Performance of adaptive linear interference suppression in thepresence of dynamic fading

Adaptive linear interference suppression for direct-sequence (DS) code-division multiple access (CDMA) is studied in the presence of time- and frequency-selective fading. Interference suppression is achieved with an adaptive digital filter which spans a single symbol interval. Both decision-directed and blind adaptive algorithms, which do not require a training sequence, are considered. Modifications to least squares adaptive algorithms are presented which are compatible with differential coding and detection. For frequency-selective fading, adaptive algorithms are presented based upon different assumptions concerning knowledge of the desired user's channel. Specifically, the cases considered are as follows: (1) perfect knowledge of the desired channel; (2) knowledge of only the relative path delays; and (3) knowledge of only one delay corresponding to the strongest path. Computer simulation results are presented which compare the performance of these algorithms with the analogous RAKE receivers. These results show that for case (3), even slow fading can cause a significant degradation in performance. Effective use of channel parameters in the adaptive algorithm reduces the sensitivity to fade rate, although moderate to fast fading can significantly compromise the associated performance gain relative to the RAKE receiver     

非频率选择性衰落信道下捕获性能的分析

对基于数字匹配滤波器(DMF)捕获系统在非频率选择性衰落信道下的捕获性能进行了深入讨论,利用状态转移图推导出单次驻留判决方式时平均捕获时间的表达式。对平坦慢衰落信道和快衰落信道下的平均捕获时间进行了数字分析,得出的主要结论是：无论信道是否存在衰落,基于DMF的捕获系统都具有很快的捕获速度;基于DMF的捕获系统在衰落信道下的捕获性能明显要比非衰落信道差;衰落速度越快,平均捕获时间就越长,系统的捕获性能就越差。因此当信道存在非频率选择性衰落时,可以在接收端采用分集接收(比如空间分集)来改善系统的捕获性能。     

Application of Adaptive Filtering to Direct-Sequence Spread-Spectrum Code Acquisition

A novel hybrid scheme utilizing an adaptive FIR filter is proposed for acquisition of DS-SS signals. Timing information on the delay offset between the incoming DS-SS signal and the locally generated replica of the spreading code is extracted from the tap-weight vector of the acquisition adaptive filter. Expressions for the mean acquisition time, detection, and false alarm probabilities for a coherent, chip synchronous DS-SS system in AWGN are derived. The improvement in acquisition performance over serial search techniques is twice the length of the adaptive filter. This is similar to that gained by other hybrid schemes that search the same number of cells at a time. However, a significant reduction in hardware complexity is obtained. The proposed system is also compared to a system utilizing a partial matched filter structure. Moreover, the same hardware could be used for code tracking and, hence, eliminating the need for a separate tracking loop.     

Comparison of the Acquisition Performance of the Matched Filter and Integrate-and-Dump Receivers in a Multipath Delay and Fading Environment

The Matched Filter and the Integrate-and-Dump circuit techniques are both attractive acquisition methods for CDMA systems. Acquisition using the matched filter technique has attracted considerable interest in recent years since it shows a better performance in the presence of Gaussian noise. On the other hand acquisition using an integrate-and-dump circuit is attractive because it is less complex to implement. However, the acquisition performance can be affected significantly by the channel characteristics. In this paper, a numerical method and a computer simulation have been developed in order to compare the two acquisition systems in different propagation environments. Results show that the acquisition using a matched filter is much faster than that using the integrate-and-dump circuit for the reception of spread-spectrum signals in the presence of Gaussian noise. However, in a multipath delay and fading environment, the results show that the mean acquisition time using the matched filter is not always faster than that using the integrate-and-dump circuit. The results show that it depends on the fading, the delay, the number of users and the signal-to-noise ratio.     

