非线性调制下MMSE盲自适应多用户接收机

主要介绍三种面向非正交多脉冲调制(NMM)系统的非相干最小均方误差(MMSE)多用户检测器,它们均采用MMSE预滤波器(per-filter),但分别满足三种不同的非相干判决准则。其中最简单的判决准则是最大(MM)准则,另外两种则需要考虑MMSE滤波后的残余多用户干扰(MAI)的二阶统计量。然后介绍一种基于随机估计方法的盲自适应算法。最后对这三种判决准则进行性能分析。     

Output MAI distributions of linear MMSE multiuser receivers inDS-CDMA systems

Multiple-access interference (MAI) in a code-division multiple-access (CDMA) system plays an important role in performance analysis and characterization of fundamental system limits. We study the behavior of the output MAI of the minimum mean-square error (MMSE) receiver employed in the uplink of a direct-sequence (DS)-CDMA system. We focus on imperfect power-controlled systems with random spreading, and establish that in a synchronous system (1) the output MAI of the MMSE receiver is asymptotically Gaussian, and (2) for almost every realization of the signatures and received powers, the conditional distribution of the output MAI converges weakly to the same Gaussian distribution as in the unconditional case. We also extend our study to asynchronous systems and establish the Gaussian nature of the output interference. These results indicate that in a large system the output interference is approximately Gaussian, and the performance of the MMSE receiver is robust to the randomness of the signatures and received powers. The Gaussianity justifies the use of single-user Gaussian codes for CDMA systems with linear MMSE receivers, and implies that from the viewpoints of detection and channel capacity, signal-to-interference ratio (SIR) is the key parameter that governs the performance of the MMSE receiver in a CDMA system     

MIMO系统中一种改进的盲MMSE空时多用户检测算法

针对MIMO系统,提出了一种改进的基于子空间的盲MMSE空时多用户检测算法。该算法结合MIMO系统的空间分集技术与Alamouti空时分组码方案,预估计MIMO信道信息并对信号子空间进行预处理,使用正交性能和稳态性能较好的NOOja算法跟踪信号子空间,在自适应过程中对特征值矩阵进行优化,去除迭代带来的噪声,解决了跟踪过程中信号特征值矩阵的近似估计会带来检测器性能恶化的问题。仿真结果表明这种算法,能有效地抑制多址干扰,抗远近效应能力强,尤其在低信噪比、远近效应明显的恶劣环境下,有稳定良好的性能表现。      

Noncoherent decision-feedback multiuser detection

The problem of noncoherent multiuser detection for multipulse modulation over the Gaussian multiuser channel is studied. It is assumed that neither the energies nor the carrier phases of the signals of any of the users are available at the receiver. Previously, a post-decorrelative generalized likelihood ratio test (GLRT) based detector was proposed as a solution to this problem. In this paper, we introduce the concept of noncoherent decision feedback (DF) that forms the basis for an improved solution to the same problem. The users are detected sequentially in some fixed order, and the decision for a particular user takes advantage of the reduction of uncertainty associated with the signal space resulting from decisions already made for previous users. In contrast to coherent DF, therefore, the feedforward and feedback transformations in the noncoherent case are themselves functions of the matched filter outputs. An efficient implementation of the noncoherent DF detector for M-ary modulation and ρ-dimensional signal space requires O(M_ρ) computational complexity per user. Upper and lower bounds on its symbol error probability are obtained. It is shown that significant improvements over the post-decorrelative GLRT-based detector are often possible. Sufficient conditions are obtained which guarantee, without ignoring error-propagation effects, that the high signal-to-noise performance of the DF strategy can be as good as its genie-aided version in which perfect past users' decisions are used     

Spectral efficiency and cutoff rate of random spreading BICM-based CDMA signals with MMSE multiuser receivers

We analyze the spectral efficiency and cutoff rate of random spreading bit-interleaved coded-modulation based synchronous code-division multiple-access signals with minimum mean-square error multiuser receivers. Both additive white Gaussian noise and frequency nonselective Rayleigh fading channels are considered. The spectral efficiency and cutoff rate are characterized as functions of the system load, modulation schemes, and signal-to-noise ratio.     

