爆炸物检测中微弱NQR信号的处理

介绍了用核四极矩共振技术检测爆炸物品的原理及检测过程中信号的处理方法。根据NQR测量中接收信号极其微弱的特点,针对以数字累加技术处理NQR信号的常规方法,提出了利用相关检测的方法来处理核四极矩共振（NQR）信号,该方法可以更有效地提高信噪比,并消除随机噪声和外界干扰对接收信号的干扰,从而提高了检测微弱NQR信号的准确性。     

New insights into optimal widely linear array receivers for the demodulation of BPSK, MSK, and GMSK signals corrupted by noncircular interferences-application to SAIC

For nonstationary observations, potentially second-order (SO) noncircular, the SO optimal complex filters are time variant and, under some conditions of noncircularity, widely linear (WL). For more than a decade, there has been an increasing interest in optimal WL filters in radiocommunications contexts involving rectilinear signals such as binary phase-shift keying (BPSK) signals. In particular, it has been pointed out that single antenna interference cancellation (SAIC) may be performed by such filters in the context of BPSK cellular networks. Recently, it has been shown that, by a simple algebraic operation of demodulation on the baseband signal, the minimum shift keying (MSK) and Gaussian MSK (GMSK) modulations can be made to approximately correspond to a BPSK modulation, allowing the application of the SAIC concept to the GSM cellular network at the mobile level, being currently studied for standardization, and offering significant improvements of the network's capacity. Despite the increasing interest in optimal WL filters in rectilinear or quasi-rectilinear contexts, many questions about their behavior and their performance have still arisen. The purpose of this paper is to gain insight into the behavior, properties, and performance of optimal WL array receivers, and thus of the SAIC technology, for the demodulation of BPSK, MSK, and GMSK signals corrupted by noncircular interferences.     

Estimation-correlation-compensation signal processing in the presence of interferences: A review

One of the directions of signal processing in the presence of interferences that involves the estimation-correlation (EC) reception technique and the interference compensation is reviewed. Optimal and quasioptimal estimation-correlation-compensation (ECC) algorithms for the detection of arbitrary signals in the presence of arbitrary interferences and noise with independent realizations are analyzed. It is demonstrated that the ECC method leads to the estimation-compensation algorithms in the case of estimating desired signals and to the compensation-correlation algorithms in the case of detecting deterministic or weak random signals. The features of the EC and ECC algorithms and the corresponding estimates are discussed for the cases of continuous and discrete observations. A comparative analysis of the structures of the optimal and quasi-optimal ECC detectors is presented. Several practical problems regarding signal processing and interference compensation that are solved via the EC and ECC reception techniques are analyzed.     

CDMA downlink interference suppression using I/Q projection

Interference suppression at the receiver can be used to improve performance and capacity in the downlink of direct-sequence code-division multiple-access systems. In this paper, an interference suppression technique is developed which uses partial knowledge of spreading sequences to cancel a group of interfering signals. Specifically, knowledge of a complex scrambling sequence is used to project the desired signal away from the interference in the in-phase/quadrature (I/Q) complex plane. A maximum likelihood receiver formulation is used, treating both the desired signal and interference as being conditionally noncircular. A zero-forcing equalization approach is used to preserve orthogonality between signals from the same base station. The development of the technique is based on the assumption that the group of signals corresponds to another base station that does not transmit the signal of interest. However, the technique is also applied to the path diversity scenario (dispersion, soft handoff, transmit delay diversity), in which the group of interfering signals includes a desired signal component. The approach, referred to as I/Q projection, provides significant gains when performance is interference limited.     

Multichannel post-filtering in nonstationary noise environments

In this paper, we present a multichannel post-filtering approach for minimizing the log-spectral amplitude distortion in nonstationary noise environments. The beamformer is realistically assumed to have a steering error, a blocking matrix that is unable to block all of the desired signal components, and a noise canceller that is adapted to the pseudo-stationary noise but not modified during transient interferences. A mild assumption is made that a desired signal component is stronger at the beamformer output than at any reference noise signal, and a noise component is strongest at one of the reference signals. The ratio between the transient power at the beamformer output and the transient power at the reference noise signals is used to indicate whether such a transient is desired or interfering. Based on a Gaussian statistical model and combined with an appropriate spectral enhancement technique, we derive estimators for the signal presence probability, the noise power spectral density, and the clean signal. The proposed method is tested in various nonstationary noise environments. Compared with single-channel post-filtering, a significantly reduced level of nonstationary noise is achieved without further distorting the desired signal components.     