Noncoherent hybrid parallel PN code acquisition for CDMA mobilecommunications

This paper proposes a noncoherent hybrid parallel pseudonoise (PN) code acquisition scheme for code-division multiple access (CDMA) mobile communication systems and analyzes the effect of multiple access interference (MAI) on the code acquisition performance for Rayleigh and Rician fading channels. The hybrid acquisition scheme combines parallel search with serial search to cover the whole uncertainty region of the input code phase. It has a much simpler acquisition hardware structure than the total parallel acquisition and can achieve the mean acquisition time slightly inferior to that of the total parallel acquisition in the case of severe MAI; on the other hand, it provides the flexibility in the tradeoff between the mean acquisition time and system complexity if no MAI is considered. The closed-form expressions of the detection and false-alarm probabilities and mean acquisition time are derived. Numerical analysis quantifies the severe performance degradation of code acquisition due to both MAI and channel fading, and demonstrates the dependence of the increase of mean acquisition time (due to MAI) on the number of users in the CDMA system, system design parameters, and channel fading statistics     

基于DMF捕获系统频率选择性信道下捕获性能的分析

对基于数字匹配滤波器(DMF)捕获系统在频率选择性信道下的捕获性能进行了深入讨论，利用状态转移图推导出单次驻留判决方式时平均捕获时间的表达式，对平均捕获时间与多径分量的关系进行了数字分析。得出的主要结论是：对于多径分量为非衰落信号时，多径分量之间的能量差别越大，系统捕获性能越好，多径分量的能量相等时，系统捕获性能最差，且随着可以分离的多径路数的增加下降；对于多径分量为慢衰落信号时，多径信道的捕获性能好于单径信道(非频率选择性衰落信道)，且多径分量之间的能量差别越大，系统捕获性能越差，当多径分量的能量相等时，捕获性能最好，这与非衰落信道的情形相反。因此在频率选择性衰落信道中，采用分集接收可以改善系统的捕获性能。     

A Novel Adaptive Antenna Array for DS/CDMA Code Acquisition

Equipped with an adaptive beamformer, existing adaptive array code acquisition still relies on the correlator structure. Due to the inherent property of the associated serial-search scheme, its mean acquisition time is large, especially in strong interference environments. In this paper, we propose a novel adaptive filtering scheme to solve the problem. The proposed scheme comprises two adaptive filters, an adaptive spatial and an adaptive temporal filter. With a specially designed structure, the spatial filter can act as a beamformer suppressing interference, while the temporal filter can act as a code-delay estimator. A mean squared error (MSE) criterion is proposed such that these filters can be simultaneously adjusted by a stochastic gradient descent method. The performance as well as the convergence behavior of the proposed algorithm are analyzed in detail. Closed-form expressions for optimum filter weights, optimum beamformer signal-to-interference-plus-noise ratio (SINR), steady-state MSE, and mean acquisition time are derived for the additive white Gaussian noise (AWGN) channel. Computer simulations show that the mean acquisition time of the proposed algorithm is much shorter than that of the correlator-based approach, and the derived theoretical expressions are accurate.     

Channel‐estimate‐based frequency‐domain equalization (CE‐FDE) for broadband single‐carrier transmission

A channel‐estimate‐based frequency‐domain equalization (CE‐FDE) scheme for wireless broadband single‐carrier communications over time‐varying frequency‐selective fading channels is proposed. Adaptive updating of the FDE coefficients are based on the timely estimate of channel impulse response (CIR) to avoid error propagation that is a major source of performance degradation in adaptive equalizers using least mean square (LMS) or recursive least square (RLS) algorithms. Various time‐domain and frequency‐domain techniques for initial channel estimation and adaptive updating are discussed and evaluated in terms of performance and complexity. Performance of uncoded and coded systems using the proposed CE‐FDE with diversity combining in different time‐varying, multi‐path fading channels is evaluated. Analytical and simulation results show the good performance of the proposed scheme suitable for broadband wireless communications. For channels with high‐Doppler frequency, diversity combining substantially improves the system performance. For channels with sparse multi‐path propagation, a tap‐selection strategy used with the CE‐FDE systems can significantly reduce the complexity without sacrificing the performance. Copyright © 2004 John Wiley & Sons, Ltd.     

自适应滤波在扩频信号分集接收中的应用研究

使用自适应滤波器完成对扩频信号的扩频码捕获后,传输信道中由于信号衰落造成的多径信号的幅度就可以从自适应滤波器收敛后的系数权值向量上体现出来.根据分集接收的理论分析,依据多径信号的信号幅度值进行分集接收在理论上是可行的,因此利用自适应滤波器对接收到的多径信号进行隐分集接收是完全可行的.使用自适应滤波器在完成扩频码捕获的同时,在不增加额外硬件支路的情况下就可以实现对多径信号的分集接收功能,从而改善在多径衰落环境下的信道传输性能.     

