Use of the Tapped Delay Line Adaptive Array to Increase the Number of Degrees of Freedom for Interference Suppression

A conjecture is made that a tapped delay line adaptive array of K elements can be used to suppress more than K - 1 noncoincidentcenter frequency interferers when the interferers operate within afraction of the signal bandwidth. This is possible because of theavailability in the increased number of degrees of freedom. Asimulation study is made verifying this conjecture for the case of atwo-element array and two interferers. Three and five tap sectionsare used in the simulation. The study indicates that the performanceimproves with increasing number of taps.     

The Effects of Gaussian Interference on Communication Systemswith Adaptive Arrays

The performance of a bandlimited binary phase-shift-keyed (BPSK) communication system is examined when the received BPSK signal is corrupted by both thermal noise and a directional Gaussian noise interfering signal. The system uses an LMS adaptive array to suppress this interference. The effects of signal power levels, arrival angles, bandwidths, and the array bandwidth are examined. The performance of a system that uses tapped delay lines for the array weights is also examined. It is shown that the performance of a system with tapped delay lines is not affected by the interference bandwidth for a single interferer.     

A New Sampling Procedure for the Synthesis of Linear Transversal Filters

In linear transversal filters for signal processing, the impulse response of the filter is syntheszed in a sampled form by means of a tapped delay line with weighted and summed taps. The synthesis procedure consists of sampling the specified impulse response at appropriate intervals and using the sample values as the tap weights. A nonuniform sampling procedure for the synthesis of the impulse response is introduced, which can produce results more conveniently than the classical Shannon sampling procedure because of the simplicity of the transversal filter implementation. According to the new procedure, in the time domain the samples are taken at points where the instantaneous phase of the impulse response is a multiple of ir radians. The spectral distribution which ensues in the frequency domain consists of a series of spectra all having the same fractional bandwidth. The transversal filters synthesized by this sampling criterion are referred to as phase-sampled impulse response (PSIR) filters. An important application of PSI R filters is to the generation and matched filtering of a large class of AM and FM waveforms.     

Adaptive Array Bandwidth with Tapped Delay-Line Processing

The bandwidth of adaptive arrays with tapped delay lines behind the elements is examined. Such processing offers improved bandw over that attainable with quadrature hybrid processing. The performance of a two-element array with four types of processing (equarature hybrids, single delay lines, 3-tap delay lines, and 5-tap delay lines) is compared. It is shown that with half-wavelength element spacing, a quadrature hybrid and single delay-line processor are inadequate at 10-percent bandwidth. A 3-tap processor is adeq however, up to 40-percent bandwidth.     

Wideband Interference Cancellation in Adaptive Sidelobe Cancellers

This paper presents numerical results from a study of sidelobe cancellers operating against wideband interfering signals. The study was designed to provide information that would be useful to system designers. Among other things, we address the question of whether it is best to enhance the bandwidth capability of the canceller by using tapped delay lines or simply by using more auxiliary elements. We also develop guidance rules to help the designer determine the number of adaptive loops required for a given environment.     

Cancellation of specular and diffuse jammer multipath using ahybrid adaptive array

It has been demonstrated that specular or diffuse jammer multipath can be canceled to a desired level by using an adaptive array that combines bandwidth partitioning with tapped delay lines. Such hybrid systems are studied. In particular, the author studies an ideal two-element array that uses bandwidth partitioning in both the main and auxiliary channels, with an Mth-order adaptive finite impulse response filter in each subband of the auxiliary. The ability of this system to cancel specular moderately diffuse and diffuse multipath is studied. The combinations of bandwidth partitioning and filter order that can achieve a specified jammer cancellation level are discussed     

Effects of Random Implementation Errors in Matched Filters for Binary Phase Code Pulse Signals

The effects of implementation errors in tapped delay line filters on the peak-to-sidelobe ratio and the signal-to-noise ratio of binary PSK pulse compression codes are considered. As error model the delay elements and weighting factors of such filters are assumed to have an error distribution with known mean and variance.     

Design of MTI Radar on the Basis of Detection Probability

A single (quadrature) channel moving target indicator (MTI) radar system employing a tapped delay line filter is analyzed. The point of view taken is that of optimal clutter rejection in conjunction with subsequent receiver decision operations. The random nature of the spread of target Doppler shifts is taken into account. Based on the above, a procedure is presented by means of which the detection probability can be numerically evaluated for an optimized filter frequency response.     

