多径衰落信道下基于子空间的RLS盲多用户检测

本文在RLS盲检测算法的基础上,利用子空间的概念,构建了基于子空间的RLS多用户盲检测算法,在仅仅需要知道目标用户的特征序列和定时的条件下,自适应地估计检测向量,通过理论分析表明,改进的检测算法在运算复杂度上低于满秩RLS算法[7].仿真结果表明,改进的检测算法收敛性能优于满秩RLS算法,同时在特征序列畸变条件下表现出健壮件也远优于满秩RLS检测算法.     

Spatio-temporal blind adaptive multiuser detection

We propose blind multiuser detection schemes with antenna arrays, which is based on signal subspace estimation. They are a multichannel extension of the decorrelating and minimum mean-square-error detectors, and therefore they share their immunity to near-far effects. The blind scheme may be seen as an extension of the results in of Wang and Poor (see IEEE Trans. Inform. Theory, vol.44, p.677-90, 1998). However, it is seen that compared with the latter results when spatial diversity is considered, the proposed spatio-temporal detectors offer, with little attendant increase in computational complexity, a better performance. A blind adaptive implementation based on a new orthogonal PAST (projection approximation subspace tracking) algorithm, which is shown to be efficient for subspace tracking, is proposed. Also, we develop a blind estimation of the spatial signature based on the orthogonality between noise and signal subspaces. It is seen that the blind adaptive multiuser detection and blind spatio-temporal signature estimation can he integrated jointly     

Blind adaptive multiuser detection in multipath CDMA channels basedon subspace tracking

A blind adaptive technique for signal demodulation in multipath code-division multiple-access (CDMA) communication channels is proposed. This technique is based on signal subspace estimation. The receiver employs a bank of linear filters (decorrelating filters or linear MMSE filters) at the front end to mitigate the multiple-access interference and the multipath interference. A channel estimator is used to estimate the channel state for diversity combining. It is shown that through the use of signal subspace estimation, both the decorrelating filterbank and the linear MMSE filterbank can be obtained blindly, i.e., they can be estimated from the received signal with the prior knowledge of only the signature waveform of the desired user. Two forms of the subspace-based linear filterbanks are developed and their equivalence in terms of the interference suppression capability is established. These subspace-based blind adaptive interference suppression techniques require, at each symbol epoch, the eigenvalues and the eigenvectors of an appropriate signal subspace, which ran be obtained using computationally efficient sequential adaptive eigendecomposition (subspace tracking) algorithms. Moreover, a blind adaptive method for estimating the channel state is developed, which also produces the postcombining decision statistic as a byproduct     

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 equalization and multiuser detection in dispersive CDMAchannels

The problem of blind demodulation of multiuser information symbols in a high-rate code-division multiple-access (CDMA) network in the presence of both multiple-access interference (MAI) and intersymbol interference (ISI) is considered. The dispersive CDMA channel is first cast into a multiple-input multiple-output (MIMO) signal model framework. By applying the theory of blind MIMO channel identification and equalization, it is then shown that under certain conditions the multiuser information symbols can be recovered without any prior knowledge of the channel or the users' signature waveforms (including the desired user's signature waveform), although the algorithmic complexity of such an approach is prohibitively high. However, in practice, the signature waveform of the user of interest is always available at the receiver. It is shown that by incorporating this knowledge, the impulse response of each user's dispersive channel can be identified using a subspace method. It is further shown that based on the identified signal subspace parameters and the channel response, two linear detectors that are capable of suppressing both MAI and ISI, i.e., a zero-forcing detector and a minimum-mean-square-error (MMSE) detector, can be constructed in closed form, at almost no extra computational cost. Data detection can then be furnished by applying these linear detectors (obtained blindly) to the received signal. The major contribution of this paper is the development of these subspace-based blind techniques for joint suppression of MAI and ISI in the dispersive CDMA channels     

基于子空间跟踪算法的盲多用户检测技术研究

本文提出一种基于压缩投影逼近子空间跟踪(PASTd)算法与Kalman滤波算法相结合的盲自适应多用户检测算法.基于本算法,仅使用期望用户的特征波形和定时信息,多用户检测器可以盲获得.仿真实验表明,在多径非频率选择性衰落信道和用户数目变化的动态环境中,本文提出的盲多用户检测算法虽然在计算量上较LMS算法略为提高,但其收敛性能却优于后者,与RLS算法和Kalman滤波算法相比,本文提出的算法具有更低的计算复杂度和更优的收敛性能.     

