Nonlinear group-blind multiuser detection

This paper is a theoretical analysis of the maximum likelihood (ML) channel estimator for orthogonal frequency-division multiplexing (OFDM) systems in the presence of unknown interference. The following theoretical results are presented. Firstly, the uniqueness of the ML solution for practical applications, i.e., when thermal noise is present, is analytically demonstrated when the number of transmitted OFDM symbols is strictly greater than one. The ML solution is then derived from the iterative conditional ML (CML) algorithm. Secondly, it is shown that the channel estimate can be described as an algebraic function whose inputs are the initial value and the means and variances of the received samples. Thirdly, it is theoretically demonstrated that the channel estimator is not biased. The second and the third results are obtained by employing oblique projection theory. Furthermore, these results are confirmed by numerical results.

The primary goal of this letter is to provide comparative assessments to the two cases where different criteria are used in joint over antenna minimum mean squared error (JA-MMSE) spatial filtering for space-time coded systems in the presence of unknown interference. It is shown that additional constraints to preserve the space-time coded signal structure in JA-MMSE spatial filtering place floor in bit error rate (BER) performance, while imposing no additional constraint places no error floor.

Despite that underwater acoustic channels are well known to contain various interferences, research on interference mitigation in underwater acoustic communications has been very limited. In this paper, we deal with a wideband orthogonal frequency division multiplexing (OFDM) transmission in the presence of an external interference which occupies partially the signal band and whose time duration is shorter than the OFDM block. We parameterize the unknown interference waveform by a number of parameters assuming prior knowledge of its frequency band and time duration, and develop an iterative receiver, which couples interference detection via a generalized likelihood-ratio-test (GLRT), interference reconstruction and cancellation, channel estimation, and data detection. In addition to simulation results, we verify the receiver performance using data sets collected from two experiments. In both time-invariant and time-varying channels, the proposed iterative receiver achieves robust performance in the presence of unknown interference.

A sensitive excitation-emission fluorescence method with a second-order calibration strategy is proposed to simultaneously determine abscisic acid (ABA) and gibberellin (GA) contents in extracts of leaves and buds of ginkgo. The methodology is based on the alternating normalization-weighed error (ANWE) and the parallel factor analysis (PARAFAC) algorithms, which make it possible that the ABA and GA concentration can be attained in extract of plants even in the presence of unknown interference from potential interfering matrix contaminants introduced during the simple pretreatment procedure. Satisfactory recoveries were obtained although the excitation and emission profiles of the analytes were heavily overlapped with each other and the background in the extracts. The limits of detection obtained for GA and ABA in leaf samples were 9.6 and 6.9 ng mL-1, respectively, which were in the concentration range (from hundreds to several ng g-1) for GA and ABA in leaves in different periods. Furthermore, in order to investigate the performance of the developed method, some statistical parameters and figures of merit of ANWE and PARAFAC are evaluated. The method proposed lights a new avenue to determine quantitatively phytohormones in extracts of plants with a simple pretreatment procedure, and may hold potential to be extended as a promising alternative for more practical applications in plant growth processes.

In this contribution we propose and study a novel multiuser detection (MUD) scheme for multicarrier direct-sequence code-division multiple-access systems employing both time (T)-domain and frequency (F)-domain spreading, which are referred to as the TF/MC DS-CDMA systems. Specifically, a reliability-aided MUD scheme is proposed, which consists of a linear MUD and a so-called L-level maximum-likelihood (ML)-MUD. The linear MUD is either a joint TF-domain linear MUD or constituted by two linear MUDs, one of which is operated in the T-domain and the other one in the F-domain. The L-level ML-MUD is a reduced ML-MUD, which only searches in a space of size 2L in order to find the optimum solutions for the L most unreliable data bits detected by the linear MUD. In this contribution the bit error rate (BER) performance of the TF/MC DS-CDMA using the reliability-aided MUD is investigated, when communicating over additive white Gaussian noise (AWGN) channels or over frequency-selective Rayleigh fading channels. Our simulation results show that the reliability-aided MUD is capable of significantly outperforming the corresponding linear MUD. It can be shown that, provided that the signal-to-noise ratio (SNR) is sufficiently high, the reliability-aided MUD can readily obtain several decibels of SNR gain over the linear MUD at the cost of slight or moderate increase of the detection complexity.