Detection Performance of the Cell Averaging LOG/CFAR Receiver

The cell averaging LOG/CFAR receiver is a special implementation of a constant-false-alarm-rate (CFAR) receiver in which the noise level estimate is derived from a set of contiguous time samples of the output of a logarithmic (LOG) detector as obtained from a tapped delay line. This CFAR receiver is capable of operating over a larger dynamic range of noise levels than a conventional cell averaging CFAR receiver, but with somewhat poorer detectability. The performance in stationary Gaussian noise of the cell averaging LOG/CFAR receiver with no post-detection integration is determined in this paper. For a small number of reference noise samples, results were obtained by a Monte Carlo simulation using the technique of importance sampling. For a large number of reference noise samples, a second moment analysis gave the desired results. Both these results can be summarized in the following simple formula, NLOG = 1.65NLIN - 0.65, which relates the number of reference samples required by each of the two receivers for equivalent performance. Thus, for the cell averaging LOG/CFAR receiver to give the same detection performance as the conventional cell averaging CFAR receiver, the number of reference noise samples has to be increased by up to 65 percent.     

Quasi-Impulse Generator for Chirp Radar

A technique for generation of a large spectral bandwidth wave form is described which, when utilized as a dispersive delay line excitation signal, provides an efficient means for generation of large-percentage-bandwidth linear FM radar transmit pulses.     

A noise insensitive solution to an ambiguity problem in spectralestimation

The instantaneous frequency measurement (IFM) receiver is capable of measuring the center frequency of single frequency pulses over a wide range (bandwidth) of center frequencies. Because of various constraints, the frequency resolution requirement results in long correlator delay times that reduce the single correlator bandwidth. A large bandwidth can be achieved only if two or more correlators are used. The problem of estimating frequency is then reduced to the simultaneous congruence problem of number theory. A design procedure is presented for solving the congruence problem for a given amount of noise protection, a stated frequency resolution, a minimum bandwidth, and a fixed level of precision (bits) in the IFM receiver     

Range Sidelobe Suppression for Barker Codes

The utility of Barker-type phase-reversal codes is extended by the use of sidelobe suppression techniques that can be easily implemented in digital form. It is shown that sidelobe suppression techniques can be found where the tapped delay line used to reduce the sidelobes has only a few distinct tap weights, in which case the complexity of the digital processor is greatly reduced. An example is given where the technique is applied to Barker codes with positive sidelobes, specifically, the 13-element Barker code. If higher pulse compression factors are desired than are obtainable with Barker codes, multistage Barker codes may be used. The sidelobes then may be suppressed for any one or all of the different coding stages.     

The effects of IF bandpass mismatch errors on adaptive cancellation

The effects of IF bandpass mismatch errors on adaptive cancellers are investigated. Frequency mismatch errors occur because of errors in the synthesis process of the bandpass filters which are designed to be identical and are in each input channel. Tapped-delay line transversal filters can be used to compensate for these frequency mismatches and thus improve cancellation performance. A pole/zero error model of the filters is developed whereby closed-form solutions of the maximum achievable average cancellation are obtained. This cancellation is a function of the order of the ideally matched frequency filters, the number of time-delay taps in the compensating transversal filter, the bandwidth-tapped time-delay product, and the constraints on these parameters. A design procedure is outlined for optimizing the canceller with respect to these parameters and their constraints; specifically, results are presented for Butterworth-type input filters. It is shown that an arbitrarily low output noise residue cannot be achieved by arbitrarily increasing the number of time-delay taps     

A sampling-based approach to wideband interference cancellation

Classical adaptive array schemes which use only complex spatial weights are inherently narrowband and consequently perform poorly when attempting to suppress wideband interference. The common solution to this problem is the use of tapped delay line filters in each spatial channel to facilitate space-time adaptive processing (STAP). The higher performance provided by the STAP architecture comes at the cost of a considerable increase in complexity. This paper presents a simpler technique based on programmable time adjustable sampling (TAS) that provides a limited number of wideband degrees of freedom. Two TAS methods are introduced: TAS-sidelobe canceler (TAS-SLC) is based on the sidelobe canceler, while TAS-minimum variance beamformer (TAS-MVB) is derived from the minimum variance beamformer. TAS is implemented by adjusting the sampling instant at selected array channels. TAS-SLC consists of controlling the sampling in the main channel of the sidelobe canceler With TAS-MVB array complex weights are substituted with TAS time delays. The performance of TAS methods with wideband interference is compared to the conventional sidelobe canceler and minimum variance beamformers. It is shown that TAS-SLC provides better performance than the sidelobe canceler, while TAS-MVB outperforms the minimum variance beamformer     