Blind multiuser detection using linear prediction

We propose a blind multiuser detection technique for array processing and code division multiple access (CDMA) systems that does not require knowledge of the array geometry or transmitter signature sequences. The technique has two key elements: an adaptive algorithm for separating the signal subspace from the noise subspace and an adaptive whitener based on linear prediction. The proposed algorithm offers low complexity, fast convergence, compatibility with shaped signal constellations, near-Wiener steady-state performance, and optimal near-far resistance     

The Subspace-Based Linear Conjugate CMA in BPSK-Modulated MC-CDMA Systems

A subspace-based linear conjugate constant modulus algorithm (CMA) for blind adaptive multiuser detection in BPSK-Modulated MC-CDMA systems is proposed. First, a linear conjugate CMA algorithm is presented which jointly elaborates the received signal and its complex conjugate to utilize the information contained in the pseudo-autocorrelation of the observable, thus is capable of achieving much better performance. Second, to further improve the performance, the subspace method is applied to the linear conjugate CMA algorithm to obtain a subspace-based linear conjugate CMA algorithm, which mitigates the influence of the noise subspace. Finally, an improved projection approximation subspace tracking with deflation algorithm is proposed for adaptive signal subspace estimation to reduce the computational complexity. The proposed algorithm provides an improved robustness against the signature waveform mismatch, good tracking capability in dynamic environment and better output signal-to-interference-plus-noise-ratio and bit error rate performance. The superiority of the proposed algorithm is demonstrated through computer simulations.     

Blind adaptive joint multiuser detection and equalization in dispersive differentially encoded CDMA channels

The problem of blind adaptive joint multiuser detection and equalization in direct-sequence code division multiple access (DS/CDMA) systems operating over fading dispersive channels is considered. A blind and code-aided detection algorithm is proposed, i.e., the procedure requires knowledge of neither the interfering users' parameters (spreading codes, timing offsets, and propagation channels), nor the timing and channel impulse response of the user of interest but only of its spreading code. The proposed structure is a two-stage one: the first stage is aimed at suppressing the multiuser interference, whereas the second-stage performs channel estimation and data detection. Special attention is paid to theoretical issues concerning the design of the interference blocking stage and, in particular, to the development of general conditions to prevent signal cancellation under vanishingly small noise. A statistical analysis of the proposed system is also presented, showing that it incurs a very limited loss with respect to the nonblind minimum mean square error detector, outperforms other previously known blind systems, and is near-far resistant. A major advantage of the new structure is that it admits an adaptive implementation with quadratic (in the processing gain) computational complexity. This adaptive algorithm, which couples a recursive-least-squares estimation of the blocking matrix and subspace tracking techniques, achieves effective steady-state performance.     

Subspace methods for blind joint channel estimation and multiuser detection in CDMA systems

Recently developed subspace techniques for blind adaptive multiuser detection are briefly reviewed first. In particular, blind methods based on signal subspace tracking for adapting linear multiuser detectors in AWGN CDMA channels are considered, as well as extensions of these techniques to frequency selective fading channels, dispersive channels, and antenna array spatial processing. In addition, subspace‐based nonlinear adaptive techniques for robust blind multiuser detection in non‐Gaussian ambient noise channels are also described. Several new techniques are then developed within the subspace framework for blind joint channel estimation and multiuser detection, under some specific channel conditions. These include (1) an adaptive receiver structure for joint multiuser detection and equalization in dispersive CDMA channels, (2) a subspace method for joint multiuser detection and equalization in unknown correlated noise, and (3) a method for joint interference suppression and channel tracking in time‐varying fading channels. This revised version was published online in August 2006 with corrections to the Cover Date.     

Robust multiuser detection in non-Gaussian channels

In many wireless systems where multiuser detection techniques may be applied, the ambient channel noise is known through experimental measurements to be decidedly non-Gaussian, due largely to impulsive phenomena. The performance of many multiuser detectors can degrade substantially in the presence of such impulsive ambient noise. We develop robust multiuser detection techniques for combating multiple-access interference and impulsive noise in CDMA communication systems. These techniques are based on the M-estimation method for robust regression. Analytical and simulation results show that the proposed robust techniques offer significant performance gain over linear multiuser detectors in impulsive noise, with little attendant increase in computational complexity. We also develop a subspace-based technique for blind adaptive implementation of the robust multiuser detectors, which requires only the signature waveform and the timing of the desired user in order to demodulate that user's signal. The robust multiuser detection technique and its blind adaptive version can be applied to both synchronous and asynchronous CDMA channels     

Robust blind multiuser detection against signature waveform mismatch based on second-order cone programming

Blind signal detection in multiuser code division multiple access (CDMA) system is particularly attractive when only the desired user signature is known to a given receiver. A problem common to several existing blind multiuser CDMA detectors is that the detection performance is very sensitive to the signature waveform mismatch (SWM) which may be caused by channel distortion. In this paper, we consider the design of a blind multiuser CDMA detector that is robust to the SWM. We present a convex formulation for this problem by using the second-order cone (SOC) programming. The resulting SOC problem can be solved efficiently using the recently developed interior point methods. Computer simulations indicate that the performance of our new robust blind multiuser detector is superior to those of many existing methods.     

