to estimate the multiple users’ symbols, where only AWGN channel was considered. In our work, we study a similar subspace-based signature waveform estimation algorithm for downlink W-CDMA systems, which use chaotic codes instead of pseudo random codes, that provide estimates of the multiuser channel by exploiting structural information of the data output at the base station. In particular, we show that the subspace of the (data+noise) matrix contains sufficient information for unique determination of channels, and hence, the signature waveforms and signal constellation. We consider Rayleigh and Rician fading channel model to quantify the multipath channel effects. Performance measures like bit error rate and root mean square error are plotted for both chaotic codes and Walsh codes under Rayleigh and Rician fading channels.     

A maximum likelihood approach to blind multiuser interferencecancellation

This paper addresses the problem of blind multiple access interference (MAI) and intersymbol interference (ISI) suppression in direct sequence code division multiple access (DS CDMA) systems. A novel approach to obtain the coefficients of a linear receiver using the maximum likelihood (ML) principle is proposed. The method is blind because it only exploits the statistical features of the transmitted symbols and Gaussian noise in the channel. We demonstrate that an adequate linear constraint on these coefficients ensures that the desired user is extracted and the resulting linearly constrained maximum likelihood linear (LCMLL) receiver can be efficiently implemented using the iterative space alternating generalized expectation-maximization (SAGE) algorithm. In order to take advantage of the diversity inherent to multipath channels, we also introduce a blind RAKE multiuser receiver that proceeds in two steps. First, soft estimates of the desired user transmitted symbols are obtained from each propagation path using a bank of appropriate LCMLL receivers. Afterwards, these estimates are adequately combined to enhance the signal-to-interference-and-noise ratio (SINR). Computer simulations show that the proposed blind algorithms for multiuser detection are near-far resistant and attain convergence using small blocks of data, thus outperforming existing linearly constrained minimum variance (LCMV) blind receivers     

A two-stage CMA-based receiver for blind joint equalization and multiuser detection in high data-rate DS-CDMA systems

The paper deals with the problem of blind mitigation of intersymbol interference (ISI) as well as multiple-access interference (MAI) in asynchronous high data-rate direct-sequence code-division multiple-access systems. A blind adaptive multiuser receiver based on the constant-modulus algorithm (CMA) is proposed, which demodulates each desired user by exploiting only the knowledge of its spreading code, without requiring estimation of the users's channels and timings. In order to overcome the CMA interference capture problem, which arises in a multiuser scenario, a two-stage adaptive receiver is adopted: In the first stage, partial MAI and ISI suppression is blindly achieved by exploiting the desired user signature structure properties; in the second stage, the residual MAI and the ISI are removed by using the CMA, and the information symbols of the desired user are reliably recovered. Theoretical analysis and simulation results show that the first stage is an effective blind adaptive strategy which allows the CMA detector in the second stage to lock on the desired-user symbol, at a particular delay. The proposed blind receiver achieves a significant performance gain in comparison with existing blind methods.     

一种基于线性预测的长码CDMA盲多用户检测方法

将一种基于线性预测的盲均衡和盲多用户方法应用在长扩频码CDMA系统中。该方法能消除多址干扰和多径条件下引起的码间串扰,是一种适用于未来实际CDMA系统的多用户检测算法。     

Adaptive step-size algorithms for blind interference suppression inDS/CDMA systems

This paper develops adaptive step-size blind LMS algorithms and adaptive forgetting factor blind RLS algorithms for code-aided suppression of multiple access interference (MAI) and narrowband interference (NBI) in DS/CDMA systems. These algorithms optimally adapt both the step size (forgetting factor) and the weight vector of the blind linear multiuser detector using the received measurements. Simulations are provided to compare the proposed algorithms with previously studied blind RLS and blind LMS algorithms. They show that the adaptive step-size blind LMS algorithm and adaptive forgetting factor blind RLS algorithm field significant improvements over the standard blind LMS algorithm and blind RLS algorithm in dynamic environments where the number of interferers are time-varying     

Blind MAI and ISI suppression for DS/CDMA systems using HOS-basedinverse filter criteria