Direct sequence spread spectrum matched filter acquisition infrequency-selective Rayleigh fading channels

This paper presents the performance of a matched filter acquisition scheme in nonselective and frequency-selective Rayleigh fading channels. This acquisition scheme employs a reference filter to estimate the variance of the interference at the output of the detecting matched filter. Expressions for probability of detection and probability of false alarm are derived and the mean acquisition time is evaluated for various channel and system parameters. The results presented show the relationship between channel and system parameters and the system performance     

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.     

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.     

Adaptive Hybrid Acquisition of PN Sequences Based on Automatic Multipath Cancellation in Frequency-Selective Rayleigh Fading Channels

A hybrid PN-code acquisition scheme is proposed and studied for direct sequence code division multiple access communications in frequency-selective Rayleigh fading channels. The most important feature of the proposed system is its ability to dynamically estimate the number of multipath signals that may lie in the reference channel. The basic idea of the considered algorithm is to sort the partial correlations and carry out successive censoring tests by updating the noise estimation until the first path synchronization. Exact expressions for the probability of false alarm and the mean acquisition time are derived. The performances of the proposed system are then studied and compared with those of the adaptive hybrid acquisition processor (AHAP) and the ordered statistic hybrid acquisition processor (OSHAP). The effects of various channel parameters on the acquisition performance, namely the number of resolvable paths, the multipath intensity profile and the signal-to-noise ratio are also investigated. Numerical results show that the proposed acquisition scheme outperforms the conventional ones which are based on AHAP and OSHAP algorithms.     

Performance of coded DS-CDMA with pilot-assisted channel estimationand linear interference suppression

We consider a direct sequence (DS-) code division multiple access (CDMA) system with orthogonally multiplexed pilot signals and minimum mean squared error (MMSE) data and channel estimation. Both flat and frequency-selective fading channels are considered. Large system analysis is used to optimize the pilot-to-data power ratio (PDR) and the code rate for a fixed bandwidth expansion. Specifically, the PDR is selected to minimize the probability of error subject to a constraint on transmitted power. When the MMSE filter estimates the channel of the desired user, but averages over the channels of the interferers (corresponding to an adaptive filter in moderate to fast fading), the optimal PDR is less than that for the matched filter (MF). That is, the MMSE filter benefits from allocating more power to the data. When the MMSE filter directly incorporates estimates of all users' channel coefficients, the optimal PDR is greater than that for the MF. System performance as a function of code rate is characterized through both probability of error and cutoff rate. The optimal code rate for the MMSE receiver is generally higher than that for the MF, and increases with load and E_b/N₀. In the presence of fading, and with channel estimation, the optimal code rate approaches zero for both MMSE and MF receivers, but the MMSE filter is more robust with respect to a suboptimal choice of code rate     

Utilization of multipaths for spread-spectrum code acquisition infrequency-selective Rayleigh fading channels

A novel acquisition scheme that utilizes multipaths to improve the acquisition performance is proposed for frequency-selective fading channels. The proposed acquisition scheme employs nonconsecutive search and joint triple-cell detection. The performance is analyzed in frequency-selective Rayleigh fading channels. Equations for the probabilities of detection and false alarm are derived, and an expression for the mean acquisition time is developed. The mean acquisition time performance of the proposed and conventional acquisition schemes is evaluated and compared. It is found that the proposed acquisition scheme significantly outperforms the conventional one. The effects of various channel parameters such as the number of resolvable paths, the shape of the multipath intensity profile (MIP) and the signal-to-interference ratio (SIR) on acquisition performance are also investigated     

Acquisition performance of CDMA systems with multiple antennas

This paper discusses the code-acquisition problem for preamble search in the code-division multiple-access (CDMA) uplink when multiple antennas are employed for signal reception. The multiple antennas are used either in the form of antenna array for beamforming or in the form of antenna diversity. The performance of a maximum-likelihood (ML) code-acquisition technique utilizing beamforming is analyzed by considering the detection, miss, and false-alarm probabilities. This analysis is based on the statistics of the CDMA noncoherent demodulator and beamformer output. In the case of a code-acquisition scheme utilizing antenna diversity, the probability density function of a decision variable is analytically derived for multiple antennas. The expressions for the probabilities of detection, miss, and false alarm are also presented. The acquisition performance and mean acquisition time for both code-acquisition techniques are numerically evaluated and compared. The code-acquisition approaches with multiple antennas are shown to provide much better performance than the corresponding scheme with a single antenna in a frequency-selective fading channel.     

