Single Jammer Research Articles

Modeling Cascading Failures for Weighted Packet Networks

Cascading failures triggered by small local shocks under special condition exist in most of networks. Here we present a simple model of cascading failures for weighted packet networks, such as the Internet. In the model, links between nodes are chosen as the main research object, and local load redistribution is taken into account. Moreover, the influence factor, which is represented by betweenness centrality of neighboring links, is introduced to characterize the affection induced by jamming links. We study the model in both ER and BA networks using numerical simulation methods. We show that, in addition to the location of congestion, cascades of a packet network perturbed by a single jamming link also depend on the intrinsic topology characteristics and the current traffic load states of the network, which are of crucial factors. This is particularly important for real networks like the Internet to avoid the spread of traffic jam in the whole network.

Independent null steering by decoupling complex weights

In this paper, we propose a structure for independent null steering by decoupling complex weights. It has the advantage of independent steering of(N-1) nulls for a linear array having N elements and the complex weights are decoupled such that each weight corresponds to a particular null. For this purpose, the position of each zero of the array factor on the unit circle in the complex plane is controlled by the corresponding weight in the structure. It means that if a single jammer changes its position, only a single corresponding complex weight has to be recalculated, which reduces computational complexity.

Time-varying AR modeling and adaptive IIR notch filter for anti-jamming DSSS receiver

Using Time-Varying AR (TVAR) model and adaptive notch filter is a new method for the non-stationary jammer suppression in Direct Sequence Spread Spectrum (DSSS). The performance of TVAR model for Instantaneous Frequency (IF) estimation will be affected by some factors such as basis functions. Focusing on this problem, the optimal basis function of TVAR model for the IF estimation of the LFM signal is obtained in this paper. Besides the depth and width of notching, the phase properties of notch filter affect the Signal-to-Interference plus-Noise Ratio (SINR) of correlation output to the narrowband jammer suppression in DSSS, in response to the problem the closed solution of correlation output SINR improvement has been derived when a single frequency jammer passes through direct IIR notch filter, and its performance has been compared with those of five coefficient FIR filters. Later, a novel method for LFM jammer suppression based on Fourier basis TVAR model and direct IIR notch filter is proposed. The simulation results show the effectiveness of the proposed method.

Dynamics of the congestion triggered by boundary

We study the congestion on a homogeneous roadway with open boundaries. Three different phases are identified. Both numerical and analytical results are presented. For stationary boundaries, three different flow can be associated with a single density on the fundamental diagram. When the congestion is characterized by numerous jams, the system is homogeneous and the traffic flow is low. When the system is dominate by a single jam, the time trajectory traces out finite cycles and a higher traffic flow can be supported. If the cycles fill up a two-dimensional area, the traffic flow remains constant as the density changes; if the cycles collapse into a one-dimensional loop, a much higher traffic flow can be achieved. We also discuss the effects of dynamical boundaries.

Performance Analysis for Impulse Radio and Direct-Sequence Impulse Radio in Narrowband Interference

The impact of narrowband interference (NBI) on two ultra wideband (UWB) systems is analyzed. The two systems are impulse radio (IR) and a variation of it, termed direct-sequence IR (DS-IR). The signal-to-noise ratio (SNR) at the decision device of a correlation receiver is computed for both systems, assuming that the NBI is wide sense stationary and that the channel is frequency-selective. The SNR is expressed by means of a simple equation involving the signal and the interference spectrum. Next, a statistical model for the interference is introduced, considering the interference as the sum of a given number of sinusoidal signals with random powers and frequencies. The bit-error rate of IR and DS-IR is derived. The results are then specialized and compared with simulations in three case studies of practical interest, where the NBI is a single jammer with deterministic power and frequency, a multitone signal with random frequencies, or a grid of interfering signals with random powers.

On the design of the target-signal filter in adaptive beamforming

This brief deals with the improvement of jammer suppression for adaptive broadband beamforming. The analysis is carried out with a two-microphone Griffiths-Jim beamformer and a single jammer signal. By examining the derived optimal filter, a design strategy for the constant target-signal filter in the main channel of the adaptive beamformer is given. This leads to a substantial reduction in the required filter length of the adaptive filter, while maintaining the same jammer suppression. Furthermore, a faster rate of convergence can be achieved, as there are fewer filter coefficients that have to be adapted. The brief concludes with measurement results from a real-time implementation of an adaptive beamformer, showing that the proposed design method is effective, even when more than two microphones are used.