Cumulant-based inverse filter criteria (IFC) using second-and higher order statistics (HOS) proposed by Tugnait et al. (1993) have been widely used for blind deconvolution of discrete-time multi-input multi-output (MIMO) linear time-invariant systems with non-Gaussian measurements through a multistage successive cancellation procedure, but the deconvolved signals turn out to be an unknown permutation of the driving inputs. A multistage blind equalization algorithm (MBEA) is proposed for multiple access interference (MAI) and intersymbol interference (ISI) suppression of multiuser direct sequence/code division multiple access (DS/CDMA) systems in the presence of multipath. The proposed MBEA, which processes the chip waveform matched filter output signal without requiring any path delay information, includes blind deconvolution processing using IFC followed by identification of the estimated symbol sequence with the associated user through a user identification algorithm (UIA). Then, some simulation results are presented to support the proposed MBEA and UIA. Finally, some conclusions are drawn     

A low‐complexity adaptive receiver for DS‐CDMA systems in unknown code delay environment

In direct‐sequence code division multiple access (DS/CDMA) multiuser communication systems in multipath channels, both intersymbol interference (ISI) and multiple‐access interference (MAI) must be considered. The multipath channel characterizes the propagation effects including the timing offset and delays, etc. Traditionally, we use the delay‐locked loop (DLL) code tracking loop to recover the timing delay. But DLL cannot work well in multipath environment. In this paper, we propose a low‐complexity adaptive receiver to suppress ISI/MAI and solve the timing offset problems without using conventional DLL code tracking loop. The proposed receiver employs an adaptive filter whose weights are adapted using a block least‐mean square error algorithm with fractional sampling. Simulations confirm the good performance, including learning curves and theoretical analysis of minimum mean‐square error, of the proposed receiver. Copyright © 2010 John Wiley & Sons, Ltd.     

DS-CDMA通信系统中基于独立分量分析的盲多用户检测

本文研究了直接序列扩频码分多址(DS-CDMA)通信系统的多用户检测问题,提出了基于独立分量分析(ICA)方法的盲多用户检测算法,实现多用户信号的盲检测。该方法不需知道用户的扩频码就可完成多用户信号的盲检测。仿真结果验证了本文提出的方法的优良性能。     

Smart antennas in wireless systems: uplink multiuser blind channeland sequence detection

Space-division multiple-access (SDMA) schemes have been proposed to increase the capacity of wireless communication systems by simultaneously transmitting and receiving multiple co-channel signals through different spatial channels. We address the uplink (remote users to the base station antenna array) blind channel and sequence identification problem for an SDMA system. We show that multiuser blind identification can be accomplished by exploiting the spatial and temporal diversities of an antenna array system. In particular, a recursive estimation algorithm is developed to recover multiple signals from intersymbol interference (ISI) and co-channel interference (CCI) by taking advantage of a special structure of the array output and the finite-alphabet property of digital communication signals. The implementation of the proposed approach in practical applications is discussed, and field experiments have been conducted to demonstrate the effectiveness of the proposed algorithm     

Adaptive feedforward/feedback architectures for multiuser detectionin high data rate wireless CDMA networks

We consider the design and performance of nonlinear minimum mean-square-error multiuser detectors for direct sequence code-division multiple-access (CDMA) networks. With multiple users transmitting asynchronously at high data rates over multipath fading channels, the detectors contend with both multiple-access interference (MAI) and intersymbol interference (ISI). The cyclostationarity of the MAI and ISI is exploited through a feedforward filter (FFF), which processes samples at the output of parallel chip-matched filters, and a feedback filter (FBF), which processes detected symbols. By altering the connectivity of the FFF and FBF, we define four architectures based on fully connected (FC) and nonconnected (NC) filters. Increased connectivity of the FFF gives each user access to more samples of the received signal, while increased connectivity of the FBF provides each user access to previous decisions of other users. We consider three methods for specifying the FFF sampling and propose a nonuniform FFF sampling scheme based on multipath ray tracking that can offer improved performance relative to uniform FFF sampling. For the FC architecture, we capitalize on the sharing of filter contents among users by deriving a multiuser recursive least squares (RLS) algorithm and direct matrix inversion approach, which determine the coefficients more efficiently than single-user algorithms. We estimate the uncoded bit-error rate (BER) of the feedforward/feedback detectors for CDMA systems with varying levels of power control and timing control for multipath channels with quasi-static Rayleigh fading. Simulations of packet-based QPSK transmission validate the theoretical BER analysis and demonstrate that the multiuser RLS adapted detectors train in several hundred symbols and avoid severe error propagation during data transmission mode     

