Adaptive MMSE Research Articles

Adaptive MMSE Equalizer for Blind Fractional Spaced CMA Channel Equalization through LMS Algorithm

Adaptive MMSE Equalizer for Blind Fractional Spaced CMA Channel Equalization through LMS Algorithm

Modulation-domain Kalman filtering for single-channel speech enhancement

In this paper, we investigate the modulation-domain Kalman filter (MDKF) and compare its performance with other time-domain and acoustic-domain speech enhancement methods. In contrast to previously reported modulation domain-enhancement methods based on fixed bandpass filtering, the MDKF is an adaptive and linear MMSE estimator that uses models of the temporal changes of the magnitude spectrum for both speech and noise. Also, because the Kalman filter is a joint magnitude and phase spectrum estimator, under non-stationarity assumptions, it is highly suited for modulation-domain processing, as phase information has been shown to play an important role in the modulation domain. We have found that the Kalman filter is better suited for processing in the modulation-domain, rather than in the time-domain, since the low order linear predictor is sufficient at modelling the dynamics of slow changes in the modulation domain, while being insufficient at modelling the long-term correlation speech information in the time domain. As a result, the MDKF method produces enhanced speech that has very minimal distortion and residual noise, in the ideal case. The results from objective experiments and blind subjective listening tests using the NOIZEUS corpus show that the MDKF (with clean speech parameters) outperforms all the acoustic and time-domain enhancement methods that were evaluated, including the time-domain Kalman filter with clean speech parameters. A practical MDKF that uses the MMSE-STSA method to enhance noisy speech in the acoustic domain prior to LPC analysis was also evaluated and showed promising results.

Performance of Adaptive MMSE Receivers Using Aided Tentative Coefficients in Flat Fading Channels

The conventional matched filter (MF) receiver is considered the optimum filter to recover the CDMA signals. One of the problems of the MF is that its performance is significantly degraded due to the channel impairments and the increase of the multiple access interference (MAI). In this paper, an adaptive minimum mean square error-maximum likelihood (MMSE-ML) receiver is introduced to overcome this problem. This receiver uses two sets of adaptive coefficients to increase the ability of tracking the time variations of the channel. The performance of the receiver is measured in terms of bit error rate (BER) and compared with other receivers over flat fading channel. It is found that the performance of theadaptive MMSE-ML receiver is much better than the performance of the other receivers in the flat fading channel.

Performance Evaluation of Adaptive MMSE Receivers in Frequency Selective Fading Channels Using Aided Tentative Coefficients

Recently, the code division multiple access (CDMA) technique has become the most widely communication technique, because of its varity of applications. The well-known matched filter (MF) is considered as the optimum filter that recovers the CDMA signals. However; its performance recently becomes unsatisfying. This is due to increasing the multiple access interference (MAI), in addition to the other channel impairments such as the fading or the multi path interference (MPI). In this paper, an adaptive minimum mean square error-maximum likelihood (MMSE-ML) receiver is presented as an alternative solution to overcome the drawbacks of the MF receiver. This adaptive MMSE-ML receiver uses adaptive aided tentative coefficients in addition to the basic adaptive coefficients to increase the tracking ability of the channel impulse response variations. The performance of the adaptive MMSE-ML receiver is compared with the traditional adaptive MMSE receiver over a frequency selective fading channel. Moreover, the performances of both the presented and traditional adaptive receivers are compared with the performance of the MF under the same condition. It is found that the performance of the adaptive MMSE-ML receiver is much improved using the proposed technique.

Long polyphase sequences for adaptive MMSE detector in asynchronous CDMA PLC system

An adaptive receiver solution for an asynchronous DS-CDMA system in an indoor powerline network is proposed. A combination of different receiver structures with binary and complex-polyphase sequences is evaluated and compared in terms of bit error rate. The simulation results based on a complete powerline channel model are reported.

A Blind Adaptive MMSE Multiuser Detector over Multipath CDMA Channels and its Analysis

Based on the refinement-only fast subspace tracking (RO-FST) algorithm to adaptively estimate the signal sub-space, a blind adaptive SVD-based minimum-mean-square-error (MMSE) multiuser detector is proposed in this paper for asynchronous direct-sequence code-division-multiple-access (DS-CDMA) systems over multipath fading channels. Numerical results show that the proposed detector gives low computational complexity and good performance in terms of signal-to-interference-plus-noise ratio (SINR) and bit-error-rate (BER). In addition, we apply the perturbation analysis of the proposed adaptive detector to derive the closed-form expressions for the mean-square error (MSE) and SINR. The analytical results are shown to match well with the simulation results.

