Systems In Multipath Fading Channels Research Articles

Frame timing estimation based on statistical analysis for orthogonal frequency division multiplexing systems in multipath fading channels

This study investigates the problem of frame timing estimation in orthogonal frequency division multiplexing systems. Conventional timing estimation methods, which take advantage of the correlation property of a given preamble, always experience performance degradation in multipath fading channels with severe channel dispersion. To achieve accurate timing estimation, the authors propose a robust threshold-based timing detection method independent of the preamble structure. Based on the autocorrelation and cross-correlation, a novel timing metric with an extended correlation length is proposed to mitigate noise and resist large carrier frequency offsets. Due to the superior statistical property of the proposed timing metric, the threshold can be easily determined with no need for the process of noise variance estimation. Simulation results under different multipath fading channels demonstrate that the proposed method achieves a remarkably improved timing accuracy compared to the existing methods.

Frequency-Domain Iterative SDFE for MIMO-OFDM Systems

This paper proposes a frequency-domain iterative soft decision feedback equalization (SDFE) scheme for multiple-input---multiple-output orthogonal frequency division multiplexing communication systems in multi-path fading channels. Error propagation can seriously affect the performance of the adaptive decision feedback equalization. We propose an iterative SDFE to not only combat error propagation but also be suitable for multilevel modulation systems. Under a Gaussian approximation of the a priori information and the SDFE output, the output of an equivalent AWGN channel and expressions for the a posteriori log likelihood ratio (LLR) and the extrinsic LLR are derived for multilevel modulation. In addition, we also investigate the iterative receivers by employing irregular low-density parity check codes (LDPC) and turbo codes. The performance of the LDPC-coded iterative receiver is better than that of the turbo-coded iterative receiver. Some simulation examples are given to show the effectiveness and comparisons of the proposed receiver.

다중경로 페이딩 채널에서 주파수 옵셋 보상 알고리즘을 적용한 디지털 M/W 전송 시스템의 성능 분석

본 논문에서는 다중 경로 페이딩 채널을 고려한 환경에서 디지털 M/W전송 시스템의 동기에 관한 모의 시뮬레이터를 구성하여 주파수 동기를 수행하였고, 송신단과 수신단의 주파수 허용 편차(오실레이터 간 오차 범위)로 발생되는 주파수 옵셋에 보상 알고리즘을 적용하여 신호 성좌도(constellation)의 성능 개선 정도를 평가하였다. 성능 분석 결과, 송 수신단의 주파수 허용 편차가 큰 상태에서는 주파수 옵셋을 보상해도 주파수 허용 편차가 이미 크기 때문에 잔류 주파수의 영향을 상대적으로 많이 받게 됨을 알 수 있었다. 결과적으로 송수신단의 주파수 허용 편차에 따라 잔류 주파수 옵셋이 시스템 성능에 큰 영향을 미치게 됨을 알 수 있었다. 본 결과물은 향후 방송중계용 시스템의 대역 이전 시 필요한 주파수 동기 기준으로 활용 가능할 것이다. In this paper, we investigated frequency synchronization through computer simulation of digital M/W transmission system in multipath fading channel. we suggested frequency offset correction algorithm against frequency offset between transmitter and receiver, then evaluated the degree of constellation performance enhancement. From the performance evaluation, in case of large frequency offset, although adopting frequency offset correction scheme, residual frequency offset degraded system performance. As a result, according to frequency offset value between transmitter and receiver residual frequency offset affects system performance significantly. The results of this paper should be utilized for frequency synchronization criterion when frequency band of broadcasting system is rearranged.

A Comparison of Normalizations for ZF Precoded MU-MIMO Systems in Multipath Fading Channels

We compare two commonly used normalizations for zero-forcing (ZF) precoded multi-user multi-input-multi-output (MU-MIMO) systems: equal transmit power (ETP) and equal receive power (ERP) normalization. Both have been proposed in the literature, but with no comparison of the performance in multipath fading channels. We show analytically, and through simulations, that the ETP normalization leads to a higher total received power at the users and a higher sum rate. We also present simulation results in a multipath fading channel to corroborate the analysis and observe that the performance spread among users increases as the delay spread decreases with ETP normalization. This can be used to choose the appropriate normalization in a particular multipath scenario.

