Linearly Constrained Constant Modulus Research Articles

Blind Capon-like adaptive ST-BC MIMO-CDMA receiver based on constant modulus criterion

In this paper, we present a new Capon-like blind receiver based on linearly constrained constant modulus (LCCM) criterion for the multiple-input multiple-output (MIMO) antennas system along with space–time block code (ST-BC) using direct-sequence code division multiple access (CDMA) modulation technique. A time-varying channel model with generalized sidelobe canceller (GSC) associated with the recursive least squares (RLS) algorithm is implemented to reduce the complexity of receiver design. In our derived algorithm, the parameter of constant modulus, α, relating to the desired user power is updated adaptively via stochastic gradient algorithm to track userʼs amplitude variation. Also we prove theoretically that in the two-branch filter bank receiver design the weight vector of one branch can be updated simply using the other one, which has been obtained with our proposed CM-GSC-RLS algorithm, with simple pre-calculated transform. Hence computation complexity of the proposed adaptive blind receiver can be further reduced significantly. Via intense simulations it reveals that our proposed scheme has robust performance against the userʼs acquisition inaccuracies comparing with current available algorithms.

Fuzzy step size least mean square algorithm tuned linearly constrained constant modulus algorithm for multi‐carrier code division multiple access blind multiuser detector

ABSTRACTAdaptive fuzzy logic step size least mean square algorithm (FLCLMS) tuning unit is designed to adjust the step size of the linear constrained constant modulus algorithm for blind multiuser detection (MUD) in multi‐carrier code division multiple access (MC‐CDMA) systems is presented. In wireless communications system, the conventional constant modulus algorithm for blind MUD has some disadvantages such as slow convergence speed and phase rotation. To overcome these shortages, a modified constant modulus algorithm was developed for blind multiuser detector. On the basis of the deterministic behavior of the FLCLMS algorithm, it has also proposed simplification over the cost function calculation, obtaining more efficient algorithm and creating new perspective for the MUD implementation. The proposed algorithm ensures the algorithm convergence to the desired user and suppresses the multiple access interference in this cost function. Thus, the performance of the system is improved. The computational experiments show that the proposed FLCLMS not only can find optimal or closed‐to‐optimal solutions but also can obtain the low computation complexity both better and more robust results than existing algorithm reported recently in the literature. Copyright © 2013 John Wiley & Sons, Ltd.

Constrained Adaptive Constant Modulus RLS Algorithm for Blind DS-CDMA Multiuser Receiver under Time-Varying Channels

The performance of the blind multiuser detector for a DS-CDMA system with linearly constrained constant modulus (LCCM) criterion is known to highly depend on the exact knowledge of the desired user amplitude; it is usually not available at receiver end. In this paper, we propose a novel LC adaptive CM RLS (LC-ACM-RLS) algorithm to adaptively implement the optimal solution of the LCCM receiver, and to track the desired user's amplitude, simultaneously. From computer simulations, we verify the superiority of the new proposed algorithm over the conventional LCCM-RLS algorithm for multiple access interference (MAI) suppression. Also, for time-varying channel during the adaptation processes, if the amplitude of desired user is not available and varies with time, such as hand-off and Rayleigh fading environments, we show that the proposed LC-ACM-RLS algorithm has better tracking capability compared with the conventional approaches.

A Multistage Hybrid Constant Modulus Array With Constrained Adaptation for Correlated Sources

The multistage constant modulus (CM) array was previously proposed for capturing multiple received signals in a cochannel signal environment. It consists of a cascade of individual CM array stages combined with adaptive signal cancelers that remove the various signals captured across the stages. However, when the received signals are mutually correlated, the signals captured by the CM array stages are not completely canceled, and previous parallel extensions of the system do not guarantee that different signals will be captured across the stages. In this paper, we present a hybrid implementation of the multistage CM array for separating correlated signals where the canceler weights in the cascade structure provide estimates of the direction vectors of the captured signals. These estimates are then used in a parallel implementation of the linearly constrained CM (LCCM) array leading to the hybrid structure. Since the direction vectors are obtained directly from the canceler weights, the hybrid implementation does not require prior knowledge of the array response matrix and is independent of the type of antennas used in the receiver. The effect of a bias in the direction vector estimates for closely-spaced signals is analyzed, and the steady-state performance of the hybrid structure is compared to that of a conventional constrained implementation for correlated sources. Computer simulations for example cochannel scenarios are provided to illustrate various properties of the system. Mean-square-error (MSE) learning curves indicate that the proposed hybrid LCCM algorithm converges faster and has lower MSE than previous implementations

Direct Implementation of Space-Time MC-CDMA Receiver with Adaptive Linearly Constrained Constant Modulus Algorithm

SUMMARY To support high data rate wireless communications, in this paper, based on the linearly constrained constant modulus (LCCM) criterion, the reverse link performance of the multi-carrier code division multiple access (MC-CDMA) receiver, with frequency combiner, and having smart antenna arrays beamformer in base station, has been investigated over the Rayleigh fading channel. By using the Kronecker product an equivalent direct formulation, which integrates the information of spatial-domain as well as temporal-domain, with constraint matrix could be obtained. In consequence, the modified normalized LCCM-gradient algorithm is devised to adaptively implement the direct constrained optimal weights solution of the fully space-time MC-CDMA detector. We show that the proposed method outperforms the constrained minimum output energy (CMOE) algorithm and is more robust against to the signal mismatch, due to imperfect channel and direction-of-arrival estimation used in the array beamformer.

