Adaptive beam forming using split-polarity transformation forcoherent signal and interference

Split-polarity transformation (SPT), which is incorporated into conventional linearly constrained minimum variance (LCMV) beamformers to decorrelate the desired signal from interference, is presented. The SPT processor does not distort the direction vectors associated with wave fronts impinging on the array, but it does reverse the phase of the signal coming from a specified direction. With the aid of SPT processing, the signal cancellation due to correlation between the desired signal and interference is almost eliminated. The design of a robust SPT processor for combating the effects of mismatch between idealized array model and actual scenario arising from causes such as sensor location, amplitude, and phase errors is discussed. A detailed performance study of the SPT-LCMV beamformer shows it to be robust against direction uncertainty in the assumed look direction. Numerical results are presented to demonstrate the effectiveness of the SPT-LCMV beamforming scheme is a coherent interference environment     

Direct-sequence spread-spectrum parallel acquisition innonselective and frequency-selective Rician fading channels

This work presents the performance of the direct-sequence spread-spectrum (DS-SS) parallel acquisition system, previously proposed by the authors (1989, 1990), for nonselective and frequency-selective Rician (i.e. specular plus Rayleigh) fading channels. The acquisition system utilizes a bank of parallel I-Q noncoherent matched filters for the search mode, and a coincidence detector for the verification mode. The probabilities of detection and false alarm are derived and the mean and variance of the acquisition time are evaluated as a measure of the system performance. The nonselective channel is a Rayleigh fast fading channel, while the frequency-selective channel model is the so-called wide sense stationary uncorrelated scattering (WSSUS), selective only on frequency. These channels are typical for aircraft-satellite and line-of-sight (LOS) communications     

Radiometric detection of direct-sequence spread-spectrum signals using interference excision

Direct-sequence spread-spectrum (DS-SS) transmission are difficult to intercept. The theoretical analysis of intercept performance against such systems has always been a difficult task if the noise samples are correlated. Thus, many results are purely experimental and lack a generality which can only be achieved from theoretical considerations. The target system described is a DS-SS communications system operating environment which consists of additive white Gaussian noise and continuous wave interference. The interceptor uses a radiometer in conjunction with interference excision techniques to detect the presence of the target signal. It is shown that much of the interceptor's effectiveness is restored by the interference excision.<>     

Frequency-diversity spread-spectrum communication system to counterbandlimited Gaussian interference

A spread spectrum system to counter bandlimited Gaussian interference is proposed. The optimum receiver for this system is easy to build. Frequency diversity which allows the receiver to distinguish unjammed signal replicas from their jammed versions is used. The system can also resist bandlimited partial-time jamming. The only choice left to a smart jammer to maximize the error probability is to spread its signal like the communicator. The optimum receiver, which jointly performs symbol detection and interference rejection, is derived. Side information needed by this receiver can easily be estimated. However, if the interference bandwidth is narrow compared to the signal bandwidth, side information on noise and interference levels is not needed by a simpler and near-optimum receiver. Bit-error probability is evaluated for quaternary phase shift keying (QPSK) modulation and compared to that of direct sequence spread spectrum. We also propose the use of polyphase filters for simple system implementation     

Direct-sequence spread-spectrum multiple-access communications with random signature sequences: a large deviations analysis

A direct-sequence spread-spectrum multiple-access bit-error probability analysis is developed using large-deviations theory. Let m denote the number of interfering spread-spectrum signals and let n denote the signature sequence length. Then the large deviations limit is as n to infinity with m fixed. A tight asymptotic expression for the bit-error probability is proven, and in addition, recent large-deviations results with the importance sampling Monte Carlo estimation technique are applied to obtain accurate and computationally efficient estimates of the bit-error probability for finite values of m and n. The large-deviations point of view is compared also to the conventional asymptotics of central limit theory and the associated Gaussian approximation. The Gaussian approximation is accurate and the ratio m/n is moderately large and all signals have roughly equal power. In the near/far situation, however, the Gaussian approximation is quite poor. In contrast, large-deviations techniques are more accurate in the near/far situation, and it is here that these methods provide some important practical insight.<>     

Direct-sequence spread spectrum narrowband interference rejectionusing property restoration

A new method of rejecting narrowband interference in direct-sequence spread spectrum (DS/SS) communications is presented. A typical approach is to reject the interference using a filter with large attenuation at the interference frequencies before despreading. The interference rejection method presented incorporates vector space projection techniques to suppress the correlated interference. Several signal characteristics are formulated which lead to constraint surfaces in the vector space of possible solutions. These constraint surfaces describe interference rejection solutions which introduce minimal distortion to the spread spectrum signal and simultaneously remove the interference. The constraint surfaces essentially correspond to spread spectrum signal estimates which, after interference rejection, conform to known characteristics of the transmitted spread spectrum signal. The formulation of the surfaces relies on prior knowledge about the spread spectrum signal correlation and spectral properties     