Blind multiuser detector based on orthogonal projection [for CDMA]

A new stable blind multiuser detector based on signal subspace orthogonal projection is presented. By carrying out a pseudoeigendecomposition (PED) analysis, a technique proposed for the first time, the signal of the interested user can be approximated by the orthogonal projection of a received multiuser signal onto the related subspace of the interested user. Simulations show that compared with the previous subspace-based MMSE blind multiuser detector, the new detector offers better performance without leading to an increase in the computational complexity     

Blind adaptive multiuser detection

The decorrelating detector and the linear minimum mean-square error (MMSE) detector are known to be effective strategies to counter the presence of multiuser interference in code-division multiple-access channels; in particular, those multiuser detectors provide optimum near-far resistance. When training data sequences are available, the MMSE multiuser detector can be implemented adaptively without knowledge of signature waveforms or received amplitudes. This paper introduces an adaptive multiuser detector which converges (for any initialization) to the MMSE detector without requiring training sequences. This blind multiuser detector requires no more knowledge than does the conventional single-user receiver: the desired user's signature waveform and its timing. The proposed blind multiuser detector is made robust with respect to imprecise knowledge of the received signature waveform of the user of interest     

On CDMA with space-time codes over multipath fading channels

We explore code-division multiple-access systems with multiple transmitter and receiver antennas combined with algebraic constellations over a quasi-static multipath fading channel. We first propose a technique to obtain transmit diversity for a single user over quasi-static fading channels by combining algebraic constellations with full spatial diversity and spreading sequences with good cross-correlation properties. The proposed scheme is then generalized to a multiuser system using the same algebraic constellation and different spreading sequences. We also propose a linear multiuser detector based on the combination of linear decorrelation with respect to all users, and the application of the sphere decoder to decode each user separately. Finally, we consider the generalization to multipath fading channels where the additional diversity advantage due to multipath is exploited by the sphere decoder, and a method of blind channel estimation based on subspace decomposition is examined.     

Blind adaptive multiuser detection based on a constrained MMSE criterion

The proposed blind adaptive multiuser detector utilizes the signature waveform and time information of the desired user. With each received sample vector, the proposed algorithm updates the detector and gives the symbol estimate in the current time slot. Such property facilitates it to track time-varying channels.     

Blind detection for multi-rate DS-CDMA system with antenna array at the base station

In this paper, we propose the blind space-time high rate multi-user detector for synchronous uplink multi-rate Direct Sequence Code Division Multiple Access （DS-CDMA） systems with antenna array at the base station. By employing antenna array at the base stations, the spatial dimension is used efficiently to suppress co-channel interference and increase the capacity for multi-rate CDMA system. After low rate physical users in the system are modeled as corresponding high rate virtual users, we construct the space-time signature vectors of virtual users. And subspace projection algorithm is employed to estimate space-time signature vectors blindly. Then a soft-decision high rate lnultiuser detector is proposed based on the estimated signature vectors, which avoids estimating the ambiguous complex factors which are necessary in traditional blind detector. Numerical simulation results evaluate the performance in terms of Bit Error Rate （BER） for the proposed scheme. Simultaneously, it demonstrates that the system capability increases two times when using twoelement antenna array.     

多速率CDMA系统的盲自适应多用户接收机

该文研究可变扩频长度(Variable Spreading Length, VSL) 多速率直接序列码分多址(Direct-Sequence Code-Division Multiple-Access, DS-CDMA) 系统中的自适应多用户接收机。利用到相交子空间的迭代投影估计得到包含信道信息的特征矢量,以此构造多用户接收机。借助于子空间跟踪算法的特殊形式,推导了特征矢量估计算法及多用户接收机的一种低复杂度的自适应实现。仿真实验证明,该算法有很好的数值稳定性,与传统算法BER性能相同的情况下,具有更低的计算复杂度。     

Blind multiuser detection for asynchronous MC-CDMA systems without channel estimation

This paper presents a blind adaptive decorrelating detector for asynchronous multicarrier code-division multiple-access (MC-CDMA) systems with Rayleigh-fading channels. This detector is derived by making use of the cross-correlation matrix between the consecutively received signals. The main attraction of the detection algorithm is its simplicity, since the detector can be implemented blindly without channel estimation, except for the synchronization of the desired user. To implement the proposed detector, a new adaptive subspace-tracking algorithm for any Hamiltonian matrices is developed, while the previously reported subspace-tracking methods can only estimate the subspace of a positive Hamiltonian matrix. Since a common drawback of the multiuser detector based on the subspace method is the existence of a phase factor that usually deteriorates the performance of the detector, a new method is developed to compensate for the phase of the proposed detector, based on the property of the complex white Gaussian noise. Results of extensive computer simulations show the performance improvement of the proposed detector.     

Advanced blind adaptive multi-user detector for communications innonstationary multipath fading channel

This paper deals with an adaptive multi-user detector for direct sequence code division multiple access (DS/CDMA) wireless communication systems, named advanced blind adaptive multi-user detector (ABA-MUD), whose main features are low complexity and joint utilization of time diversity and adaptive blind processing techniques. Differently from known blind adaptive detectors, the proposed scheme achieves remarkable performance even in critical time-varying environments by means of a suitable window reprocessing technique. The ABA-MUD avoids the use of training sequences and only needs knowledge of timing and signature waveform of the desired user, number of active users and a rough evaluation of the signal-to-interference ratio (SIR) for a proper setting of the detection algorithm. Numerical results, shown here in terms of bit error rate (BER), highlight good behavior and remarkable near-far resistance of the proposed ABA-MUD receiver with respect to different alternatives, in particular, in the case of worst fading environments