Time-varying Multipath Fading Channels Research Articles

Radial basis function network assisted single-user channel estimation by using a linear minimum mean square error detector under impulsive noise

This paper proposes a novel robust channel estimator for direct-sequence code-division multiple-access systems with time-varying multipath fading channels under impulsive noise. The channel estimation approach presented is based on the linear single-user minimum mean square error criteria and uses a radial basis function network to eliminate the effect of impulsive noise. A threshold mechanism is also developed to eliminate the disadvantage of the proposed estimator under Gaussian noise. Simulation results clearly show that the proposed estimator eliminates the effect of impulsive noise and improves the performance of channel estimation under impulsive noise.

4S 해상 통신을 위한 채널 추정 알고리즘 비교 연구

In this paper, we compare the existing channel estimation technique for 4S (Ship to Ship, Ship to Shore) maritime communications under AWGN channel model, Rician fading channel model, and Rayleigh fading channel model respectively. In general, the received signal is corrupted by multipath and ISI (Inter Symbol Interference). The estimation of a time-varying multipath fading channel is a difficult task for the receiver. Its performance can be improved if an appropriate channel estimation filter is used. The simulation is performed in MATLAB. In this simulation, we use the popular estimation algorithms, LMS (Least Mean Square) and RLS (Recursive Least-Squares) are compared with respect to AWGN, Rician and Rayleigh channels.

H∞Channel Estimation for DS-CDMA Systems: A Partial Difference Equation Approach

In the communications literature, a number of different algorithms have been proposed for channel estimation problems with the statistics of the channel noise and observation noise exactly known. In practical systems, however, the channel parameters are often estimated using training sequences which lead to the statistics of the channel noise difficult to obtain. Moreover, the received signals are corrupted not only by the ambient noises but also by multiple-access interferences, so the statistics of observation noises is also difficult to obtain. In this paper, we will investigate theH∞channel estimation problem for direct-sequence code-division multiple-access (DS-CDMA) communication systems with time-varying multipath fading channels. The channel estimator is designed by applying a partial difference equation approach together with the innovation analysis theory. This method can give a sufficient and necessary condition for the existence of anH∞channel estimator.

Robust Channel Estimation Under Impulsive Noise for Multipath Fading CDMA Systems

This paper proposes a novel fuzzy channel estimator for direct-sequence code-division multiple-access systems with time-varying multipath fading channels under impulsive noise. The proposed estimator combines a least-squares channel estimator with a fuzzy median filter which is based on fuzzy rank ordering of samples. Simulation results clearly show the superior channel estimation success of the proposed estimator under highly impulsive noise.

Channel Estimation and Interference Cancellation for OFDM Systems Based on Total Least Squares Solution

In this paper, we study the effects of interference to channel estimation and present an iterative total least-squares algorithm for channel estimation by using the conjugate gradient method in time-varying multipath fading channels. The Doppler shift of fast-fading channels will generate inter-carrier interference (ICI), hence, degrade the performance of orthogonal frequency-division multiplexing (OFDM) systems. Furthermore, when the length of channel impulse response (CIR) overstep the duration of cyclic prefix (CP), which will cause inter-symbol interference (ISI) and degrade system performance severely. Conventional channel estimation and equalization schemes, if applied to this case of insufficient CP, suffer significant performance degradation. By ISI and ICI analysis, an Interference elimination technique is proposed for channel estimation. After analyzing perturbation of least squares problems, we present total least-squares (TLS) scheme to eliminate the ICI, ISI and noise. We present a short theoretical overview of the TLS problem. In our estimation, residual ISI and ICI are not being handled as a noise. Since the introduction of interference, the operation of matrix becomes perplexed. We implement the Conjugate gradient total least-squares method. Simulation results confirm the performance of Conjugate gradient total least-squares method and our theoretical analysis.