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     

Error probabilities for DS spread-spectrum systems using an ALE fornarrow-band interference rejection

The effects of the least-mean-squares adaptive line enhancer (ALE) weight misadjustment errors on the bit error rate are investigated for a direct-sequence spread-spectrum binary communication system in the presence of strong narrowband interference. The converged ALE weights are modeled as the parallel connection of a deterministic FIR (finite impulse response) filter and a random FIR filter. The statistics of the random filter are derived, assuming the output of the random filter to be primarily due to the jammer convolved with random filter weights. This output is shown to be nonGaussian and to cause significant error rate degradation in comparison to a Gaussian model. Error probability expressions are derived for the bit error rate, evaluated numerically, and compared to the corresponding error probabilities for a Gaussian model for the random filter output. For some typical system parameter values and error probabilities it is shown that the Gaussian model yields performance results that are too optimistic by several decibels     

Concatenated coding for frequency-hop spread-spectrum withpartial-band interference

Concatenated coding techniques are applied to slow frequency-hop packet radio communications for channels with partial-band interference. Binary orthogonal signaling (e.g., binary FSK) is employed with noncoherent demodulation, Reed-Solomon codes are employed for the outer code while both block and convolutional codes are considered for the inner code. Hard-decision and soft decision decoding methods are considered for the inner codes. A method is devised for estimating the reliability of the outer Reed-Solomon code symbols, and this estimate is used to determine which code symbols should be erased. Comparisons are made between the performance of concatenated codes and the performance of Reed-Solomon codes alone     

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     

Filtering out intersymbol interference in spread-spectrum communication systems

In this letter the transfer characteristic of the optimum filter suitable for eliminating intersymbol interference in pseudo-noise spread-spectrum communication systems is determined. Such a characteristic is independent of the code sequence employed.     

面向IR-UWB基于直接序列设计抑制窄带干扰算法

针对窄带系统对超宽带冲激无线电系统(Impulse Radio Ultra-wideband,IR-UWB)的干扰问题,提出基于Rayleighritz理论的直接序列(Direct Sequence,DS)设计方案,记为RRDS。通过设计DS序列,使得IR-UWB频谱在窄带干扰频率点上产生陷波,从而实现抑制干扰的目的,同时,通过RRDS方案,提高IR-UWB系统的误码率。RRDS方案先建立目标矩阵,并求解最小特征值及对应的特征向量。如果特征向量里面的元素有非+1或-1的值,则通过符号函数将特征向量转变为全部由+1,-1组成。若最小特征值是多重,先将所有的特征向量通过符号函数转变为全部由+1或-1组成的向量,并在这些向量里找出使目标矩阵的值最小的特征向量,将此向量作为直接序列DS。仿真结果表明,提出的RRDS方案有效地抑制窄带干扰,并提高了IR-UWB系统的误码率。     

Investigation into the interference potential of spread-spectrum clock generation to broadband digital communications

Since its inception, spread-spectrum clocks have been a valuable technology for the purposes of EMI reduction and EMC compliance. This study evaluates the interference potential of spread spectrum clocks to broadband digital communications such as high definition television (HDTV). This is accomplished by comparing the spread spectrum clock interference susceptibility of digital television (DTV) receivers to the interference susceptibility of analog television receivers. This study shows that DTV receivers are more immune to the same level of interference than existing analog television receivers by 16 dB. Since industry has shown that analog television has had negligible interference from information technology equipment, including spread spectrum clocks, then the digital systems should have even less.     

Adapted demodulation for spread-spectrum receivers which employtransform-domain interference excision

It has been shown previously that the performance of a direct-sequence spread-spectrum (DS/SS) receiver which incorporates transform-domain excision of narrowband interference is improved by the use of time-weighting when the interference-to-signal power ratio (ISR) is relatively large, but is degraded by its use otherwise. The demodulator employed in this receiver is a filter which is matched to the chip sequence. While the matched filter gives minimum probability of error for additive white Gaussian noise (AWGN) in the absence of time-weighting and excision processing, the matched filter is suboptimal in their presence. Alternative demodulation rules which are adapted to time-weighting and excision processing are developed in the present paper. It is shown that, for the weighting functions considered, the matched filter requires as much as 3 dB more signal power than an adapted demodulator, to obtain a given probability of error. Furthermore, the performance with an adapted demodulator is generally superior to that for a receiver which does not use time-weighting when the ISR is at least moderately large, and is comparable otherwise. The potential benefit that may be derived from the use of an adapted demodulator in a CDMA network overlay is discussed     