Adaptive MMSE decision feedback equalizer for asynchronous CDMA with erasure algorithm

Adaptive decision feedback equalizer with erasure algorithm (E-DFE) is proposed for asynchronous code division multiple access (CDMA) transmission, which not only combats intersymbol interference (ISI) and multiple access interference (MAI) but also reduces the effects of error propagation in the presence of Gaussian background noise. To reduce the possibility of feeding back the wrong decisions, the output of the feedforward filter of the E-DFE is processed before it is fed back to the feedback filter. Specifically, the focus is on the performance of E-DFE using soft-slicer based on a novel erasure algorithm. In addition, the fully connected feedback filter of E-DFE has been used to eliminate ISI due to other active users. Comparison of the performance of conventional decision feedback equalizer (C-DFE) and E-DFE over slowly varying frequency selective fading channel is presented to show the advantages of E-DFE in terms of reduced average bit error rate performance. Simulation results are also presented to show the substantial improvement in its performance under near-far situations and the sudden change in the signal power of the desired user.

Fast Converging Adaptive MMSE Multiuser Detector for DS-CDMA System

A fast converging adaptive minimum-mean-squared-error (MMSE) multiuser detector is proposed for direct-sequence code-division multiple-access (DS-CDMA) systems with severe near-far problem where the convergence rate of adaptive MMSE detectors for distinct users can be very different. It is shown that by successively cancelling the interference signals of strong power users, the convergence rate of the proposed detectors for weak power users can be significantly increased, which helps to reduce the length of training sequence for tracking. It is also shown that the order of cancellation and several important parameters required for interference cancellation can be determined from the convergence behavior of the proposed detector. Numerical results are presented to show that the proposed detector offers improved performance in various DS-CDMA environments.

Blind adaptive channel equalization using multichannel linear prediction-based cross-correlation vector estimation

Blind equalization of transmission channel is important in communication areas and signal processing applications because it does not need training sequences, nor does it require a priori channel information. In this paper, an adaptive blind MMSE channel equalization technique based on second-order statistics is investigated. We present an adaptive blind MMSE channel equalization using multichannel linear prediction error method for estimating cross-correlation vector. They can be implemented as RLS or LMS algorithms to recursively update the cross-correlation vector. Once cross-correlation vector is available, it can be used for MMSE channel equalization. Unlike many known subspace methods, our proposed algorithms do not require channel order estimation. Therefore, our algorithms are robust to channel order mismatch. Performance of our algorithms and comparison with existing algorithms are shown for real measured digital microwave channel.

A low complexity partially adaptive CDMA receiver for downlink mobile satellite communications

AbstractA novel CDMA receiver with enhanced interference suppression is proposed for pilot symbols assisted mobile satellite systems in the presence of frequency offset. The design of the receiver involves the following procedure. First, adaptive correlators are constructed at different fingers, based on the scheme of generalized sidelobe canceller (GSC), to collect the multipath signals and suppress multi‐access interference (MAI). In particular, a partially adaptive (PA) realization of the GSC correlators is proposed based on the Krylov subspace technique, leading to an efficient algorithm without the need of complicated matrix computations. Second, pilot symbols assisted frequency offset estimation, channel estimation and RAKE combining give the estimate of signal symbols. Finally, further performance enhancement is achieved by an iterative scheme in which the signal is reconstructed and subtracted from the GSC correlators input, leading to faster convergence of the receiver. The proposed low complexity PA receiver is suitable or the downlink of mobile satellite CDMA systems, and shown to outperform the conventional fully adaptive MMSE receiver by using a small number of pilot symbols. Copyright © 2003 John Wiley & Sons, Ltd.

Adaptive MMSE receiver for multirate CDMA systems

We consider the adaptive receiver for multirate code division multiple access (CDMA) systems under a fading channel environment. The main difficulty that arises in the use of the adaptive receiver for multirate CDMA systems is that the adaptation should resume after the rate change. Hence, the adaptive receiver may not provide a reasonable performance during the transient after the rate change. In order to overcome this difficulty, we investigate an approach that allows updating the weight vectors for all rates simultaneously. For example, in a dual-rate system, the weight vector for the lower rate (the higher rate) can be updated during the period of the higher rate (resp., the lower rate) to avoid the transient after the rate change. The resulting adaptive receiver has multiple parallel adaptive filters. The adaptive filters for each rate can carry out the adaptation simultaneously, regardless of what the current rate is. As a result, the performance is not degraded by the rate change.