Adaptive blind receivers for MC-CDMA communication systems

In this paper, we propose three adaptive blind algorithms for multiuser multicarrier code division multiple-access systems in multipath fading channels. The proposed adaptive blind receivers are based on the property of the discreteness of the input data symbol and are updated in every symbol interval. We also use the concept that the variance of the output signal approaches to the variance of the desired signal to get the cost function. The three proposed receiver structures are the traditional finite impulse response FIR structure, the despreading DES filter structure and the generalized sidelobe canceller GSC structure. The advantage of the FIR filter is that the length of the filter weights does not have to be the same length as the spreading code. For the DES filter, the combination of the adaptive weight and the despreading code has the simplest structure than the other two proposed receiver structures. The constrained GSC filter is superior to the other two proposed receiver structures in the environments dominated by multiple-access interference. By this constraint, the blind GSC filter can guarantee to converge to the desired solution. Simulation results are given to show the effectiveness and comparison of the proposed adaptive blind receivers. Copyright ©2012 John Wiley & Sons, Ltd.

Adaptive low-rank channel estimation for multi-band OFDM ultra-wideband communications

In this paper, an adaptive channel estimation scheme based on the reduced-rank (RR) Wiener filtering (WF) technique is proposed for multi-band (MB) orthogonal frequency division multiplexing (OFDM) ultra-wideband (UWB) communication systems in multipath fading channels. This RR-WF-based algorithm employs an adaptive fuzzy-inference-controlled (FIC) filter rank. Additionally, a comparative investigation into various channel estimation schemes is presented as well for MB-OFDM UWB communication systems. As a consequence, the FIC RR-WF channel estimation algorithm is capable of producing the bit-error-rate (BER) performance similar to that of the full-rank WF channel estimator and superior than those of other interpolation-based channel estimation schemes.

Channel length assisted symbol synchronization for OFDM systems in multipath fading channels

Despite the promising role of orthogonal frequency-division multiplexing (OFDM) technology in communication systems, its synchronization in multipath fading channels remains an important and challenging issue. This work describes a novel synchronization algorithm that exploits channel length information for use in OFDM systems. A timing function that can identify the ISI-free region and subsequently the channel length is also developed based on both the redundancy of the cyclic prefix (CP), and the drastic increase in intersymbol interference (ISI) that rises with symbol timing error in multipath fading channels. Knowledge of the channel length information allows the symbol timing to be safely set in the middle of the ISI-free region, without any ISI. Simulation results indicate that the maximum value of the timing function occurs at the correct timing offset when the signal-to-noise ratio (SNR) is high. From low- to medium SNRs, the correct timing offset is guaranteed when the signal power induced by the channel tap is more significant than the noise power. Furthermore, an efficient search algorithm is derived to reduce the search complexity (search time and computation complexity).

Performance Analysis of Multistage Interference Cancellation for Asynchronous TH-UWB Systems in Multipath Fading Channels

In this paper, we present an accurate analytical method for an asynchronous time-hopping (TH) ultrawideband (UWB) system using multistage interference cancellation (MIC) in multipath fading channels. To model the asynchronous transmission, we first investigate the chip-asynchronous case and extend the results of chip-asynchronous transmission into completely asynchronous transmission as a more general environment. Specifically, the approximate closed-form expression is derived for numerically calculating the average bit-error probability (BEP) of the MIC receivers, which are based on the hard-decision (HD) and soft-decision (SD) detections, respectively. In performing the analysis, the effect of multiple-access interference (MAI) is modeled as a Gaussian process. The results of an interference cancellation (IC) receiver as a function of the ratio between the two types of processing gain-1) pulse combining gain <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">s</sub> and 2) pulse spreading gain <i xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">N</i> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">c</sub> -are analyzed and compared under the constraint that the total processing gain of the system is large and fixed. To build up intuitive knowledge, some remarks on the analytic results are presented to describe the design criterion for the interference suppression. In numerical results, the theoretical analysis is verified via comparison with simulation results in terms of the number of IC stages, the set of cancellation parameters, and the number of users.

An adaptive multiuser detector for DS-CDMA systems in multipath fading channels

In this paper, we investigate the performance of an adaptive multistage detection scheme for direct-sequence code-division multiple-access (DS-CDMA) systems. The first stage consists of an adaptive...

A Hybrid Receiver Scheme for Multiuser Multicode CDMA Systems in Multipath Fading Channels

In this paper, we present a system model for multiuser multicode code-division multiple access (CDMA) in a multipath fading channel and propose a hybrid receiver scheme for the system. Conventional receivers for CDMA systems utilized interference cancellation (IC) schemes, with the RAKE receiver as an initial data estimator. However, the RAKE receiver generates inaccurate data estimates due the interference that is caused by the cross correlation between the spreading codes. This inaccurate data estimate leads to significant error propagation to the subsequent stages and thus degrades the bit error rate (BER) performance. Our proposed scheme overcomes the problem by using a more accurate initial data estimator. It consists of an equalizer for obtaining initial data estimation, followed by several stages of parallel IC (PIC). Computer simulation results show that the BER curves of our proposed scheme do not encounter an error floor under all channel conditions; in contrast, the BER curves of the conventional RAKE-PIC receiver are constrained by an irreducible error floor. In addition, in an uplink four-path channel and at a BER of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-3</sup> , our simulation results show that a one-stage PIC achieves a 2.5-dB gain over the first stage equalization receiver.

