Robust approach to channel estimation and detection for OFDM systems

A robust channel estimation and detection scheme has been developed that explicitly accounts for channel estimation error by optimising the worst-case performance over a properly selected bounded uncertainty set. Numerical results show improved performance using the proposed robust approach over the one that ignores the prior estimation errors.     

一种频选衰落信道下的Turbo多用户检测算法

联合MAP多用户检测与信道解码的迭代多用户检测(MUD)技术可显著提高宽带移动CDMA系统的容量和性能.在多径时变衰落的编码信道下,提出一种迭代实现干扰抑制、符号估计、信道解码的Turbo多用户检测算法.在每次迭代中,MUD自适应地实现干扰抑制并输出符号估计的软信息,软输入软输出的信道解码器使用LOG MAP方法实现信道解码并反馈符号估计的软信息作为下一次TurboMUD迭代的先验信息.仿真结果证实了该算法在频选衰落信道下经两次迭代就能逼近单用户编码CDMA系统的接收性能.     

Robust Waveform Design for MIMO Radar with Imperfect Prior Knowledge

Waveform optimization for multi-input multi-output radar usually depends on the initial parameter estimates (i.e., some prior information on the target of interest and scenario). However, it is sensitive to estimate errors and uncertainty in the parameters. Robust waveform design attempts to systematically alleviate the sensitivity by explicitly incorporating a parameter uncertainty model into the optimization problem. In this paper, we consider the robust waveform optimization to improve the worst-case performance of parameter estimation over a convex uncertainty model, which is based on the Cramer–Rao bound. An iterative algorithm is proposed to optimize the waveform covariance matrix such that the worst-case performance can be improved. Each iteration step in the proposed algorithm is solved by resorting to convex relaxation that belongs to the semidefinite programming class. Numerical results show that the worst-case performance can be improved considerably by the proposed method compared to that of uncorrelated waveforms and the non-robust method.     

Consistent estimation of Rayleigh fading channel second-orderstatistics in the context of the wideband CDMA mode of the UMTS

We address the problem of estimating the second-order statistics of a frequency-selective Rayleigh channel in the context of the wideband CDMA mode of the UMTS. The data to be transmitted are sent over slots on which the channel is assumed to remain constant. Each slot contains a pilot symbol sequence from which it is possible to estimate the current value of the channel. The covariance matrix of the channel is usually estimated by a denoised version of the empirical covariance matrix of the trained channel estimate. However, in the UMTS, this estimate is not consistent in the sense that if the number of slots used to estimate it tends to infinity, it does not converge to the true covariance matrix of the channel. We propose a new consistent estimate of the covariance matrix and evaluate the performances of two Wiener-like channel estimation schemes based on the proposed estimate. The performances of the new approach are evaluated by means of the bit error rate provided by a RAKE receiver based on the proposed channel estimates. It is shown that our estimate of the covariance matrix allows significant improvement in the performance of the RAKE receiver     

Multiuser detection of DS-CDMA signals using partial parallel interference cancellation in satellite communications

Multiuser detection (MUD) using parallel interference cancellation (PIC) technique provides a good complexity, latency, and performance compromise. This technique is suitable for satellite systems using either code-division multiple-access (CDMA) or a combination of time-division multiple-access (TDMA) and CDMA. We offer a new scheme that is a combination of soft and hard PIC detectors whose performance is superior to that of the other famous suboptimal detectors. In soft partial parallel interference cancellation (PPIC), in the first few stages, when the performance is still poor, the accurate knowledge of power and phase cannot be of much use. However, in the following stages, accurate power and phase estimation can improve the performance. This coincides with the time when the decisions are reliable enough to be used for parameter estimation. In our scheme, after a few stages of soft interference cancellation (IC), estimation of the parameters will start. Having these estimates, in the subsequent stages hard IC is performed. The complexity of this scheme grows linearly with the number of users. Moreover, this scheme is much faster than other receivers such as successive interference cancellation (SIC). PIC detectors are usually studied in equal-power case, i.e., a perfect power control scheme is assumed. In this paper, PIC detector in a near-far condition where user signals arrive at the receiver with different power levels is also investigated.     