Adaptive processing at the subarray level

This paper describes a class of partially adaptive arrays with adaptive processing applied to the outputs of steered subarrays. The problem is to detect a signal or estimate its direction of arrival in the presence of jammers. The advantage of applying adaptive processing to subarrays is that it requires much less CPU time than the corresponding fully adaptive processing. The subarray processing equations for the two kinds of problem are described. In this paper, we compare partially adaptive processing performance with fully adaptive processing performance in the case of the following antenna and signal sources: • - array with regularly spaced sensors ; • - between 20 and 100 elements ; • - between 3 and 20 subarrays ; • - a single jammer ; • - desired signal from the antenna zenith. We suggest a method for determining the optimal subarray configurations in this case. An example is given to show that the performance of an antenna with five subarrays is comparable to that of a fully adaptive thirty-element array for eliminating a single jammer with a target at the zenith.

Performance of single-auxiliary-element adaptive sidelobe cancellers for multiple jammer environments

The performance is examined of a single-auxiliary-element adaptive sidelobe canceller (SLC) for multiple jammer environments. It is shown that multiple jammers can be equivalent to a single jammer for a single-auxiliary-element SLC system. This equivalence leads to an important result in that a single-auxitiary-element SLC system can reject multiple jammers. Computer simulations are given to confirm the theoretical analysis.

Evaluation of an adaptive beamforming method for hearing aids

In this paper evaluations of a two-microphone adaptive beamforming system for hearing aids are presented. The system, based on the constrained adaptive beamformer described by Griffiths and Jim [IEEE Trans. Antennas Propag. AP-30, 27-34 (1982)], adapts to preserve target signals from straight ahead and to minimize jammer signals arriving from other directions. Modifications of the basic Griffiths-Jim algorithm are proposed to alleviate problems of target cancellation and misadjustment that arise in the presence of strong target signals. The evaluations employ both computer simulations and a real-time hardware implementation and are restricted to the case of a single jammer. Performance is measured by the spectrally weighted gain in the target-to-jammer ratio in the steady state. Results show that in environments with relatively little reverberation: (1) the modifications allow good performance even with misaligned arrays and high input target-to-jammer ratios; and (2) performance is better with a broadside array with 7-cm spacing between microphones than with a 26-cm broadside or a 7-cm endfire configuration. Performance degrades in reverberant environments; at the critical distance of a room, improvement with a practical system is limited to a few dB.

Evaluation of a real‐time adaptive‐beamforming hearing aid

A real‐time two‐microphone monaural hearing aid, based on the constrained adaptive beamformer proposed by Griffiths and Jim [IEEE Trans. Antennas Propag. AP‐30, 27–34 (1982)], has been implemented and evaluated. The beamformer adapts to preserve the target signal (assumed to be straight ahead) and to minimize the power of jammer signals arriving from all other directions. The basic Griffiths‐Jim algorithm is augmented with a method to inhibit adaptation in the presence of strong target signals [Greenberg et al., J. Acoust. Soc. Am. Suppl. 1 85, S26 (1989)]. The real‐time system employs the Motorola DSP56001 and was evaluated using a speech target and a single jammer for a variety of free‐space and head‐mounted microphone configurations in anechoic and reverberant environments. The system demonstrates very good performance (30‐ to 40‐dB gain from input to output in an intelligibility‐weighted spectral average of target‐to‐jammer ratio) for several broadside microphone configurations in the anechoic environment. Adaptation inhibition provides a robust insensitivity to target misalignment at high‐input target‐to‐jammer ratios. As expected, performance degrades with increasing reverberation; at the critical distance, the system provides 5‐ to 10‐dB gain. In extreme reverberation, the system performs at least as well as a delay‐and‐sum beamformer. [Work supported by NIH.]

A planar version of a 4.0 GHz reactively steered adaptive array

-A planar (two-dimensional) reactively steered adaptive array (RESAA) including a single active microstrip element and eight closely coupled parasitic microstrip elements has been designed and tested. Steering of a null toward a single incident jammer is accomplished by adaptive control of reactive terminations on the parasitic elements using a guided random search algorithm. The planar array can steer a null in both the elevation and azimuth directions with a depth of 30 dB.