An adaptive multiuser detector for DS‐CDMA systems in multipath fading channels

AbstractIn this paper, we investigate the performance of an adaptive multistage detection scheme for direct‐sequence code‐division multiple‐access (DS‐CDMA) systems. The first stage consists of an adaptive multiuser detector which is based on the linear constrained minimum variance (LCMV) criterion. The interference cancellation (IC) occurs in the second stage. The performance of the iterative receiver over both flat and frequency‐selective fading channels is investigated and compared to the single‐user bound. In all cases, and under heavy system loads with near‐far problems, the iterative receiver is shown to offer substantial performance improvement and large gain in user‐capacity relative to the standard LCMV. In flat‐fading channels, our results show that the performance of the iterative detector is very close to the single‐user bound. For the frequency‐selective channel, this performance is noted to be in the order of 1 dB far from the single‐user bound. Copyright © 2007 John Wiley &amp; Sons, Ltd.

Performance and Optimization of Multistage Partial Parallel Interference Cancellation for Wideband CDMA Systems in Multipath Fading Channels

In this paper, a closed-form bit error rate (BER) of hard-decision multistage partial parallel interference cancellation (PPIC) having perfect and imperfect channel estimation for complex spreading code-division multiple access (CDMA) over multipath Rayleigh channels was derived. The effects of receiving BER, power ratio of pilot to data channels, multiple-access interference (MAI) and multipath interference, additive white Gaussian noise, and other factors affecting channel estimation error are analyzed. The expression determining the optimal interference cancellation weight at path level for PPIC is first derived. Then, the oscillatory behavior of user-level-optimized PIC and the convergence of perfect channel estimation PIC and path-level-optimized PIC are validated. Results indicate that all factors determining the optimal weight certainly affect the BER performance of PPIC and that the lower BER limit of multistage PPIC depends on the adjustable factors, which include power ratio, observation length, and signal-to-noise ratio, and an unchangeable factor, i.e., number of users. In addition, the variance of MAI for complex spreading sequence is obtained, and the analytical expression can also be employed to predict the performance of multicode CDMA

Performance analysis of space-time MMSE multiuser detection for coded DS-CDMA systems in multipath fading channels

This paper investigates the use of convolutional coding in space-time minimum mean-square-error (MMSE) multiuser-based receivers over asynchronous multipath Rayleigh fading channels. We focus on the performance gain attained through error control coding when used with binary-phase-shift-keyed modulation (BPSK) and multiuser access based on direct sequence-code-division multiple access (DS-CDMA). In our analysis, we derive an approximation for the uncoded probability of bit-error in multipath fading channels. This bit-error rate (BER) approximation is shown to be very accurate when compared to the exact performance. For a convolutionally coded system, we obtain a closed form expression for the bit-error rate upper bound. This error bound is noted to be tight as the number of quantization levels increased beyond eight. Using our theoretical results, we obtain an estimate for the achieved user-capacity that accrues due to error control coding. It is found that using convolutional coding with 3-bit soft-decision decoding, a user-capacity gain as much as 300% can easily be achieved when complete fading state information plus ideal channel interleaving are assumed.

On the Asymptotic Performance of Threshold-Based Acquisition Systems in Multipath Fading Channels

The asymptotic performance of timing acquisition systems having fixed dwell time in multipath fading channels is investigated. The detrimental effect of the multipath channel fading on the acquisition performance is isolated by considering the asymptotic performance as the average signal-to-noise ratio (SNR) increases. It is found that for any threshold such that the average probability of false alarm is less than a given tolerance, the channel fading results in a lower bound on the asymptotic average probability of miss which is nontrivial for a variety of fading scenarios. A threshold-based direct-sequence spread-spectrum signal acquisition system is considered and it is found that the detrimental effect of channel fading on asymptotic acquisition performance, albeit nontrivial, is not very significant. The asymptotic acquisition performance of two threshold-based acquisition schemes for ultra-wideband (UWB) signals with time-hopping (TH) spreading are also evaluated and compared. For both schemes, the detrimental effect of the channel fading on the asymptotic acquisition performance turns out to be significant.