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     

A full-rank regularization technique for MMSE detection in multiuser CDMA systems

In multiuser code-division multiple-access (CDMA) environments, the ill-conditioning of the covariance matrix of the received signal may degrade the performance of minimum mean-squared error (MMSE) detectors, especially when few samples are available for the covariance matrix estimation. In order to mitigate this performance degradation, we propose a full-rank regularized MMSE detector based on the covariance matrix tapering (CMT) technique. Simulation results show the effectiveness of the proposed technique at high signal-to-noise ratio (SNR).     

An Adaptive Multiuser Detection Algorithm for CDMA Systems Using Antenna Diversity

Based on the minimum mean squared error (MMSE) between the data stream and the linear combiner output, a new multiuser detection (MUD) algorithm that combines space–time (ST) processing and antenna array on direct-sequence CDMA signals is proposed. The proposed ST-MUD algorithm is proved to be equivalent to two existing MMSE-based ST-MUD algorithms, and the theoretical BER performances for all the three algorithms are the same. The most attractive feature of the new ST-MUD algorithm is based on the fact that the new method does not require explicit estimation of channel and signaling information. This avoids any channel estimation error, and the method is thus more robust and more accurate than the other two ST-MUD algorithms in practical implementation. Adaptation of the proposed ST-MUD algorithm is implemented by using training sequences. Performance of this new multiuser detector is compared with that of two existing MMSE multiuser detectors and the conventional single-user space–time rake receiver through simulations. The proposed ST-MUD algorithm provides a performance better than existing algorithms and is especially suitable for practical CDMA systems.     

基于SVD的LC_CDMA通信系统信道估计研究

在LC-CDMA通信系统中，由于在多径衰落信道下使用了时变扩频码，使得信道估计非常困难，而对接收机的设计往往需要信道的先验知识。本文建立了长码条件下的CDMA通信系统模型和多径频率选择性衰落信道的FIR滤波器模型，利用子空间分解的方法，推导了长码条件下CDMA通信系统的信道估计算法，并仿真分析算法的性能。     

Robust Competitive Estimation With Signal and Noise Covariance Uncertainties

Robust estimation of a random vector in a linear model in the presence of model uncertainties has been studied in several recent works. While previous methods considered the case in which the uncertainty is in the signal covariance, and possibly the model matrix, but the noise covariance is assumed to be completely specified, here we extend the results to the case where the noise statistics may also be subjected to uncertainties. We propose several different approaches to robust estimation, which differ in their assumptions on the given statistics. In the first method, we assume that the model matrix and both the signal and the noise covariance matrices are uncertain, and develop a minimax mean-squared error (MSE) estimator that minimizes the worst case MSE in the region of uncertainty. The second strategy assumes that the model matrix is given and tries to uniformly approach the performance of the linear minimum MSE estimator that knows the signal and noise covariances by minimizing a worst case regret measure. The regret is defined as the difference or ratio between the MSE attainable using a linear estimator, ignorant of the signal and noise covariances, and the minimum MSE possible when the statistics are known. As we show, earlier solutions follow directly from our more general results. However, the approach taken here in developing the robust estimators is considerably simpler than previous methods     

Maximum-likelihood synchronization of a single user forcode-division multiple-access communication systems