Coherent equalization for underwater acoustic communications: A performance evaluation of adaptive MMSE and time-reversal techniques

A simple analytical expression is presented for evaluating and comparing the performance of channel-estimate-based coherent equalizers. A channel-estimate-based equalizer is one whose filter coefficients are calculated as a function of an explicit estimate of the acoustic channel impulse response. This is in contrast to a direct adaptation equalizer whose coefficients are calculated directly as the solution to a data estimation error minimization problem. The equalizer performance expression is different from other commonly used expressions because it is formulated from the viewpoint of the estimate of the channel impulse response rather than the actual channel impulse response. The expression allows the dependence of the performance of coherent equalizers on channel complexity and the quality of the channel estimate to be separately evaluated. The expression is used to evaluate and compare the performance of adaptive MMSE and time-reversal equalizers. [Work supported by ONR Ocean Acoustics.]

Turbo soft interference cancellation for coded asynchronous ds/cdma

In this paper we propose a Multiuser receiver architecture which removes the Multiple Access Interference (Mai) by means of a Soft Input Soft Output (siso) multiuser detector followed by a bank of channel decoders. The extrinsic information at the output of the channel decoders are used to estimate the values of the interfering bits and the SISO multiuser detector subtracts the vector of the interfering bits from the incoming signal. The resulting vector is then filtered by an adaptive mmse filter to help remove the residual mai. The paper examines the use of such a receiver in the context of a synchronous awgn channel and then we generalize to the case of asynchronous multipath fading channels.

Differentially Coherent Blind Adaptive Receiver for Rayleigh Fading CDMA Channels

Interest in code division multiple access (CDMA) has increased dramatically in both the industry and academia. This is due to the potential increase in user capacity and interference, multipath fading, and jam-resistant capabilities. Different approaches of multiuser detection schemes have shown improvement over the conventional receiver. However, multiuser detectors require system parameters and adaptive multiuser receivers require training sequences. These parameters are difficult to obtain in multipath fading channels with a degradation in performance. A blind adaptive multiuser receiver for flat fading and frequency-selective fading environments is studied in this paper. This receiver alleviates the problem of multiuser detectors and adaptive multiuser receivers by using a blind adaptive equalization method. The structure of the receiver is similar to that of the adaptive MMSE receiver which consists of an adaptive equalizer and an adaptive mechanism that uses a standard least mean-squared (LMS) algorithm, but uses a matched filter to generate a reference signal. The receiver is shown to be effective in different fading channels with perfect power control and a near-far effect. Furthermore, the simulation results show that the blind MMSE receiver outperforms the conventional receiver and the RAKE receiver.

Information Capacity of the Space Division Multiple Access Mobile Communication System

An expression for the Shannon capacity lower bound of the space division multiple access (SDMA) mobile communication channel with interference is shown. The bound is tightly approaching the Shannon capacity of the orthogonal SDMA system over Gaussian channel with no interference. The information capacity of the SDMA system with adaptive MMSE receivers is presented as well. The SDMA system with MMSE receivers achieves at least 50% of the orthogonal SDMA system capacity for the input signal to noise ratio of practical interest in cellular mobile communications (10–30dB).

Adaptive power control and MMSE interference suppression

Power control algorithms assume that the receiver structure is fixed and iteratively update the transmit powers of the users to provide acceptable quality of service while minimizing the total transmitter power. Multiuser detection, on the other hand, optimizes the receiver structure with the assumption that the users have fixed transmitter powers. In this study, we combine the two approaches and propose an iterative and distributed power control algorithm which iteratively updates the transmitter powers and receiver filter coefficients of the users. We show that the algorithm converges to a minimum power solution for the powers, and an MMSE multiuser detector for the filter coefficients.

An adaptive linear MMSE matrix filter algorithm using updated incorrect a priori signal knowledge (Ph.D. Thesis abstr.)

An adaptive linear MMSE matrix filter algorithm using updated incorrect a priori signal knowledge (Ph.D. Thesis abstr.)