Adaptive nonlinear filters for narrow-band interference suppressionin spread-spectrum CDMA systems

This paper presents a novel nonlinear filter and parameter estimator for narrow band interference suppression in code division multiple access spread-spectrum systems. As in the article by Rusch and Poor (1994), the received sampled signal is modeled as the sum of the spread-spectrum signal (modeled as a finite state independently identically distributed (i.i.d.) process-here we generalize to a finite state Markov chain), narrow-band interference (modeled as a Gaussian autoregressive process), and observation noise (modeled as a zero-mean white Gaussian process). The proposed algorithm combines a recursive hidden Markov model (HMM) estimator, Kalman filter (KF), and the recursive expectation maximization algorithm. The nonlinear filtering techniques for narrow-band interference suppression presented in Rusch and Poor and our proposed HMM-KF algorithm have the same computational cost. Detailed simulation studies show that the HMM-KF algorithm outperforms the filtering techniques in Rusch and Poor. In particular, significant improvements in the bit error rate and signal-to-noise ratio (SNR) enhancement are obtained in low to medium SNR. Furthermore, in simulation studies we investigate the effect on the performance of the HMM-KF and the approximate conditional mean (ACM) filter in the paper by Rusch and Poor, when the observation noise variance is increased. As expected, the performance of the HMM-KF and ACM algorithms worsen with increasing observation noise and number of users. However, HMM-KF significantly outperforms ACM in medium to high observation noise     

New nonlinear algorithms for estimating and suppressing narrowbandinterference in DS spread spectrum systems

It has been shown that the narrowband (NB) interference suppression capability of a direct-sequence (DS) spread spectrum system can be enhanced considerably by processing the received signal via a prediction error filter. The conventional approach to this problem makes use of a linear filter. However, the binary DS signal, that acts as noise in the prediction process, is highly non-Gaussian. Thus, linear filtering is not optimal. Vijayan and Poor (1990) first proposed using a nonlinear approximate conditional mean (ACM) filter of the Masreliez (1975) type and obtained significant results. This paper proposes a number of new nonlinear algorithms. Our work consists of three parts. (1) We develop a decision-directed Kalman (DDK) filter, that has the same performance as the ACM filter but a simpler structure. (2) Using the nonlinear function in the ACM and the DDK filters, we develop other nonlinear least mean square (LMS) filters with improved performance. (3) We further use the nonlinear functions to develop nonlinear recursive least squares (RLS) filters that can be used independently as predictors or as interference identifiers so that the ACM or the DDK filter can be applied. Simulations show that our nonlinear algorithms outperform conventional ones     

扩频系统中自适应窄带干扰抑制滤波器对PN码捕获性能的影响

本文通过详细的理论推导,分析了扩频系统中自适应窄带干扰抑制滤波器对PN码捕获性能的影响;并根据滤波器理论,给出了自适应窄带干扰抑制滤波器的幅频响应、群延迟响应和相应的仿真结果.理论分析与仿真结果表明,自适应干扰抑制滤波器在抑制窄带干扰的同时,也抑制了部分扩频信号,从而形成较大的相关旁瓣,对PN码捕获性能产生严重的影响.     

On the use of a suppression filter for CDMA overlay

This paper is concerned with a direct-sequence code-division multiple-access (DS-CDMA) system operating over a Rayleigh fading channel and sharing a common spectrum with a narrow-band waveform. A suppression filter at the receiver is employed to reduce the narrow-band interference. We evaluate the average up-link bit error rate (BER) performance and investigate how the performance is influenced by various parameters, such as the number of taps of the suppression filter, the number of multiple-access users, the ratio of narrow-band interference bandwidth to the spread-spectrum bandwidth, the interference power to signal power ratio, the ratio of the offset of the interference carrier frequency from the spread-spectrum carrier frequency to the half spread-spectrum signal bandwidth, and so on     

一种直扩通信窄带干扰抑制的自适应滤波技术

文中研究了自适应非线性滤波在直扩通信中抑制窄带干扰的应用,采用修正LMS(MLMS)自适应滤波算法对窄带干扰进行抑制,并运用ACM滤波非线性函数进行非线性处理,使滤波性能明显改善.最后用MATLAB仿真自适应滤波过程,验证了算法的滤波性能.     

Closed-form analytical results for interference rejection ofnonlinear prediction filters in DS-SS systems

Closed-form analytical expressions for narrow-band interference rejection of nonlinear prediction filters in direct-sequence spread-spectrum (DS-SS) systems are established. It is shown that nonlinear prediction filters outperform linear ones     

Closed-Form Analytical Results for the Rejection of Narrow-Band Interference in PN Spread-Spectrum Systems--Part II: Linear Interpolation Filters

The performance of PN spread-spectrum communication systems in the presence of narrow-band interference is studied when linear interpolation filters are employed for the estimation and subsequent suppression of the interference. Closed-form analytical expressions for the system's performance are established for a broad class of interference processes. The advantages of linear interpolation filters over predictive filters with identical number of taps are examined analytically and some unexpected results are obtained. The analytical results are illustrated by examples.     