Widely Linear MVDR Beamformers for the Reception of an Unknown Signal Corrupted by Noncircular Interferences

For nonstationary observations, potentially second-order (SO) noncircular, the SO optimal complex filters are time variant (TV) and, under some conditions of noncircularity, widely linear (WL). Moreover, for applications such as spectrum monitoring or passive listening, the sources' waveforms are unknown and no training sequence or spreading code is a priori available. In this context, this paper aims at introducing the time invariant (TI) WL minimum variance distortionless response (MVDR) beamformer for the optimal reception of an unknown signal, whose waveform is unknown but whose steering vector is known, corrupted by potentially noncircular interferences. Its properties, performance, and adaptive implementation in noncircular contexts are analyzed in this paper. This optimal beamformer is shown to always improve, in the steady state, the performance of the well-known Capon's beamformer for noncircular interferences. Moreover, it should be noticed that this optimal beamformer allows the processing of up to 2 (N-1) rectilinear interferences from an array of N sensors. Finally, at the end of this paper, a TV extension of this TI WL MVDR beamformer is presented to process SO noncircular interferences having a nonnull carrier residue or frequency offset.     

Effects of Rician faded and log-normal shadowed signals on spectrumefficiency in microcellular radio

An assessment of spectrum efficiency for a microcellular land mobile radio system is presented by considering the desired signal as (fast) Rician fading with (slow) log-normal shadowing and cochannel interfering signals as uncorrelated (fast) Rayleigh fading superimposed over (slow) log-normal shadowing. Spectrum efficiency is defined in terms of reuse distance, i.e., cluster size, traffic intensity, bandwidth of the system, and area of a cell by considering cochannel interference probability. The expression for cochannel interference probability is derived using appropriate path-loss law for microcells for four different cases: Rician plus log-normal desired signal and Rayleigh plus log-normal interfering signals; Rician desired signal and Rayleigh fading plus log-normal shadowing interfering signals; Rician desired signal and Rayleigh interfering signals; and both desired and interfering signals as Rician fading. The performance of a microcellular system is compared with that of a conventional macrocellular system     

Adaptive time-varying cancellation of wideband interferences inspread-spectrum communications based on time-frequencydistributions

The paper proposes an adaptive method for suppressing wideband interferences in spread-spectrum (SS) communications. The proposed method is based on the time-frequency representation of the received signal from which the parameters of an adaptive time-varying interference excision filter are estimated. The approach is based on the generalized Wigner-Hough transform as an effective way to estimate the instantaneous frequency of parametric signals embedded in noise. The performance of the proposed approach is evaluated in the presence of linear and sinusoidal FM interferences plus white Gaussian noise in terms of the SNR improvement factor and bit error rate (BER)     

Impact of TH‐UWB interference on MB‐OFDM UWB systems: interference modeling and performance analysis

In this paper, the coexistence issue between multiband‐orthogonal frequency‐division multiplexing (MB‐OFDM) and time‐hopping ultra‐wideband (TH‐UWB) networks is widely analyzed. For this purpose, to study and model the TH‐UWB interference, an analytical framework which describes key features of the interference distribution is provided. The interference distribution is studied in the context of TH‐UWB's signaling parameters. Our results reveal that the interference distribution highly depends on its time‐hopping parameters. Therefore, choosing proper time‐hopping parameters leads to less destructive interferences. The Generalized Gaussian and the Symmetric‐α‐Stable (SαS) distributions are used to model the interference‐plus‐noise signal. The maximum likelihood and a characteristic function‐based regression‐type methods are adopted to estimate parameters of Generalized Gaussian and SαS distributions, respectively. Moreover, the interference channel effects on the impulsive behavior of the TH‐UWB signal is studied. It is shown that impulsive behavior of the faded interference signals highly depends on the channel time‐dispersiveness. Furthermore, an exact performance of a multiband‐orthogonal frequency‐division multiplexing system impaired by a TH‐UWB system is derived. The comparison of the analytical performance, the empirical simulation, and the approximation results show that both approximation methods are valid for low interference‐to‐noise‐ratio, while SαS provides a more accurate approximation for high interference‐to‐noise‐ratio. Copyright © 2015 John Wiley & Sons, Ltd.     