Code-division multiple access (CDMA) has emerged as an access protocol well-suited for voice and data transmission. One significant limitation of the conventional CDMA system is the near-far problem where strong signals interfere with the detection of a weak signal. Multiuser detectors assume knowledge of all of the modulation waveforms and channel parameters, and exploit this information to eliminate multiple-access interference (MAI) and to achieve near-far resistance. A major problem in practical application of multiuser detection is the estimation of the signal and channel parameters in a near-far limited system. We consider maximum-likelihood estimation of users delay, amplitude, and phase in a CDMA communication system. We present an approach for decomposing this multiuser estimation problem into a series of single-user problems. In this method the interfering users are treated as colored non-Gaussian noise. The observation vectors are preprocessed to be able to apply a Gaussian model for the MAI. The maximum-likelihood estimate (MLE) of each user's parameters based on the processed observation vectors becomes tractable. The estimator includes a whitening filter derived from the sample covariance matrix which is used to suppress the MAI, thus yielding a near-far resistant estimator     

直扩卫星通信系统中多用户检测算法比较

实际在CDMA系统中,多址干扰(MAI)是主要的干扰来源,多用户检测技术完全可以抑制这种多址干扰。论文分析了几种典型的多用户检测技术,并在高斯信道环境下对解相关检测器、最小均方误差检测器和传统检测器进行了仿真和性能比较。     

Autocorrelation-based blind spreading-factor detection for CDMA

Spreading factor (SF) in code-division multiple-access (CDMA) systems depends on the data rate of the user, and is, therefore, unknown a priori for a communication receiver. In this letter, the blind SF-detection problem in an additive white Gaussian noise channel is studied, and a novel one-step autocorrelation-based SF detector is proposed. Three decision rules are derived to find simple and robust SF detectors for practical communication receivers. Performance of the detectors is studied and compared with the optimal detector via Monte Carlo computer simulations. The autocorrelation-based SF detector appears to give performance close to the optimal detector, assuming perfect knowledge of signal-to-noise ratio (SNR). It is also found to be significantly more robust to SNR estimation errors than the optimal detector.     

MBER Space-Time Decision Feedback Equalization Assisted Multiuser Detection for Multiple Antenna Aided SDMA Systems

This paper proposes a space-time decision feedback equalization (ST-DFE) assisted multiuser detection (MUD) scheme for multiple receiver antenna aided space division multiple access systems. A minimum bit error rate (MBER) design is invoked for the MUD, which is shown to be capable of improving the achievable bit error rate performance and enhancing the attainable system capacity over that of the standard minimum mean square error (MMSE) design. An adaptive implementation of the MBER ST-DFE assisted MUD is proposed using a stochastic gradient-based least bit error rate algorithm, which is demonstrated to consistently outperform the classical least mean square (LMS) algorithm, while achieving a lower computational complexity than the LMS algorithm for the binary signalling scheme. Our simulation results demonstrate that the MBER ST-DFE assisted MUD is more robust to channel estimation errors as well as to potential error propagation imposed by decision feedback errors, compared to the MMSE ST-DFE assisted MUD.     

多径衰落下多天线CDMA系统信道容量研究

该文对准静态Rayleigh衰落下MIMO-CDMA系统信道容量进行了研究,分析了扩频码为Walsh函数,基于多码检测的极大似然检测器(MLD)和解相关检测器的性能,并和MMSE多用户检测器(MUD)进行了比较;分析了对不同检测器在一定的信噪比下系统天线数目和信道容量的关系。仿真结果表明：在相同的情况下,基于Walsh码CDMA系统容量按MUD、干扰方差已知的MLD,干扰方差未知的MLD和解相关检测器递减,并且后面的3种检测器在大信噪比的时候均有渐进的平台效应;在信噪比较大且一定时,CDMA系统信道容量与天线数目呈线性关系。     

DS-CDMA卫星通信系统线性多用户检测算法性能比较

CDMA是新一代移动通信系统的关键技术,现代卫星移动通信也采用CDMA技术。多址干扰(MAI)的存在严重影响了CDMA系统的性能,需要采用多用户检测技术进行抑制。本文分析了卫星移动通信系统的模型,将线性多用户检测算法用于卫星移动通信系统,并对传统检测器、解相关检测器、MMSE检测器等线性检测器进行了性能比较。计算机仿真结果表明采用线性多用户检测技术使卫星移动通信系统的性能得到了明显的改善。     