Noncoherent MMSE interference suppression for DS-CDMA

A novel robust noncoherent receiver for minimum mean-squared error (MMSE) interference suppression for direct-sequence code-division multiple access (DS-CDMA) is proposed. The receiver consists of a linear MMSE filter and a decision-feedback differential detector (DF-DD). The performance of the proposed scheme is investigated analytically and by computer simulations. It is shown that the loss compared to coherent MMSE interference suppression is limited and can be made arbitrarily small by increasing the observation window used for calculation of the reference symbol of the DF-DD. Hence, the regarded noncoherent receiver is near-far resistant. For adjustment of the MMSE filter coefficients three noncoherent adaptive algorithms are proposed. In contrast to coherent adaptive algorithms, these noncoherent algorithms have the important advantage that they also converge if the channel phase is time-variant     

Optimal sequences, power control, and user capacity of synchronousCDMA systems with linear MMSE multiuser receivers

There has been intense effort in the past decade to develop multiuser receiver structures which mitigate interference between users in spread-spectrum systems. While much of this research is performed at the physical layer, the appropriate power control and choice of signature sequences in conjunction with multiuser receivers and the resulting network user capacity is not well understood. In this paper we will focus on a single cell and consider both the uplink and downlink scenarios and assume a synchronous CDMA (S-CDMA) system. We characterize the user capacity of a single cell with the optimal linear receiver (MMSE receiver). The user capacity of the system is the maximum number of users per unit processing gain admissible in the system such that each user has its quality-of-service (QoS) requirement (expressed in terms of its desired signal-to-interference ratio) met. This characterization allows one to describe the user capacity through a simple effective bandwidth characterization: users are allowed in the system if and only if the sum of their effective bandwidths is less than the processing gain of the system. The effective bandwidth of each user is a simple monotonic function of its QoS requirement. We identify the optimal signature sequences and power control strategies so that the users meet their QoS requirement. The optimality is in the sense of minimizing the sum of allocated powers. It turns out that with this optimal allocation of signature sequences and powers, the linear MMSE receiver is just the corresponding matched filter for each user. We also characterize the effect of transmit power constraints on the user capacity     

Noncoherent detection in asynchronous multiuser channels

The noncoherent demodulation of multiple differentially phase-shift-keyed signals transmitted simultaneously over an asynchronous code-division multiple-access (CDMA) channel with white Gaussian background noise is considered. A class of bilinear detectors is defined with the objective of obtaining the optimal bilinear detector. The optimality criterion considered is near-far resistance that denotes worst-case asymptotic efficiency over the signal energies and phases which are unknown at the receiver. The optimal bilinear detector is therefore obtained by solving a minimax optimization problem. In the finite packet length case, this detector is shown to be a time-varying multiinput multioutput linear decorrelating filter followed by differential decision logic. In the limit as packet lengths go to infinity, the time-varying decorrelating detector is replaced by a time-invariant multiinput multioutput decorrelating filter. Several properties of the optimally near-far resistant detector are established     

Noncoherent receivers for differential space-time modulation

In this paper, noncoherent receivers for differential space-time modulation (DSTM) are investigated. It is shown that the performance of the previously proposed conventional differential detection (DD) receiver is satisfactory only for very slow flat fading channels. However, conventional DD suffers from a considerable loss in performance even for moderately fast fading, especially if more than one transmit antenna is used. In order to overcome this problem, two improved noncoherent receivers are considered. The first one is the multiple-symbol detection (MSD) receiver. Because of the high computational complexity of MSD, also a low-complexity decision-feedback differential detection (DF-DD) receiver is derived. Analytical and simulation results confirm that both receivers perform equally well and can take full advantage of the enhanced diversity provided by multiple transmit antennas even for fast fading     