An efficient iterative frequency domain equalization for ATSC DTV receiver

In this paper, we present an efficient single carrier frequency domain equalization for Advanced Television Systems Committee (ATSC) digital television receiver. The proposed scheme employs iterative cyclic prefix reconstruction (CPR) combined with block-overlapping process to reduce inter-carrier interference (ICI) component caused by the absence of cyclic prefix in multipath fading channel. In addition, trellis decoder aided iterative decision feedback frequency domain equalizer is derived to further mitigate inter-symbol interference (ISI). Finally, low-complex iterative frequency domain channel estimation is proposed to predict and track the time-varying multipath fading channel coefficients. The computer simulation results reveal that the proposed scheme has a significant performance improvement in multipath fading channel with respect to the previous time domain equalization (TDE) and frequency domain equalization (FDE).

Robust MC-CDMA Channel Tracking for Fast Time-Varying Multipath Fading Channel

An unscented Kalman filter (UKF)-based channel-tracking method is proposed for a fast time-varying multipath fading channel in a multicarrier code-division multiple-access (MC-CDMA) system. The mobile radio channel is modeled as an autoregressive (AR) random process. The parameters of the AR process and the channel gain are simultaneously estimated by the proposed method. One-step-ahead prediction can also be obtained during channel estimation. It is useful for the decision-directed channel-tracking design, particularly in the fast-fading channel. Meanwhile, the estimated parameters can enhance the minimum mean-square error (MMSE) equalizer for symbol detection. The simulation results show that the enhanced equalizer based on the proposed estimation algorithm performs much better than that based on the conventional channel estimators in symbol error rate.

RBF network assisted adaptive path selective decorrelating detector under impulsive noise for multipath fading CDMA systems

This paper proposes a novel radial basis function assisted adaptive path selective decorrelating detector for direct sequence code division multiple access systems with time-varying multipath fading channels under impulsive noise. The proposed detector combines adaptive path selective decorrelating detector (APSDD) with a radial basis function (RBF) network to eliminate the effect of impulsive noise. Simulation results clearly show that the proposed detector eliminates the effect of impulsive noise and improves the performance of APSDD under high impulsive noise.

An Energy-Efficient Adaptive Transmission Protocol for Ultrawideband Wireless Sensor Networks

An energy-efficient adaptive transmission protocol for ultrawideband (UWB) wireless sensor networks (WSNs), called ATP-UWSN, is described and evaluated over time-varying multipath fading channel conditions and multinode interference. Since wireless channels may greatly change from one transmission to the next, the use of fixed transmission parameters for such channels results in wasted energy when channel conditions are good. Adaptation of transmission parameters reduces energy consumption and interference. The proposed ATP-UWSN permits each of the sensor nodes to adapt its code rate and spreading code length (the number of pulses used per data bit) to match the predicted channel-state information (CSI). The transmission parameters are selected in accordance with the predicted CSI characteristics of the communication links in the network to achieve the desired quality of service given by the target bit-error probability. Subsequently, we develop a mathematical adaptation framework for the analytical evaluation of ATP-UWSN. Communication performance is evaluated in terms of average information throughput efficiency, which is a relevant progress-related measure for multi- hop WSNs that is associated with the energy consumption and throughput of a packet transmission. The analytical expression of this performance metric is defined and is used to assess the benefits of the proposed ATP-UWSN when compared with a nonadaptive transmission protocol (NATP) with fixed transmission parameters. Our results show that the ATP-UWSN outperforms the NATP and can improve the robustness of the link and reduce energy consumption by adapting the transmission parameters in response to changes in the predicted CSI.

Adaptive DFE Multiuser Receiver for CDMA Systems using Two-Step LMS-Type Algorithm: An Equalization Approach

This paper presents an adaptive decision feedback equalizer (DFE) based multiuser receiver for code division multiple access (CDMA) systems over smoothly time-varying multipath fading channels using the two-step LMS-type algorithm. The frequency-selective fading channel is modeled as a tapped-delay-line filter with smoothly time-varying Rayleigh-distributed tap coefficients. The receiver uses an adaptive minimum mean square error (MMSE) multiuser channel estimator based on the reduced Kalman least mean square (RK-LMS) algorithm to predict these tap coefficients (Kohli and Mehra, Wireless Personal Communication 46:507---521, 2008). We propose the design of adaptive MMSE feedforward and feedback filters by using the estimated channel response. Unlike the previously available Kalman filtering algorithm based approach (Chen and Chen, IEEE Transactions on Signal Processing 49:1523---1532, 2001), the incorporation of RK-LMS algorithm reduces the computational complexity of multiuser receiver. The computer simulation results are presented to show the substantial improvement in its bit error rate performance over the conventional LMS algorithm based receiver. It can be inferred that the proposed multiuser receiver proves to be robust against the nonstationarity introduced due to channel variations, and it is also beneficial for the multiuser interference cancellation and data detection in CDMA systems.