Worst-case error probability of a spread-spectrum system inenergy-limited interference

We consider a communication channel corrupted by thermal noise and by an unknown and arbitrary interference of bounded energy. For this channel, we derive a simple upper bound to the worst-case error probability suffered by a direct sequence (DS) communication system with error-correction coding, pseudorandom interleaving, and a correlation receiver. This bound is exponentially tight as the block length of the error correcting code becomes large. Numerical examples are given that illustrate the dependence of the bound on the choice of error correcting code, the type of interleaving used, and the relative energy of the Gaussian noise and arbitrary interference     

Single tone interference rejection of code-phase multiplexeddirect-sequence spread-spectrum signaling

This paper presents the code-phase multiplexed direct-sequence spread spectrum signaling system, which employs parallel transmissions of several data streams modulated by different phase shifts of the same pseudo-noise (PN) code. System performance against single tone jammers is analyzed. Theoretical and simulation results are presented to show that the design increases the processing gain without reducing the data rate     

Narrow-band interference excision in spread-spectrum systems usingself-orthogonalizing transform-domain adaptive filters

Among many transform-domain interference excision techniques, transform-domain adaptive filtering has many advantages. It is based on a true optimization of some particular performance parameters such as the bit-error rate (BER). Moreover, it is insensitive to jammer frequency. However, transform-domain adaptive filtering also has the drawback of being incapable of tracking a rapidly changing interference because most adaptive algorithms require time to converge to the optimal solution. In this paper, a self-orthogonalizing transform-domain least mean square (SO-TRLMS) algorithm is used to speed up the convergence. Compared to a traditional transform-domain least mean square (TRLMS) algorithm, the SO-TRLMS algorithm can significantly improve the convergence rate of the LMS algorithm, thus making the transform-domain adaptive filtering technique more suitable for real-time processing. In order to show how the system performance is affected by various factors such as interference power and the transform used, this paper presents an analytical result for the BER performance that is applicable for arbitrary orthogonal linear transforms. Simulation results are also presented to demonstrate the validity of the analysis     

A GLRT-based spread-spectrum receiver for joint channel estimationand interference suppression

The author considers the problem of demodulating a direct-sequence (DS) spread-spectrum signal in the presence of narrowband interference and multipath. A receiver is considered that is based on the generalized likelihood-ratio test (GLRT), in which the interferer is modeled as an Nth-order circular Gaussian autoregressive (AR) process and the multipath channel is represented by a tapped-delay line. The maximum-likelihood joint estimator for the channel coefficients and interferer AR parameters is then derived. Analytical expressions for bit-error rate are presented for GLRT receiver, under the assumption of perfect estimates of the channel and interferer parameters. The performance of the GLRT receiver is compared to that of a DS receiver using a transversal equalizer. It is shown that the GLRT receiver consistently outperforms the equalizer-based receiver by 2-3 dB. The performance of an adaptive GLRT receiver is evaluated where the recursive least-squares algorithm is used to jointly estimate the interferer and channel parameters     

The performance of a direct-sequence spread-spectrum acquisition system using a linear prediction filter for narrowband interference suppression

The coarse acquisition performance of a direct sequence spread-spectrum receiver is analyzed when a linear prediction filter is used for narrowband interference suppression. We show that once an appropriate matching strategy is identified, the linear prediction filter can provide favorable performance when narrowband interference is present over a considerable range of both interference power and bandwidth. In addition, the presence of the filter dramatically improves the performance over the case where there is no filter, except when the interference bandwidth and the power are both small (i.e., when the processing gain provides sufficient interference immunity without the filter). If long spreading sequences are used with moderately sized observation windows, the acquisition performance can be severely degraded when a parallel acquisition scheme is used due to the linear predication filter. We show, however, that a slower serial receiver will provide reliable performance.This work was supported in part by the Office of Naval Research under contract ONR N00014-91-J-1234, the Army Research Office under contract ARO DAAL03-91-0071, and the NSF Center for Ultra-High Speed Circuits and Systems (ICAS).     

On a forming of cosecant square beam using a curved leakywavestructure

A simple method of forming the cosecant square beam by using the curved leakywave structure based on the periodic structure is proposed. This method is a low-cost and easy design for beam shaping and is applicable as an antenna for relatively short-range communication systems at millimeter-wave band, including various wireless indoor LANs