Specification and Performance of Passive Sonar Spectral Estimators

The application of existing estimation theory to the problem of specification and performance of passive sonar spectral estimators is considered. The classification function is addressed, so that the signal is assumed to be present, and so that the energy arrival angle is known. The spatial filter considered is a line array of M equally spaced omnidirectional hydrophones. Signal and ambient noise are both zero-mean, wide-sense, stationary Gaussian random processes that differ in their spatial correlation across the face of the array. The signal is a plane wave that can be made totally spacially corrected between array elements by inserting delays between sensors to invert the signal propagation delay. The noise correlation is a function of frequency, bandwidth, element separation, and the relative time delay between sensors. Under these assumptions, the Cramer-Rao lower bound is derived for the class of unbiased estimates of signal power in a narrow frequency band at the hydrophone in the presence of correlated ambient noise of known power. The bound is examined numerically, resulting in a threshold phenomenon with M that constitutes a new design consideration. In addition, there is a striking insensitivity to realistic values of ambient noise correlation, and there are ranges in signal-to-noise ratio for which one gains more by increasing M than by increasing the bandwidth-time product. Specific processors, including a new unbiased estimator when noise power is unknown, are developed.     

频率不变宽带波束形成权重系数的稀疏优化

在基本的傅里叶变换频率不变波束形成(FIB)基础上,从减少抽头权重系数数量,降低FIB运算量角度出发,提出基于最小l0范数的抽头权重系数稀疏优化模型,并采用正交匹配追踪(OMP)算法来求解该优化问题。在高增益、等波纹、低旁瓣FIB方向图的要求下,所提出的优化方法使得稀疏率降低到3.53%,且能够保证稀疏优化后的方向图误差小于1%。接着进一步开展阵元数量的稀疏优化,有效地减少了阵元通道数,进一步降低了算法实现的硬件复杂度。仿真结果验证了所提方法的正确性和有效性。     

Electronically Variable Time-Delay Network for Broad-Band Phased-Array Steering

This paper describes an electronically variable time-elay network that has been developed specffically for time-delay steering of a phased-array antenna. The network consists of microwave dispersive delay lines and two broad-band mixers; its delay is a function of the mixers' local oscillator frequency. The particularly advantageous feature of this network is that its phase is invariant with delay. It can, therefore, be used on a subarray basis, and at IF, without affecting the phase progression across the array. Alternative techniques, such as electron-beam delay lines and digitally switched line lengths, require external phase correction. A breadboard model of the network has been built and tested. Low-loss folded-tape meader lines provide the dispersive delay. The center frequency of the model is 3 GHz; it handles an instantaneous bandwidth of 300 MHz. Delay variation of 25 ns is provided. The spurious responses of the network are 30 dB down.     

性能保证条件下航空电子高速交换机的加速方法

 以保证航空电子互连网络的确定性延迟和高带宽为目的,建立航空电子交换式以太网（AFDX）承载于波分复用（WDM）之上的架构,兼容AFDX的延迟确定性和WDM的带宽可扩性,满足航空电子互连网络对实时性和高带宽的要求。通过对实时通信中周期性数据帧的分析,提出光波聚合加速方法,对输入端到达的数据帧进行重新分配和整型,充分利用每条光波的带宽资源,减少交换使用的波长数。在性能保证条件（100%通过率下保证有界延迟）下,对使用光波聚合方法的交换机制进行了数学推导。最后通过计算机仿真,以网络延迟时间率和加速因子为性能衡量指标推证了该加速方法的优越性和可行性。     

Stabilized Linear FM Generator

A technique for generating high-linearity chirp (linear FM) signals is presented. It uses a delay line and mixer to sense the slope of the output of a swept oscillator, and the difference from desired slope is fed back to correct the oscillator frequency. The technique also improves the coherence of the oscillator. An analysis is performed of the amount of linearization that can be achieved for three possible loop configurations. Experimental results are presented of tests run on a backward wave oscillator (BWO) operating over a bandwidth of 240 MHz with time duration of 120 ?s. The results indicate a linearity corresponding to a phase error (from parabolic) of 30 degrees (peak) and coherence within 5 degrees.     

On Optimizing Array Reception of Multipath

This paper reviews system configuration requirements and analyzes detectability performance characteristics for maximum likelihood array reception of multipath. Performance is analyzed to determine the effects of channel multipath structure (multipath delay and signal power division among the paths), space-time correlation properties of the incident processes, and the array spacing. It is shown by a series of case studies, that for single element coupling, as well as array coupling, an increased multipath delay factor results in decreased system detectability for fixed signal and noise intensity levels. The performance capacity is degraded as the available signal power tends to distribute more uniformly between the paths. These effects are attributed to the loss of effective signal energy concentration, resulting in a lower effective pre-detection signal-to-noise ratio. An investigation of the effects upon system performance, due to array element spacing, shows that performance is enhanced by increasing the spacing relative to the multipath delay factor and the reciprocal signal bandwidth. The former is the result of a more directive detectability (beam) pattern arising from the increased spacing. In effect, with increased spacing, the main lobe of the pattern is narrowed, while the side lobes are optimally suppressed by the required noise related array element link, frequency filters (weights).