A Maximum Likelihood Fine Timing Estimation for Wireless OFDM Systems

Orthogonal frequency division multiplexing (OFDM) systems are more sensitive to timing synchronization than single carrier systems. This paper presents a fine timing synchronization scheme which utilizes the channel impulse response (CIR) estimated from frequency-domain samples at OFDM receivers. A maximum likelihood estimate (MLE) of timing offset is derived based on the probability distribution of the estimated CIR under time-varying multipath fading. The ML timing scheme is further developed for both integer precision and real-valued precision implementations. In the real-valued timing precision case, a delay locked loop (DLL) structure is devised as an effective way to implement the MLE. Both analysis and simulations of the proposed MLE showed significant improvement over existing schemes under time-varying multipath fading channels.

Transmitter pre-filtering for the downlink of a time-division-duplex/direct sequence-code division multiple access system over time-varying multipath fading channels

In this paper, we investigate an optimised transmitter pre-filtering technique for downlink time-division-duplex (TDD) code division multiple access (CDMA) communications, which employs the conventional matched filter (MF) detector at the mobile receivers. The proposed pre-filtering technique eliminates the multiple-access interference and intersymbol interference (MAI/ISI) effects by applying a very simple transmission scheme that combines a signal transformation with a cyclic prefix strategy under a power constraint condition. Two constrained pre-filtering transformations are suggested depending on the information required at the mobile unit. An open-loop transmitter pre-filtering is first formulated; however, this solution does not consider the properties of the noise at the mobile receiver. A second solution is then presented via a closed-loop transmitter pre-filtering that includes an optimum gain for a given transmit and noise power. Some associated issues such as system efficiency, computational complexity and channel estimation errors are also addressed. Simulation results show that the proposed transmitter pre-filtering scheme can be used to increase the system performance and capacity. In addition, its performance is compared with another similar transmit pre-processing scheme in order to evaluate the performance improvement by the proposed algorithm.

Joint Data Detection and Channel Tracking for OFDM Systems With Phase Noise

This paper addresses the problem of data detection in orthogonal frequency division multiplexing (OFDM) systems operating under a time-varying multipath fading channel. Optimal detection in such a scenario is infeasible, which makes the introduction of approximations necessary. The typical joint data-channel estimators are decision directed, that is, assume perfect past data decisions. However, their performance is subject to error propagation phenomena. The variational Bayes method is employed here, which approximates the joint data and channel distribution as a separable one, greatly simplifying the problem. The data detection part of the resulting algorithm provides soft data estimates that are used for channel tracking. The channel itself is modeled as an autoregressive process allowing for a Kalman-like tracking algorithm. According to the developed algorithm, both data and channel estimates are exchanged and updated in an iterative manner. The performance of the proposed algorithm is evaluated by simulations. Furthermore, since OFDM is extremely sensitive to the presence of phase noise, the algorithm is extended to operate under severe phase noise conditions, with moderate performance degradation.

Adaptive path selective fuzzy decorrelating detector under impulsive noise for multipath fading CDMA systems

This letter proposes a novel adaptive path selective fuzzy decorrelating (APSFD) detector for direct-sequence code- division multiple-access (DS-CDMA), systems with time-varying multipath fading channels under impulsive noise. The proposed detector combines adaptive path selective decorrelating (APSD) detector with a fuzzy median filter which is based on fuzzy rank ordering of samples to eliminate the effect of impulsive noise. Simulation results clearly show that the proposed detector eliminates the effect of impulsive noise and improves the performance of APSD detector under high impulsive noise.