Robust Joint OFDM Timing and Frequency Offset Estimator

A robust joint symbol timing and fractional frequency offset estimator for OFDM systems in multipath fading channels is proposed based on cyclic shifting and autocorrelation properties of PN codes. A new timing metric is also introduced by considering the delay spread to improve the robustness of the estimator in the multipath fading channels.

On Beamforming for Space-Time Block Coded OFDM Systems in Multipath Fading Channels.

Recently, in order to improve high speed data transmission and spectral efficiency in wireless communication systems, the combination of OFDM and space-time coding is being actively studied. In order to maximize the system efficiency, the problem of co-channel interference must be solved. One technique to overcome the co-channel interference and to increase the system capacity is to use adaptive antennas. Conventional beamforming techniques for single antenna cannot be applied directly to STBC-OFDM systems, because the signals transmitted from the two transmit antennas are superposed at the receive antenna and the interference between signals of the two transmit antennas occurs. In this paper, we present the MMSE beamforming technique using training sequence for STBC-OFDM systems in reverse link and evaluate the performance by using various parameters such as the number of training blocks, cluster sizes and angle spreads in Two-ray, TU and HT channels. From the simulation results, we show the best cluster sizes and the number of training blocks corresponding to these cluster sizes.

Adaptive Receivers for DS/CDMA Multiuser Communication in Multipath Fading Channels

In direct-sequence code division multiple access (DS/ CDMA) multiuser communication systems with multipath channels, both intersymbol interference (ISI) and multiple-access interference (MAI) must be considered. The multipath effect usually changes the characteristics of the spreading codes. Modification of the conventional receiver structure is needed to account for the interference of the multipath fading. This paper proposes four adaptive receivers for such multiuser DS/CDMA systems in multipath fading channels. We employ least mean square (LMS) and recursive least squares (RLS) algorithms for both finite impulse response (FIR) and infinite impulse response (IIR) receiver structures. Mean square error (MSE) and convergence analysis are also given in this paper. Simulation results show the performance comparisons of the four proposed receivers.

Iterative mmse Receivers for Space–Time Trellis-Coded CDMA Systems in Multipath Fading Channels

We explore code-division multiple-access (CDMA) systems with multiple transmit and receive antennas combined with space-time trellis codes over a frequency-selective channel. The conventional noniterative multiuser minimum mean square error (NIMU-mmse) detector is generalized to accommodate multiple antennas and multiple paths and then extended to include the turbo principle in an iterative fashion, allowing interference regeneration and cancellation at the receiver. Iterative multiuser mmse (IMU-mmse) receivers employing chip- and symbol-level detectors are derived and their equivalence is demonstrated. Computer simulations show that the proposed iterative mmse equalizers completely remove the interference of the other users in a multiantenna environment; they provide a significant improvement over the NIMU-mmse detector and they effectively achieve the single-user performance, even in a fully loaded system. Two suboptimal iterative mmse detectors, which allow a computational complexity reduction of up to three orders of magnitude compared to the IMU-mmse and still outperform the NIMU-mmse detector, are introduced. The proposed iterative mmse equalizers are analyzed and supported by extensive computer simulations.

Tracking of time misalignments for OFOM systems in multipath fading channels

In this paper, new time offset tracking techniques for OFDM are proposed without devoting specific pilot symbols for synchronization. This can be attained since, in coherent OFDM, channel estimation is usually performed employing pilot symbol assisted modulation (PSAM), based on inserting known symbols in the time-frequency grid. We show how this two dimensional embedded signalling can also be used for tracking time-frequency offsets. A two-stage procedure is proposed, focusing this work on the time correction step. The ML estimator has been derived, but though optimal, may not be practical due to its complexity. Hence, a frequency-domain low-complex estimator has been proposed, based on only the samples at pilot positions. Also, the stability of the generated estimate can be improved by averaging over a few number of OFDM symbols. This estimator has been proven to be efficient against time dispersive channels.

Channel estimation for ofdm systems using adaptive radial basis function networks

We propose a new scheme for pilot-symbol-aided channel estimation in orthogonal frequency-division multiplexing (OFDM) systems in multipath fading channels, that does not require knowledge of the channel statistics (e.g., Doppler or power spectrum). It is based on using the radial basis function (RBF) network to model the dynamics of the fading process. Both one-dimensional and two-dimensional RBF networks are proposed to exploit the channel correlation in the time domain and in the time-frequency domain. The proposed RBF networks are essentially nonlinear interpolators of the pilot channels. Compared with the existing OFDM channel estimation methods based on linear filtering, the proposed new techniques offer both robustness to fading rate, and a better performance especially in relatively fast fading channels.