A new family of MMSE multiuser receivers for interferencesuppression in DS/CDMA systems employing BPSK modulation

We deal with interference suppression in asynchronous direct-sequence code-division multiple-access (CDMA) systems employing binary phase-shift keying modulation. Such an interference may arise from other users of the network, from external low-rate systems, as well as from a CDMA network coexisting with the primary network to form a dual-rate network. We derive, for all of these cases, a new family of minimum mean-square-error detectors, which differ from their conventional counterparts in that they minimize a modified cost function. Since the resulting structure is not implementable with acceptable complexity, we also propose some suboptimum systems. The statistical analysis reveals that both the optimum and the suboptimum receivers are near-far resistant, not only with respect to the other users, but also with respect to the external interference. We also present a blind and a recursive least squares-based, decision-directed implementation of the receivers wherein only the signature and the timing of the user to be decoded and the signaling time and the frequency offset of the external interferer are assumed known. Finally, computer simulations show that the proposed adaptive algorithm outperforms the classical decision-directed RLS algorithm     

MMSE receivers for multirate DS-CDMA systems

Minimum-mean squared error (MMSE) receivers are designed and analyzed for multiple data rate direct-sequence code-division multiple-access (DS-CDMA) systems. The inherent cyclostationarity of the DS-CDMA signal is exploited to construct receivers for asynchronous multipath channels. Multiple- and single-bandwidth access are treated for both single and multicarrier scenarios. In general, the optimal receiver is periodically time-varying. When the period of the optimal receiver is large, suboptimal receivers are proposed to achieve a lower complexity implementation; the receivers are designed as a function of the cyclic statistics of the signals. In multiple chipping rate systems, the complexity of receivers for smaller bandwidth users can also be controlled by changing their front-end filter bandwidth. The effect of front-end filter bandwidth on receiver performance and system capacity is quantified for a variable chipping rate system. Analysis and simulation show that significant performance gains are realized by the periodically time-varying MMSE receivers over their time-invariant counterparts     

Noncoherent multistage multiuser detection of M-ary orthogonal signals

In recent years, a great deal of research has been performed on methods of alleviating the performance degradations suffered by code division multiple access (CDMA) systems due to multiple access interference. In this paper, we consider a multistage detector for noncoherent CDMA using M-ary orthogonal signals. Using a decorrelating detector as its first stage, the detector iteratively estimates the multiple access interference affecting the desired signal, subtracts the estimated interference, and forms symbol estimates for each of K users. Through numerical examples, the bit error performance of the proposed detector is demonstrated to be superior to that of previous detectors for the same signalling scheme. This revised version was published online in June 2006 with corrections to the Cover Date.     

Linear multiuser receivers in random environments

We study the signal-to-interference (SIR) performance of linear multiuser receivers in random environments, where signals from the users arrive in “random directions.” Such a random environment may arise in a DS-CDMA system with random signature sequences, or in a system with antenna diversity where the randomness is due to channel fading. Assuming that such random directions can be tracked by the receiver, the resulting SIR performance is a function of the directions and therefore also random. We study the asymptotic distribution of this random performance in the regime where both the number of users K and the number of degrees of freedom N in the system are large, but keeping their ratio fixed. Our results show that for both the decorrelator and the minimum mean-square error (MMSE) receiver, the variance of the SIR distribution decreases like 1/N, and the SIR distribution is asymptotically Gaussian. We compute closed-form expressions for the asymptotic means and variances for both receivers. Simulation results are presented to verify the accuracy of the asymptotic results for finite-sized systems     

Noncoherent adaptive linear MMSE equalisation for 16DAPSK signals

A novel adaptive noncoherent linear minimum mean squared error equaliser for 16-level differential amplitude/phase shift keying signals is proposed. A novel modified least mean square algorithm is derived which allows an efficient equaliser adaptation. Simulations confirm the high performance of the proposed noncoherent equaliser and its robustness against carrier phase variations     