A subspace-based direction finding algorithm using fractional lowerorder statistics

We propose several classes of fractional lower order moment (FLOM)-based matrices that can be used with MUSIC to estimate the DOAs of independent circular signals embedded in additive SαS (symmetric α stable) noise (e.g., sea clutter). We run simulations with different choices of the FLOM parameter p for our FLOM-based matrices and conclude that when the noise is SαS with unknown α≠2, FLOM-multiple signal classification (MUSIC) with p close to unity yields good performance. The performance of FLOM-MUSIC and robust covariation-based (ROC)-MUSIC are similar. Three scenarios that contain circular signals (phase modulation (PM), circularly symmetrical Gaussian, and quaternary phase-shift keying (QPSK)) and one scenario that contains noncircular signals (binary phase-shift keying (BPSK)), all embedded in the same SαS noise, are tested. These simulation results reveal that the scenario containing BPSK signals leads to poor performance, indicating that FLOM-MUSIC is presently limited to circular signals     

Digitized speech transmission at VHF using existing FM mobile radios

An extensive study of digitized speech transmission over existing VHF FM mobile radio sets is presented. Objective criteria expressed in terms of bit error rate (BER) are determined for the performance evaluation of analog FM radios when transmitting digital voice. These criteria are used to define the technical characteristics such as the receiver sensitivity, co-channel rejection, and adjacent channel selectivity in the case of digital FM transmission. The concept of performance measurements of existing sets in the digital mode of operation is suggested. The results of performance measurements reported here include the following: 1) radiated FM spectra, 2) bit error rate curves, 3) receiver sensitivity, 4) BER performance in the presence of analog or digital FM interference on the same or adjacent channel, 5) signal plus noise plus distortion to noise plus distortion ratio (SINAD) performance in the presence of co-channel or adjacent channel interference for both types of interfering signals, 6) co-channel rejection and adjacent channel selectivity for various combinations of the wanted and interfering signals, 7) receiver selectivity for both modes of operation, and 8) adjacent-signal selectivity for various combinations of the desired and undesired signals. Optimum values of design parameters of digital voice radio system are deduced from the obtained results. Finally, the implications of digitized speech transmission on the technical characteristics, operating range, channel reuse distance, and adjacent channel interference performance of analog FM radios are discussed.     

Incoherent Method of Optical Interference Cancellation for Radio-Frequency Communications

In this paper, we describe a system for cancelling radio-frequency (RF) interference using optical techniques. Specifically, we attempt to receive a weak RF signal—which we assume to be of the order of microwatts—in the presence of high-power local RF interference. This local interference is a signal whose power is of the order of 100 W and is generated in close proximity to the receiver. We wish to emphasize that the nature of the interfering signal is completely known to us in practice, since we are generating it for communications purposes. This knowledge of the interfering signal will prove to be useful in our attempts to cancel it, as will be shown. We refer to this technique as optical interference cancellation, or opto-cancellation. We have demonstrated that this opto-cancellation system can cancel a simple sinusoid at 3 GHz, as well as broadband interference of approximately 100-MHz bandwidth centered at 3 GHz. We have also demonstrated cancellation of sinusoids and broadband signals at other center frequencies as well. In the case of sinusoidal signals, we have demonstrated optical cancellation over 70 dB; and in the case of the $sim$ 100-MHz signal, we have demonstrated optical cancellation over 30 dB.      

SMI adaptive antenna arrays for weak interfering signals

The performance of adaptive antenna arrays in the presence of weak interfering signals (below thermal noise) is studied. It is shown that conventional adaptive antenna arrays sample matrix inversion (SMI) algorithm is used are unable to suppress such interfering signals. To overcome this problem, the SMI algorithm is modified. In the modified algorithm, the covariance matrix is redefined such that the effect of thermal noise on the weights of adaptive arrays is reduced. Thus, the weights are dictated by relatively weak signals. It is shown that the modified algorithm provides the desired interference protection.     

Adaptive antenna arrays for weak interfering signals

The performance of adaptive antenna arrays in the presence of weak interfering signals (below noise level) is studied. It is shown that conventional adaptive arrays are unable to suppress such interfering signals. To overcome this problem, the feedback loops controlling the array weights are modified. In the modified feedback loops, the noise level in the feedback loops is reduced by reducing the correlation between the noise components of the two inputs to the loop correlator. Two techniques to decorrelate these noise components are discussed. It is shown that adaptive arrays with the modified feedback loops provide the desired interference suppression. An expression is given for the amount of noise decorrelation required to achieve a specified interference suppression.     