基于自适应IIR陷波滤波器的窄带干扰抑制技术

在存在窄带干扰时，扩频通信系统的性能可以通过使用各种不同的抗干扰滤波器来进一步提高。文中研究了二阶自适应格型IIR陷波滤波器在直扩通信中抗窄带干扰的应用，推导了系统信噪比改善因子和误比特率的闭合表达式。计算机仿真表明了该分析结果的正确性。     

MMSE receiver for multicarrier CDMA overlay in ultra-wide-band communications

Multicarrier direct-sequence code-division multiple-access (MC-CDMA) overlay has been proposed to be used for ultra-wide-band (UWB) communications. Interference reduction and interference suppression are the key issues for sharing the spectrum in harmony between the established narrow-band systems and the overlaid UWB system. In this paper, investigation is carried out on the use of compromising measures incorporated to the MC-CDMA overlay to meet these goals, as well as on their impacts to the involving parties. At the transmitter, interference reduction to the established narrow-band systems is done by using notch filters. A multipath Nakagami fading channel is assumed. At the receiver, the interference suppression from those narrow-band systems is fulfilled by minimum mean square error (MMSE) detection technique. Numerical results show that precombining MMSE with selective-maximal combining provides the UWB system with much better performance than the receiver made up of notch filter in cascade with code correlator.     

Performance of CDMA overlay system using filtering and diversity

This correspondence considers an interference suppression scheme for a direct-sequence code-division multiple-access overlay system operating in a multipath Nakagami fading environment. The receiver employs suppression filters and diversity to mitigate the effect of multipath and narrow-band jammer interference. The system performance is presented in terms of the bit error rate with different system parameters such as bit energy, jammer-to-signal power, multipath, diversity, ratio of interference bandwidth to spread-spectrum bandwidth, and number of active users. Numerical results indicate substantial improvement in system performance.     

Rejection of multiple tone interference in DS spread-spectrumsystems employing minimum redundant transversal filters

A direct-sequence spread-spectrum (DSSS) receiver using a new transversal filter structure called minimum redundant transversal filter (MRTF), for combating narrow-band interference is proposed. The signal-to-noise (SNR) ratio improvement factors are derived under the conditions of multiple tone interference. The effectiveness of the new suppression filter is examined and the results obtained reveal the fact that the MRTF structure leads to superior performance especially when the frequencies of the jamming tones are near the carrier frequency of the modulated signal     

一种改进的窄带干扰抑制算法

针对窄带强干扰对直扩系统造成的严重影响,给出了变换域干扰抑制技术的实现原理框图;在分析传统N-SIGMA算法和中值滤波算法的基础性,提出了改进的中值滤波算法;最后,使用MATLAB对其干扰抑制能力及抑制前后误码性能进行仿真及比较分析,仿真及分析结果表明改进的中值滤波算法在性能上比传统方法有较大的提高,比N-SIGMA算法硬件实现简单,更具有工程推广价值。     

Nonlinear techniques for the joint estimation of cochannel signals

Cochannel interference occurs when two or more signals overlap in frequency and are present concurrently. Unlike in spread-spectrum multiple-access systems where the different users necessarily share the same channel, cochannel interference is a severe hindrance to frequency- and time-division multiple-access communications, and is typically minimized by interference rejection/suppression techniques. Rather than using interference suppression, we are interested in the joint estimation of the information-bearing narrow-band cochannel signals. Novel joint estimators are proposed that employ a single-input demodulator with oversampling to compensate for timing uncertainties. Assuming finite impulse-response channel characteristics, maximum likelihood (ML) and maximum a posteriori (MAP) criteria are used to derive cochannel detectors of varying complexities and degrees of performance. In particular, a (suboptimal) two-stage joint MAP symbol detector (JMAPSD) is introduced that has a lower complexity than the single-stage estimators while accruing only a marginal loss in error-rate performance at high signal-to-interference ratios. Assuming only reliable estimates of the primary and secondary signal powers, a blind adaptive JMAPSD algorithm for a priori unknown channels is also derived. The performance of these nonlinear joint estimation algorithms is studied through example computer simulations for two cochannel sources     

Narrow-band interference suppression in spread-spectrum CDMAcommunications using pipelined recurrent neural networks

This paper investigates the application of pipelined recurrent neural networks (PRNN's) to the narrow-band interference (NBI) suppression over spread-spectrum (SS) code-division multiple-access (CDMA) channels in the presence of additive white Gaussian noise (AWGN) plus non-Gaussian observation noise. Optimal detectors and receivers for such channels are no longer linear. A PRNN that consists of a number of simpler small-scale recurrent neural network (RNN) modules with less computational complexity is conducted to introduce best nonlinear approximation capability into the minimum mean-squared error nonlinear predictor model in order to accurately predict the NBI signal based on adaptive learning for each module from previous non-Gaussian observations. Once the prediction of the NBI signal is obtained, a resulting signal is computed by subtracting the estimate from the received signal. Thus, the effect of the NBI can be reduced. Moreover, since those modules of a PRNN can be performed simultaneously in a pipelined parallelism fashion, this would lead to a significant improvement in its total computational efficiency. Simulation results show that PRNN-based NBI rejection provides a superior signal-to-noise ratio (SNR) improvement relative to the conventional adaptive nonlinear approximate conditional mean (ACM) filters, especially when the channel statistics and exact number of CDMA users are not known to those receivers     