$H_{\infty}$ Channel Estimator Design for DS-CDMA Systems: A Polynomial Approach in Krein Space

This paper considers the problem of robust channel estimation for direct-sequence code-division multiple-access (DS-CDMA) systems with time-varying multipath fading channels. Due to the lack of accurate knowledge of model dynamics and statistics of system noises, a polynomial channel estimator is developed by virtue of the Krein space theory. The estimator is calculated by performing a J-spectral factorization. In this paper, under some conditions, we obtain a closed-form solution to this J-spectral factorization, which greatly reduces the computational load of the estimator design. The simulation results show that the proposed estimator provides a robust estimation performance against various types of noises and modeling errors.

Joint Doppler-Frequency Diversity for OFDM Systems Using Hybrid Interference Cancellation in Time-Varying Multipath Fading Channels

In time-varying fading channels, orthogonal frequency division multiplexing systems suffer severe performance degradation caused by interchannel interference (ICI). We utilize this ICI as a source of diversity by using a new ICI-cancellation algorithm and Wiener filter. The proposed algorithm can support joint Doppler-frequency diversity gain. In addition, both the computational complexity and total detection time are reduced.

A Novel Two-Step Channel-Prediction Technique for Supporting Adaptive Transmission in OFDM/FDD System

This paper introduces a novel channel-prediction technique for an orthogonal frequency-division multiplexing (OFDM)/frequency-division duplex (FDD) system to support an adaptive-modulation and coding (AMC) scheme over a rapidly time-varying multipath fading channel. Most channel-prediction techniques assume that there is no channel variation in the predefined time duration, e.g. the slot. As a result, those techniques cannot compensate for the degradation of packet-error-rate (PER) performance, resulting from the rapid channel variation over the slot duration. In this paper, we propose a novel two-step channel-prediction technique that considers the time-varying nature of a channel over the duration of interest. Simulation results show that the AMC scheme of the OFDM/FDD system utilizing the proposed channel-prediction technique can guarantee the target PER of 1% without any loss of system throughput compared with a case supported by a conventional channel prediction under ITU-R Veh A 30 km/h.

Adaptive MC-CDMA Multiple Channel Estimation and Tracking Over Time-Varying Multipath Fading Channels

Multicarrier code-division multiple access (MC-CDMA) combines multicarrier transmission with direct-sequence (DS) spread spectrum techniques. In this study, several possible channel models of MC-CDMA systems are considered as a multi-state switching Markov process to match the time-varying multipath fading channel. A novel multiple channel estimator, which consists of several possible channel models and can be self-adjusted to fit different channel uncertain conditions, is proposed for MC-CDMA systems to effectively estimate channel coefficients in the frequency domain over time-varying multipath fading channels. Based on the proposed channel estimation, an enhanced minimum mean-square error (MMSE) equalizer is employed for symbol detection of MC-CDMA systems. Simulation results demonstrate that the proposed multiple channel estimator can track the channel more accurately than the conventional channel estimators. With the consideration of channel estimation error as a channel noise to improve symbol detection, the enhanced equalizer based on the proposed multiple channel estimator performs much better than those based on the conventional channel estimators in bit error rate (BER).

Genetic algorithm-based MMSE receiver for MC-CDMA systems transmitting over time-varying mobile channels

A novel multi-user detector, based on a genetic algorithm-assisted per-carrier MMSE criterion, is proposed for MC-CDMA systems transmitting over time-varying multipath fading channels. The analysed multi-user detector outperforms state-of-the-art adaptive receivers based on deterministic gradient algorithms, particularly for an increasing number of users.

Evolutional Algorithm Based Learning of Time Varying Multipath Fading Channels for Software Defined Radio

Software defined radio, which uses reconfigurable signal processing devices, requires the determination of multiple unknown parameters to realize the potential capabilities of adaptive communication. Evolutional algorithms are optimal multi dimensional search techniques, and are well known to be effective for parameter determination. This letter proposes an evolutional algorithm for learning the mobile time-varying channel parameters without any specific assumption of scattering distribution. The proposed method is very simple to realize, but can provide precise channel estimation results. Simulations of an OFDM system show that for an example of OFDM communication under the time-varying fading channel, the proposed learning method can achieve the better BER performance.