On the Convexity of the LCCM Cost Function for DS-CDMA Blind Multiuser Detection

The linearly constrained constant modulus algorithm (LCCMA) applied to blind DS-CDMA multiuser detection was widely considered in the last few years. In some papers, global convergence of the linearly constrained constant modulus (LCCM) algorithm was incorrectly claimed, after which corrections were published by other authors (see the Introduction section for references). In this letter, analysis of the LCCM criterion is extended to prove convexity of the LCCM cost function between any point and the global minimum, subject to certain conditions, by using the definition of a continuous convex function.

Adaptive MC-CDMA receiver with constrained constant modulus IQRD-RLS algorithm for MAI suppression

The multicarrier code division multiple access (MC-CDMA) technique is considered to be one of the attractive candidates to achieve high data-rate for future wireless communication systems. In this paper, based on linearly constrained constant modulus (LCCM) least square (LS) criterion, a new robust adaptive constrained filtering algorithm, referred to as the LCCM inverse QRD-RLS (IQRD-RLS) algorithm, is devised for MC-CDMA detector. The proposed robust LCCM IQRD-RLS algorithm can be used to estimate the weights of the combining process to combat the multiple access interference (MAI), effectively, and is more robust to against the imperfect channel estimation error. By this approach we require only the knowledge of code sequences of desired user rather than the code sequences of other users. The superiority of the proposed algorithm for estimating the weights in the combining process is verified by evaluating the performance, in terms of output signal to interference and noise ratio (SINR) and bit error rate (BER). From computer simulation results we showed that it outperformed the conventional techniques, such as the maximum ratio combining (MRC), blind adaptation algorithm, least mean square algorithm with partitioned linear interference canceller structure (PLIC-LMS) and LCCM-gradient based approaches.

A Non-Canonical Linearly Constrained Constant Modulus Algorithm for a Blind Multiuser Detector

We investigate an alternative blind adaptive multiuser detection scheme based on a non-canonical linearly constrained constant modulus (LCCM) criterion and prove that, under the constrained condition, the non-canonical linearly constrained constant modulus algorithm (LCCMA) can completely remove multiple-access interference. We further demonstrate that the non-canonical LCCM criterion function is strictly convex in the noise-free state, and that under the constrained condition, it is also strictly convex even where small noise is present. We present a simple method for selecting the constant as well as a stochastic gradient algorithm for implementing our scheme. Numerical simulation results verify the scheme's efficiency.

Comments on "A linearly constrained constant modulus approach to blind adaptive multiuser interference suppression"

For original paper see IEEE Commun. Lett., vol.2, no.8, p.217-19 (1998 August). A linearly constrained constant modulus (LCCM) receiver was presented in the above paper by Miguez et al. In this letter, we point out that the analysis in the above paper is incomplete and show in the noise-free case that the LCCM approach can completely remove multiple-access interference if and only if the desired user amplitude is no less than the critical value 1/spl radic/3. The simulation examples demonstrate the results.

Closed-form analysis of linearly constrained CMA-based blind multiuser detector

In this letter, we present a performance analysis of the blind adaptive multiuser detector (MUD) proposed in Miguez and Casredo (1998), which is based on the linearly constrained constant modulus (LCCM) criterion. Global convergence of the algorithm with additive white Gaussian noise (AWGN) is proved and two approximate closed-form solutions of the optimum tap-weight vector are derived. Also the validity of analysis is demonstrated by comparing analytical results with simulated results.

A robust algorithm for untrained adaptive signal processing

A new adaptive signal processing technique is described that does not require a training or reference signal for adaptation. The approach taken is to minimize variations in the complex envelope of an adaptive processor output while subjecting the processor coefficients to a set of linear constraints. This Linearly-Constrained Constant-Modulus (LCCM) method is motivated by the drawbacks associated with two existing untrained algorithms in certain signal environments. The problems specifically addressed by LCCM are the signal cancellation problem of Linearly-Constrained Power Minimization (LCPM) and the signal ambiguity problem of the Constant-Modulus Algorithm (CMA). A complete adaptive implementation of the LCCM algorithm is presented including its stability and convergence properties. In addition, adaptive performance of LCCM is demonstrated in a comparison with CMA and LCPM.