Blind adaptive DS/CDMA receivers for multipath and multiuser environments

This paper proposes several blind adaptive receivers to eliminate multiple-access interference (MAI), intersymbol interference (ISI), and interchip interference (ICI) in direct-sequence code-division multiple access (DS/CDMA) downlink multiuser systems. We use the following concepts to formulate the cost function: 1) the variance of the despreading output approaches to the variance of the desired signal and 2) the discreteness property of the input signal. The proposed approaches are called variance-oriented approaches (VOAs). The VOA is then applied to three proposed receiver structures, especially the generalized sidelobe canceller (GSC) scheme that is generally the concept of spatial domain in beamforming system, to eliminate the MAI by one particular constraint in temporal domain. Besides, by this constraint, GSC filter possesses the property of global convergence in multipath environment once the channel estimation is appropriate. Simulation examples are shown to demonstrate the effectiveness and comparison of the proposed blind adaptive receivers.     

Adaptive detection in asynchronous code-division multiple-accesssystem in multipath fading channels

In code-division multiple-access systems transmitting data over time-varying multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) arise. In this paper, we address interference suppression, multipath diversity and processing gain protection for multiuser detection with less noise enhancement by using a parallel cancelling scheme. The proposed detector consists of a RAKE filter, forward filter, and feedback filter with different functions for each filter. The RAKE filter increases the signal-to-noise ratio by taking the advantage of multipath and code diversities. The forward filter is proposed, in combination with the feedback filter, to remove the effects of MAI and ISI by parallel cancellation. In order to avoid performance deterioration due to unreliable initial estimation in the parallel cancellation, a cost function with proper weighting is introduced to improve the performance of the proposed detector. In the proposed design method, a recursive least square algorithm is employed to update the tap-coefficients of all filters for MAI and ISI cancellation. Finally, the performance of the proposed detector is analyzed and compared with other detectors     

Genetically modified multiuser detection for code division multipleaccess systems

The problem of multiple access interference (MAI) and intersymbol interference (ISI) suppression in code division multiple access (CDMA) systems is considered. By combining the theory of multiuser detection (MUD) and evolutionary computation, a hybrid genetic engine is proposed, suitable for the detection of CDMA signals in the presence of MAI and ISI. The proposed hybrid detector structure can be extended to most multiuser detectors and used as the base detector within the structure. Using random selection, mutation and crossover operators and a unique chromosome structure, the genetic algorithm evolves the base detector to a group of more efficient detectors in terms of bit-error rate performance. First, a new packet-level genetic MUD technique, using a conventional single user detector as the base detector, is proposed for asynchronous CDMA (ACDMA) with negligible ISI. Then the signal-subspace-based minimum mean square error detector is chosen as a base detector and wrapped inside the hybrid genetic engine to evolve to a better structure nearly to eliminate both ISI and MAI. The novelty of the proposed structure is the way the deterministic closed-form solution of the base detector is mapped to a genetic engine resulting in a group of more efficient and adaptive detectors     

一种子空间盲自适应多用户检测技术在天线阵CDMA系统中的应用

多用户检测技术是ＤＳ－ＣＤＭＡ中的一项关键技术,而天线阵也是一种减轻多用户干扰的方法,本文将两种技术结合在一起,提出了将子空间方法应用于天线阵多用户检测技术,这种技术可适用于低速移动台在慢衰落信道下的盲检测,仿真结果表明这种算法比单纯作最佳二维Ｒａｋｅ接收要好,和有训练序列的先做ＣＭＯＥ再做最佳合并的性能差不多。     

A Multiuser Detection Receiver Using Blind Antenna Array and Adaptive Parallel Interference Cancellation