Multiuser channel estimation and tracking for long-code CDMA systems

Channel estimation techniques for code-division multiple access (CDMA) systems need to combat multiple access interference (MAI) effectively. Most existing estimation techniques are designed for CDMA systems with short repetitive spreading codes. However, current and next-generation wireless systems use long spreading codes whose periods are much larger than the symbol duration. We derive the maximum-likelihood channel estimate for long-code CDMA systems over multipath channels using training sequences and approximate it using an iterative algorithm to reduce the computational complexity in each symbol duration. The iterative channel estimate is also shown to be asymptotically unbiased. The effectiveness of the iterative channel estimator is demonstrated in terms of squared error in estimation as well as the bit error rate performance of a multistage detector based on the channel estimates. The effect of error in decision feedback from the multistage detector (used in the absence of training sequences) is also shown to be negligible for reasonable feedback error rates using simulations. The proposed iterative channel estimation technique is also extended to track slowly varying multipath fading channels using decision feedback. Thus, an MAI-resistant multiuser channel estimation and tracking scheme with reasonable computational complexity is derived for long-code CDMA systems over multipath fading channels.     

Large system performance of linear multiuser receivers in multipathfading channels

A linear multiuser receiver for a particular user in a code-division multiple-access (CDMA) network gains potential benefits from knowledge of the channels of all users in the system. In fast multipath fading environments we cannot assume that the channel estimates are perfect and the inevitable channel estimation errors will limit this potential gain. We study the impact of channel estimation errors on the performance of linear multiuser receivers, as well as the channel estimation problem itself. Of particular interest are the scalability properties of the channel and data estimation algorithms: what happens to the performance as the system bandwidth and the number of users (and hence channels to estimate) grows? Our main results involve asymptotic expressions for the signal-to-interference ratio of linear multiuser receivers in the limit of large processing gain, with the number of users divided by the processing gain held constant. We employ a random model for the spreading sequences and the limiting signal-to-interference ratio expressions are independent of the actual signature sequences, depending only on the system loading and the channel statistics: background noise power, energy profile of resolvable multipaths, and channel coherence time. The effect of channel uncertainty on the performance of multiuser receivers is succinctly captured by the notion of effective interference     

Group-blind multiuser detection for uplink CDMA

Previously developed blind techniques for multiuser detection in code division multiple access (CDMA) systems lead to several near-far resistant adaptive receivers for demodulating a given user's data with the prior knowledge of only the spreading sequence of that user. In the CDMA uplink, however, typically the base station receiver has the knowledge of the spreading sequences of all the users within the cell, but not that of the users from other cells. In this paper, group-blind techniques are developed for multiuser detection in such scenarios. These new techniques make use of the spreading sequences and the estimated multipath channels of all known users to suppress the intracell interference, while blindly suppressing the intercell interference. Several forms of group-blind linear detectors are developed based on different criteria. Moreover, group-blind multiuser detection in the presence of correlated noise is also considered. In this case, two receiving antennas are needed for channel estimation and signal separation. Simulation results demonstrate that the proposed group-blind linear multiuser detection techniques offer substantial performance gains over the blind linear multiuser detection methods in a CDMA uplink environment     

On the second-order statistics of the weighted sample covariance matrix

The second-order statistics of the sample covariance are encountered in many covariance based processing algorithms. This paper derives closed-form expressions for the covariance of the weighted sample covariance matrix with an arbitrary weight for both a real system and complex system. Given a system model, the results explicitly rely on the second-order and fourth-order statistics of the channel noise and inputs. They are shown to coincide with the existing results when the channel inputs and noise are Gaussian distributed. Our results can be directly applied to analyze the statistical properties of subspace-based channel estimation methods for single-input multiple-output (SIMO) systems and code-division multiple access (CDMA) systems. Numerical examples are provided to further verify analyses.