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 phenomenon. This is due to 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 CDMA communication systems. This detector is essentially a robust version of the linear decorrelating multiuser detector. The robustification is based on the Hampel's M-estimator found in robust statistics. The approach is corroborated with simulation results. Simulation results show that the proposed approach yields significant performance gain over the linear decorrelating multiuser detector with little attendant increase in the computational complexity. Further, it offers better gain over the Huber's M-estimator.

This paper is focused on the cross-layer design problem of joint multiuser detection and power control for energy-efficiency optimization in wireless data networks through a game-theoretic approach. Unlike previous studies in this area, which have considered the application of game theory to achieve energy-efficiency in idealized synchronous code division multiple access systems, more realistic assumptions including system asynchronism, the use of band-limited chip-pulses, and the multipath distortion induced by the wireless channel are explicitly incorporated in the development of this paper. Several noncooperative games are proposed wherein each user may vary its transmit power and uplink receiver in order to maximize its utility, which is defined here as the ratio of data throughput to transmit power. In particular, the case in which a linear multiuser detector is adopted at the receiver is considered first, and then, the more challenging case in which a nonlinear decision feedback multiuser detector is employed is considered. Via large system analysis (LSA), a decentralized implementation of the power allocation game requiring very little prior information on the interference background is proposed. LSA is also used to compare the energy efficiency of several linear multiuser detectors, and to obtain the optimal (i.e., utility-maximizing) length of the training sequence of each data frame. Numerical results show the effectiveness of the proposed solutions, as well as a very satisfactory agreement of the LSA-based analysis with simulation results obtained for systems with finite (and not so large) numbers of users.

Novel concepts are introduced for finding the relationship between multiuser detection (MUD) and multiuser transmission (MUT), so that the study in MUT can benefit from the well-built theory in MUD. Our study shows that, for any given linear MUD scheme, there exists a counterpart linear MUT scheme, which can be readily designed from the original linear MUD. Based on our observations, a range of MUT algorithms are derived in correspondence with the well-known linear processing schemes in the fields of MUD and array processing, including minimum mean-square error (MMSE), minimum variance distortionless response (MVDR), minimum power distortionless response (MPDR) and maximum signal-to-interference-plus-noise ratio (MSINR), respectively. Furthermore, the physical principles behind the considered MUT schemes are explained with the aid of the concepts built in this contribution. In order to gain insight into the effect of noise power on the achievable performance of a MUT scheme, a noise-suppression factor is introduced to those MUT schemes, such as MMSE, requiring the knowledge of noise variance. Our study shows that the achievable performance of the MUT is generally loosely depended on the noise-suppression factor. This finding may result in extremely low-complexity wireless transceivers in time- division duplex (TDD) cellular systems, where the uplink MUD and downlink MUT can be implemented based on one adaptive signal processing scheme.

The problem of iterative multiuser detection in single-carrier broadband multiple-input multiple-output (MIMO) system is studied in this paper. Two minimum mean squared error (MMSE) multiuser receivers are proposed for space-time trellis coded systems in frequency selective channels. The first receiver uses the MMSE criterion both for multiuser detection and equalization. The second one uses the MMSE criterion only for multiuser detection, while the equalization part is an optimal maximum a posteriori (MAP) equalizer. The second receiver significantly outperforms the first one both in the presence and in the absence of the unknown co-channel interference, at the expense of increased complexity.

This paper presents robust blind multiuser detection in direct sequence-code division multiple access systems (DS-CDMA) over Nakagami-m fading channels with impulsive noise including maximal-ratio combining diversity. An M-estimator based structure for non-coherent demodulation of differentially phase shift keyed (DPSK) signals transmitted over a synchronous CDMA flat-fading Nakagami channels with impulsive noise is studied and analyzed. A new M-estimator is proposed for robustifying the detector and an approximate expression for average probability of error of non-coherent DPSK signals is derived using an approximation to generalized Marcum-Q function over single-path Nakagami-m fading channels. Simulation results are presented to evaluate the performance of the proposed robust blind multiuser detector in comparison with the linear decorrelating detector, Huber and Hampel estimator based detectors.