DS-CDMA移动通信系统的捕获技术

PN (PseudoNoise)码捕获技术是DS CDMA (DirectSequence CodeDivisionMultipleAccess)蜂窝移动通信系统的关键技术。文中提出了一种基于能量窗的捕获技术 ,有效地利用了CDMA系统固有的多径分集能力 ,捕获对象是多径信号 ,研究结果表明 :对有效多径成分进行捕获有比单径捕获和跟踪有更好的性能。     

宽带CDMA中一种新的自适应信道估计方法

在衰落信道中进行相干解调必须知道瞬时的信道参数，但由于信道是时变的，所以信道估计器的频率响应和带宽应该是由衰落信道的统计特性和系统的信干比决定。利用瞬时信道估计的自相关函数，本文提出了信道估计的一种新的自适应算法。在平坦瑞利信道中的计算机仿真表明本方法在3G标准支持的所有多普勒频率范围内都能获得良好的均方误差性能。     

Performance of coded DS‐CDMA system using reduced rank MMSE receiver in Rayleigh fading channels

This paper examines the performance of a reduced rank minimum mean square error (MMSE) receiver‐based direct sequence code division multiple access (DS‐CDMA) system. For such system, when a large processing gain is employed, substantial time is consumed in computing the filter tap weights. Many schemes for reducing the complexity of the MMSE have been proposed in recent years. In this paper, computational complexity reduction of the MMSE receiver is achieved by using the K‐mean classification algorithm. The performance of the uncoded and coded systems are investigated for the full rank MMSE receiver and reduced rank MMSE receiver and results are compared in terms of bit error rate at different loading levels in both AWGN and fading channels. A system with the matched filter (MF) receiver is also presented for the purpose of comparison and an analytical pair‐wise error bound for the coded system is derived. In the adaptive implementation of the receivers, results show that good performance is achieved for the reduced rank receiver when compared to the full rank receiver in both coded and uncoded systems, while in the optimum implementation of the tap weights, the reduced dimension receiver performance experiences degradation when compared to the full rank scheme. Over the band‐limited channels considered, results for the reduced rank receiver also reiterate the fact that higher code rates tend to yield lower BER than that of low rate codes. Copyright © 2006 John Wiley & Sons, Ltd.     

Complexity reduction of the MLSD/MLSDE receiver using the adaptivestate allocation algorithm

The idea of adaptive state allocation (ASA) algorithm is used in this paper to substantially reduce the computational complexity of the maximum-likelihood sequence detection and estimation (MLSD/MLSDE) receiver without a significant degradation in its performance. In the ASA algorithm, the total number of states assigned to the trellis and the number of states selected from the entire set are changed adaptively based on the short-term power of the channel impulse response (CIR) or its estimate. The ASA algorithm is a combination of two methods: adaptive threshold (AT) and adaptive state partitioning (AP). In the AT method, a threshold value is formulated based on the probability of removing the correct state in the trellis diagram. At each time, only the paths whose costs are less than the minimum cost (corresponding to the best survivor path) plus the threshold value are retained and are extended to the next trellis stage. The AT method significantly reduces the computational complexity of the regular MLSDE mostly at high signal-to-noise ratio (SNR) with a negligible loss in performance. Simulation results for fading channels show that the AT method typically selects one trellis state (the minimum possible number of states) at high SNRs. In the AP method, the branch metrics are fused and diffused adaptively by using the Kullback-Leibler (KL) distance metric invoked for quantifying the differences between the probability density functions of the correct and incorrect branch metrics in the trellis. The adaptation is done such that the channel coefficients with short-term power less than a threshold are assumed to be zero in computing the branch metrics. The AP method decreases the computational complexity of the regular MLSDE at low SNRs