A New Approach for Estimating the Number of Sources Under the Coexistence of Circular and Various Noncircular Sources

Estimation of source number is a fundamental problem of direction-of-arrival (DOA) estimation. In the problem of DOA estimation under the coexistence of circular and various noncircular signals, the source number should be estimated in order to distinguish the signal subspace from the noise subspace. Thus, a new method for source number estimation is proposed in this paper. Using the approach of k-means clustering, the projections of a one-dimensional reduced covariance matrix are divided into two categories. Then the signal subspace and the noise subspace are separated by the optimal classification boundary of those two categories so as to obtain the equivalent source number. Simulation results show that the proposed method has relatively better performance even in low SNR or in a colored noise environment.     

随机共振瓦斯微弱信号检测方法研究

为了解决检测煤矿复杂环境中的瓦斯信号时易受周围噪声干扰以至微弱信号被掩埋或产生异常数据的问题,提出一种基于随机共振的微弱瓦斯信号检测方法。采用欠采样原理对大频率信号尺度变换及粒子群算法优化系统结构参量,对大参量微弱信号在随机共振系统中的共振效果进行了理论分析和研究。结果表明,该方法可以以较低的采样频率,自适应地达到较好的共振效果;可有效地滤除噪声并增强系统辨识微弱信号的灵敏度以及信号检测的动态范围。该研究为瓦斯突出信息的早期辨识提供了一定的理论依据。     

ALTBOC调制信号抗干扰性能研究

由于环境中存在诸多电磁辐射干扰,因此如何有效评估全球卫星导航系统（GNSS）信号的抗干扰性能是一个亟待解决的问题。首先,本文利用干扰误差包络曲线对交替二进制偏移载波（ALTBOC）信号进行了在单音干扰和白噪声干扰两种情况下的抗干扰仿真分析,并和其他调制信号的抗干扰性能进行对比,仿真结果可为后续的信号体制研究提供参考。     

Co-channel interference mitigation in the time-scale domain: theCIMTS algorithm

In many communication systems, the problem of co-channel interference is encountered when along with the signal of interest (SOI), and one or more interfering signals are present in a common receiver. The SOI and the interference are correlated, possess similar characteristics and power, and share the same region of support both in the time and frequency domains. In this paper, we present co-channel interference mitigation in the time-scale domain (CIMTS) algorithm (for MPSK signals) which estimates the SOI and the interfering signal from their superposition in the presence of additive noise. This method is inspired by the reconstruction of interference from the null space of the SOI in the time-scale domain. Once the null space of the SOI is determined, the interfering signal is reconstructed via a set of linear operations. Therefore, the SOI is estimated by a simple subtraction of the estimated interference from the observations     

Blind wideband spatio-temporal filtering based on higher-order cyclostationarity properties

In this paper, a new method for blind minimum mean-square error (MMSE) spatio-temporal signal filtering is proposed. To obtain a blind version of the MMSE filter, the proposed method utilizes the signal selectivity property of the higher order cyclostationary statistics (HOCS) exhibited by the signal of interest. An accurate estimate of the transfer function of the MMSE filter is obtained also in the presence of strong frequency-overlapped interfering signals, which would render standard stationarity-based techniques ineffective. The method is useful when the desired signal cannot be accurately extracted by exploiting its second-order cyclostationarity properties because they are too weak and/or they are shared by the interference and noise signals as well. The performance analysis, which is carried out by computer simulations in the case of quadrature amplitude modulated (QAM)-modulated signals with partially overlapping bands, substantiates the effectiveness of the proposed method.     

用于MIMO单载波频域均衡的块BLAST算法

单载波频域均衡(SC-FDE)是多输入多输出(MIMO)无线通信中克服多径衰落的一种重要技术。本文基于V-BLAST信号检测的原理,对MIMO-SC-FDE系统提出了以块为单位进行分层检测的块BLAST算法。在该算法中,系统按一定的准则确定对各路发送信号进行频域均衡和检测的顺序,较先检测出的信号被变换到频域作为干扰从接收信号中去除,使得对以后各路信号进行频域均衡和检测的信干噪比得以提高,从而大大改善了MIMO-SC-FDE系统的误码性能。仿真实验结果显示,块BLAST算法取得了大大优越于MIMO-SC-FDE常规检测算法的误码性能。