Interference suppression to aid acquisition in direct-sequencespread-spectrum communications

The acquisition and tracking systems of a spread-spectrum receiver are probably the most critical components of the receiver, since if they fail to function properly, it is doubtful that the desired signal can be successfully detected. This means that the affect of interference (such as jamming) on the receiver while it is attempting to learn the correct phase position of the incoming code might be especially harmful, since the interference might not allow the receiver to acquire the signal. To address this problem, a narrow-band interference suppression filter is used to enhance the performance of a serial search acquisition scheme for a direct-sequence spread-spectrum receiver. Analytical expressions for the probabilities of error in both the search and lock modes are derived, and numerical results are used to illustrate the sensitivity of the receiver to various system parameters. It is shown that the presence of the rejection filter can be significantly improves the performance of the acquisition system     

Performance Analysis of Narrow-Band Interference Rejection Techniques in DS Spread-Spectrum Systems

Linear least squares estimation (LLSE) techniques can provide an effective means of suppressing narrow-band interference in direct sequence (DS) spread-spectrum systems. In the results presented here, analytical expressions for bit error rate are derived for two DS spread-spectrum systems under the conditions of either tone or narrowband Gaussian interference. It is shown that the most common LLSE filter design can lead to performance inferior to that of various other filter designs. However, results are also presented demonstrating that an LLSE filter design motivated by the structure of the maximum-likelihood receiver leads to consistently superior performance. The performance of a system using this new design criterion is compared with that of an approximation to the maximum-likelihood (ML) receiver for the tone interference model and with that of the exact ML receiver for the Gaussian interference. Finally, it is shown that the bit error rate estimate obtained from application of a Gaussian approximation for the test statistic is overly pessimistic for the systems studied here.     

MMSE interference suppression for direct-sequence spread-spectrumCDMA

We consider interference suppression for direct-sequence spread-spectrum code-division multiple-access (CDMA) systems using the minimum mean squared error (MMSE) performance criterion. The conventional matched filter receiver suffers from the near-far problem, and requires strict power control (typically involving feedback from receiver to transmitter) for acceptable performance. Multiuser detection schemes previously proposed mitigate the near-far problem, but are complex and require explicit knowledge or estimates of the interference parameters. In this paper, we present and analyze several new MMSE interference suppression schemes, which have the advantage of being near-far resistant (to varying degrees, depending on their complexity), and can be implemented adaptively when interference parameters are unknown and/or time-varying, Numerical results are provided that show that these schemes offer significant performance gains relative to the matched filter receiver. We conclude that MMSE detectors can alleviate the need for stringent power control. In CDMA systems, and may be a practical alternative to the matched filter receiver     

Automatic suppression of narrow-band interference indirect-sequence spread-spectrum systems

This paper addresses the problem of narrow-band interference (NBI) cancellation in direct-sequence spread-spectrum systems. The proposed procedure amounts to a preliminary nonlinear processing, wherein, upon projection of the received signal onto a Fourier basis, a number of samples having the largest modula are excluded from further processing. The structure of the optimum detector operating on censored observations is obtained, showing that the optimum detector performs matched filtering on the censored data. The performance assessment demonstrates that this receiver is able to suppress narrow-band interferers, no matter what their structure, provided that the censoring depth is properly chosen. A blind version of such a receiver is presented also, and a comparative performance assessment demonstrates that, unlike other suppression procedures, the proposed system allows suppression of NBI with no prior knowledge on its structure     

Wavelet Notch Filter Design of Spread-Spectrum Communication Systems for High-Precision Wireless Positioning

Spread-spectrum communication systems are now commonly used in the field of cellular telephone positioning. However, wireless positioning systems by low-power spread-spectrum communication are extremely vulnerable to high-power interference, which limits achievable measurement precision. In this paper, a bandwidth variable wavelet notch filter design method is proposed to suppress a large number of jammers in multiple locations with noise interfering with spread-spectrum systems. The filter uses combinations of Gaussian wavelets with optimal time-frequency localization and computational efficiency for real-time operation of denoising. The performance of the adaptive filter has been evaluated by experiments associated with a spread-spectrum communication system model employing a reliable noise detector to locate the filter notch. Experimental results demonstrate that the proposed wavelet notch filter removes the narrow-band interference in accordance with the corrupted frequency contents while minimizing signal distortion and information loss, which leads to high-precision wireless positioning.     

基于DFT的抗窄带干扰研究

在接收端通过频域干扰抑制技术,去除窄带干扰能量,从而使直接扩频调制的抗窄带干扰能力得到明显的改善。在直接扩频通信系统中,基于傅里叶变换的门限估计常用的有N-sigma法、λ算法和中值滤波3种。对其进行了详细的理论分析,并进行了MATLAB仿真对比,其结果表明3种方法都能使窄带干扰得到一定抑制,但其硬件实现的难易不同。