Suppression of Narrow-Band Interference in a PN Spread-Spectrum Receiver Using a CTD-Based Adaptive Filter

Analytical results have shown that adaptive filtering can be a powerful tool for the rejection of narrow-band interference in a spreadspectrum receiver. However, the complexity of adaptive filtering hardware has hindered the experimental verification of these results. This paper describes a new adaptive filter architecture for implementing the Widrow-Hoff LMS algorithm while using only two multipliers regardless of filter order. This hardware simplification is achieved through the use of a burst processing technique. A 16-tap version of this adaptive filter constructed using charge-transfer devices (CTD's) is used to suppress a single tone jammer in a direct sequence spread-spectrum receiver. Probability of error measurements demonstrating the effectiveness of the adaptive filter for suppressing the single tone jammer along with simulation results for the optimal Weiner-Hopf filter are presented and discussed.

The Steady-State Antenna Patterns of Adaptive Arrays

The antenna pattern of a receiving adaptive array of arbitrary three-dimensional geometry operating in an environment of K sources, one desired signal and (K - 1) jammers, is considered. It is shown that the adapted (voltage) antenna pattern of the array is a linear combination of K (or less) basis patterns, each of which is a function of one source only. We find that these basis patterns have a simple physical meaning, namely, the kth basis pattern is the pattern realized by the array when the transmission of source k is considered a desired signal and all other sources are turned off. When the array elements are isotropic, these basis patterns are retrodirective (that is, the mainlobe of the kth basis pattern points at source k). It had been shown that this property is also exhibited by a different decomposition of the adapted pattern in the special case of a single jammer (K = 2). In contrast, our decomposition which is simpler than the earlier one, yields retrodirective beams for all K. The simple, physically meaningful, pattern decomposition developed here is quite significant in the insight it provides regarding the basic underlying principles of adaptive arrays. It is also instrumental in elucidating their capabilities and limitations.

Canceler Performance Degradation Due to Estimation Noise

The multiple sidelobe canceler (MSLC) configured using the gradient descent algorithm converges to an approximation to the optimum weight vector. Since the weight vector estimate is obtained from a finite data set, there is a random error that increases the steady state residue power. An exact analysis of the steady state residue power for a digital implementation is performed. The analysis predicts that as the algorithm's gain (step size) is increased, the steady state residue power tends towards infinity. The gain that causes infinite power is smaller than the gain that causes instability for the average weight vector analysis. It is shown that the steady state residue power is equal to the minimum residue power multiplied by a factor that depends upon ratios of the eigenvalues. The steady state residue power is evaluated for some special cases. It is shown that the value of the factor is greatest when the interference is due to a single jammer.

Anti-Intercept Margins of Relay-Augmented Data Links

A quantitative measure of the anti-intercept improvement (or margin) of a relay data net is described. It is assumed that the link relays are located to obtain the maximum anti-jam advantage against a line-of-sight jammer. The anti-intercept margin is defined as the ratio of the respective spatial power densities available to the intercept receiver when no relay and relays are used, and can be as large as 10-20 dB. It is shown that the relay anti-intercept (AI) margin for a spatially uniform jamming field intensity is lower than for a single jammer environment, and requires the same form of relay spacing needed to maximize the AI margin in the absence of jamming. It is also shown that operation in a lossy medium offers an opportunity for enhancing anti-intercept capability. A measure of overall AI performance that illustrates this enhancement is discussed.

13—THE BIAXIAL LOAD-EXTENSION BEHAVIOUR OF A MODEL OF PLAIN WEFT-KNITTING—PART I

An analysis is presented of a model of plain weft-knitting subjected to uniaxial loading in either direction and also to biaxial loading. For each case, calculated load–extension curves, which are given for a range of values of fabric tightness, illustrate biaxial fabric deformation. Each curve shows a marked dependence of fabric modulus on the prevailing jamming conditions. In particular, where jamming in the direction of the application of the load is present, the relaxed fabric behaves as a pre-stressed structure. Whereas uniaxial loading leads ultimately to a state of single jamming with near-constant moduli, biaxial loading may lead to a more complex situation, which is outlined in the paper.

Single Sidelobe Canceller: Theory and Evaluation

A sidelobe canceller (SLC) can be efficiently employed to reduce the undesirable effects of jammers on a search radar system. In this paper a functional block circuit is described for a single-sidelobe canceller (SSLC) apt to a single Jammer suppression; a detailed analysis is also presented. It is shown that the theory of stochastic differential equations and the covariance analysis describing function (CADET) technique can be used to evaluate the SSLC performances when two Gaussian colored and mutually correlated input processes are considered. The statistical properties of the weight can be obtained. The last results, which are very important in the case of nonstationary jammers, cannot be obtained without this theory. System implications can be drawn from the abovementioned analysis when the input process consists of one jammer or two jammers and/or thermal noise.