The main objective of this paper is to investigate linear multiuser detection in an UMTS network in terms of system capacity and system coverage. This includes the impact of linear multiuser detection on radio resource management concepts as load control and power control. One issue the paper deals with is joint optimization of linear multiuser detection with power control. In order to keep practical UMTS issues in mind, we describe an iterative two-stage power control multiuser detection algorithm for UMTS application. With this algorithm detailed system level simulation will be carried out to show coverage and system capacity gains. As an example, we work out the maximum allowable cell radius for a given cell load and outage probability. The main result indicates for UMTS that optimum minimum mean squared error (MMSE) multiuser detection yields moderate gains compared to single-user-matched filtering. The linear decorrelator is even inferior to the single-user-matched filter.

The problem of joint channel estimation, equalization, and multiuser detection for a multiantenna DS/CDMA system operating over a frequency-selective fading channel and adopting long aperiodic spreading codes is considered in this paper. First of all, we present several channel estimation and multiuser data detection schemes suited for multiantenna long-code DS/CDMA systems. Then, a multipass strategy, wherein the data detection and the channel estimation procedures exchange information in a recursive fashion, is introduced and analyzed for the proposed scenario. Remarkably, this strategy provides, at the price of some attendant computational complexity increase, excellent performance even when very short training sequences are transmitted, and thus couples together the conflicting advantages of both trained and blind systems, that is, good performance and no wasted bandwidth, respectively. Space-time coded systems are also considered, and it is shown that the multipass strategy provides excellent results for such systems also. Likewise, it is also shown that excellent performance is achieved also when each user adopts the same spreading code for all of its transmit antennas. The validity of the proposed procedure is corroborated by both simulation results and analytical findings. In particular, it is shown that adopting the multipass strategy results in a remarkable reduction of the channel estimation mean-square error and of the optimal length of the training sequence.

For practical wireless DS-CDMA systems, channel estimation is imperfect due to noise and interference. The impact of channel estimation error on multiuser detection (MUD) is analyzed under the framework of the replica method. System performance is obtained in the large system limit for optimal MUD, linear MUD and turbo MUD, and is validated by the numerical results of finite systems.

The performance of blind multiuser detection methods is known to degrade severely in the presence of even small mismatches between the actual and the presumed desired user signatures. Such mismatches may occur in practical situations due to an imperfect knowledge of the channel impulse response. We propose a new robust approach to blind multiuser detection in the presence of unknown arbitrary-type mismatches of the desired user signature. Two different formulations of a robust multiuser receiver are considered. The proposed formulations are based on the explicit modeling of uncertainties in the covariance matrix of the desired user signature and/or data covariance matrix and optimization of the worst-case performance. Simple closed-form solutions to the considered robust multiuser detection problems are derived. The proposed methods have a computational complexity comparable to that of the traditional blind multiuser detection algorithms, and, at the same time, offer an improved robustness and faster convergence rates.

The main objective of this paper is to investigate asynchronous linear multiuser detection (MUD) in mixed service UMTS networks in terms of system capacity as well as power reduction at mobile stations. This especially requires us to theoretically describe the interaction of linear multiuser detection with radio resource management concepts like power control and load control for asynchronous multirate transmissions. For system capacity assessments we apply a two-stage power control multiuser detection algorithm where the users are detected by a minimum mean square error (MMSE) block detection. By detailed system level simulations we evaluate the UMTS load control parameter noise rise and also show corresponding system capacity bounds for different service mixes. It is important to note that for typical UMTS services the signal-to-noise operating points of the MMSE detector are very low (less than 0 dB). These low SINR operating points severely limit multiuser detection performance gains. Compared to simple single-user matched filtering the MMSE leads to fairly modest capacity gains of up to 60% dependent on the service mix assumed.