In this paper, the problem of multiuser detection for synchronous code division multiple access systems in both additive white Gaussian noise and multipath channels is addressed. A new multiuser detection receiver that uses an adaptive blind array along with an adaptive parallel interference canceler is proposed. The replacement of a conventional antenna array with a two-dimensional RAKE receiver is also considered for frequency selective Rayleigh fading channels. By using a constrained optimization criterion along with the gradient-projection algorithm, a blind algorithm for the adaptation of the array response vector is proposed. The new algorithm is superior to a few typical blind algorithms in the literature in terms of both performance and computational complexity. The proposed receiver has the ability to cancel very strong multiple access interference coming from the same direction as the desired signal. Simulation results are presented to show the excellent performance of the proposed combination scheme in comparison to that of using either a multiuser detection or adaptive antenna arrays in a severe near-far situation.     

Code-aided interference suppression for DS/CDMA communications. II.Parallel blind adaptive implementations

For pt.I see ibid., vol.45, no.9, p.1101-11 (1997). An adaptive code-aided technique for the simultaneous suppression of narrow-band interference (NBI) and multiple-access interference (MAI) in direct-sequence code-division multiple-access (DS/CDMA) networks is proposed. This technique is based on the recursive least-squares (RLS) version of the minimum mean-square error (MMSE) algorithm for multiuser detection. The convergence dynamics of the RLS blind adaptive algorithm for suppressing the combined NBI and MAI are analyzed. The steady-state performance of this algorithm in terms of the signal-to-interference ratio (SIR) is also derived. Systolic array structures for parallel implementations of the RLS adaptive interference suppression algorithms are then proposed. Versions of the rotation-based QR-RLS algorithms for both the blind adaptation mode and the decision-directed adaptation mode are derived. These algorithms exhibit high degrees of parallelism, and can be mapped to VLSI systolic arrays to exploit massively parallel signal processing computation     

Pilot to data channel power allocation for PCA-DS/CDMA withinterference canceler

The purpose of this letter is to examine channel power assignment for multiuser direct sequence/code division multiple access (DS/CDMA) uplinks. The channels under consideration are pilot channels and data channels. A multiuser detector is also included. The pilot channel estimates are used for multiple access interference (MAI) regeneration, as well as for data channel detection. Simulation results allowed us to pinpoint optimum pilot-to-data channel power ratios (PDRs) for the best bit error rate (BER) performance. In addition, the optimum PDRs of the receivers with the multiuser detector are high compared with those of conventional receivers without the multiuser detector. This study may prove helpful in the effort to optimize the channel power assignment for channel estimation and coherent detection of systems with the multiuser detector     

基于子空间盲多用户检测算法的设计和仿真

随着数字移动通信技术的飞速发展,以码分多址(CDMA)为核心的第三代移动通信(3G)技术已经走入人们的日常生活。CDMA系统具有容量大、软容量、抗多径衰落强等优点,但多址干扰(MAI)的存在却严重的影响了系统的性能和容量,因此如何消除和抑制这种干扰就成了CDMA技术研究的热门话题之一,第三代移动通信系统已经将多用户检测技术(MUD)作为克服多址干扰的有效方法之一。文中主要研究了直接序列扩频码分多址(DS-CDMA)系统中的盲多用户检测算法,深入的研究了基于卡尔曼滤波的盲多用户检测算法,通过在卡尔曼滤波算法上引入子空间提高了原有卡尔曼滤波算法的效率。     

Performance of multiuser detectors based on EM-like method for ultra-wideband communications systems in multipath fading channel

An adaptive multiuser detector (MUD) is proposed for direct-sequence ultra-wideband (DS-UWB) multiple access communication systems to suppress both multiple access interference (MAI) and inter-symbol interference (ISI). In this contribution, considering the MUD from a combination viewpoint, we proposed a MUD based on electromagnetism-like (EM) method, which applied the concept of EM search to Hopfield neural network (EMHNN) for solving optimization problems. We analyze the performance of the EMHNN MUD in multipath fading channel, and compare it with the optimum detector and several suboptimum schemes such as conventional, decorrelator detector (DD), minimum-mean-squared error (MMSE) and HNN MUD. Simulation results will demonstrate that the proposed EMHNN MUD, which alleviates the detrimental effects of the MAI problem, can significantly improve the system performance.