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     

Probability of error in MMSE multiuser detection

The performance analysis of the minimum-mean-square-error (MMSE) linear multiuser detector is considered in an environment of nonorthogonal signaling and additive white Gaussian noise. In particular, the behavior of the multiple-access interference (MAI) at the output of the MMSE detector is examined under various asymptotic conditions, including: large signal-to-noise ratio; large near-far ratios; and large numbers of users. These results suggest that the MAI-plus-noise contending with the demodulation of a desired user is approximately Gaussian in many cases of interest. For the particular case of two users, it is shown that the maximum divergence between the output MAI-plus-noise and a Gaussian distribution having the same mean and variance is quite small in most cases of interest. It is further proved in this two-user case that the probability of error of the MMSE detector is better than that of the decorrelating linear detector for all values of normalized crosscorrelations not greater than ½√(2+√3)≅0.9659     

Adaptive MMSE maximum likelihood CDMA multiuser detection

The well-known code division multiple access maximum likelihood receiver (MF-ML) uses a bank of matched filters as a generator of sufficient statistics for maximum likelihood detection of users transmitted symbols. In this paper, the bank of matched filters is replaced by a bank of adaptive minimum mean squared error (MMSE) filters as the generator of sufficient statistics. This formal replacement of the MF bank by the adaptive MMSE filter bank has significant conceptual consequences and provides improvement by several performance measures. The adaptive MMSE-ML receiver's digital implementation is significantly computationally simplified. The advantages of the proposed adaptive MMSE-ML receiver over the MF-ML receiver are: (1) ability to perform joint synchronization, channel parameter estimation, and signal detection where the signal is sent over an unknown, slowly time-varying, frequency-selective multipath fading channel; (2) increased information capacity in a multicellular environment; and (3) significantly improved bit error rate (BER) performance in a multicellular mobile communications environment. The information capacity and the BER of the proposed MMSE-ML receiver are analyzed. Numerical results showing the BER performance of the MMSE-ML receiver in a multipath channel environment are presented     

Iterative receivers for multiuser space-time coding systems

Space-time coding (STC) techniques, which combine antenna array signal processing and channel coding techniques, are very promising approaches to substantial capacity increase in wireless channels. Multiuser detection techniques are powerful signal processing methodologies for interference suppression in CDMA systems. In this paper, by drawing analogies between a synchronous CDMA system and an STC multiuser system, we study the applications of some multiuser detection methods to STC multiuser systems. Specifically, we show that the so-called “turbo multiuser detection” technique, which performs soft interference cancellation and decoding iteratively, can be applied to STC multiuser systems in flat-fading channels. An iterative multiuser receiver and its projection-based variants are developed for both the space-time block coding (STBC) system and the space-time trellis coding (STTC) system. During iterations, extrinsic information is computed and exchanged between a soft multiuser demodulator and a bank of MAP decoders, to achieve successively refined estimates of the users' signals. Computer simulations demonstrate that the proposed iterative receiver techniques provide significant performance improvement over conventional noniterative methods in both single-user and multiuser STC systems. Furthermore, the performance of the proposed iterative multiuser receiver approaches that of the iterative single-user receiver in both STBC and STTC systems     

Convergence of linear interference cancellation multiuser receivers

We consider the convergence in norm of several iterative implementations of linear multiuser receivers, under the assumption of long random spreading sequences. We find that asymptotically, linear parallel interference cancellation diverges for systems loads of greater than about 17%. Using known results from the theory of iterative solutions for linear systems we derive optimal or near-optimal relaxation parameters for parallel (first- and second-order stationary, Chebyshev) and serial cancellation (successive relaxation) methods. An analytic comparison of the asymptotic convergence factor for the various methods is given. Simulations are used to verify results for finite size systems     

Robust blind adaptive multiuser detector

A modified version of the constrained mean output energy (CMOE) detector, called a multiuser detector (MUD), is presented. With estimation of the channel noise variance, a significant reduction in performance can be avoided when signature mismatch exits, which is one of main problems of the CMOE detector. Numerical results are presented which confirm the validity of the method