An anticipative adaptive array for frequency-hopping communications

To fully utilize the theoretical processing gain achievable when an adaptive array and frequency hopping are combined, frequency compensation is required. Improved versions of an anticipative adaptive array are examined that provide efficient compensation by adapting the complex weights at each antenna element to the appropriate values for a carrier frequency before that frequency is received. The underlying adaptive algorithm used is the maximum algorithm. Computer simulation results are used to compare the different versions of anticipative processing. These results show that an appropriate version ensures the rapid convergence of weights to values that provide wideband nulling of the interference and noise     

Cryptographic Digital Communication

This paper reviews the fundamentals of cryptographic digital systems and defines and discusses the two basic types of ciphers. The remainder of this article examines the word and bit error rates in digital al communication systems with block or stream ciphers. Upper bounds and ensemble averages of the error rates are obtained for both ciphers. The effect of modulation type on the degradation in communication system performance caused by the addition of cryptography is analyzed. A comparison is made between block and stream ciphers with respect to their effects on degradation.     

Frequency-hopping signal detection using partial band coverage

The performance of a channelized radiometer in detecting a frequency-hopping signal is analyzed for a variable number of parallel radiometers not necessarily covering the entire hopping band. The full band may not be covered because of an attempt to avoid interfering signals, limited radiometer resources, lack of knowledge of the band frequency location, or combinations of these factors. The analysis provides for calculation of the value of the signal-to-noise ratio (SNR) required to achieve a given probability of detection for a specified false-alarm rate, assuming an observation interval equivalent to N hops using either a fixed or a moving observation window. The dependence of the probability of detection on a misalignment of the detector observation intervals with the hop transitions is also analyzed. Numerical results are presented and discussed. Applied to a typical slow-hopping VHF radio, the results imply that a 150-hop transmission can be detected by a channelized radiometer covering half the hopping band when the SNR is about 2 dB     

The Uncertainty of Pulse Position Due to Noise

The time at which a received signal crosses a certain level fluctuates in the presence of noise. In this paper, a theoretical formula for the standard deviation of this thresholding time is obtained. The formula is applied to the detection of a pulse perturbed by Gaussian noise. Two practical detection schemes, the peak amplitude estimator and the double differentiator, are theoretically analyzed and compared. Also, a formula is derived which may be used to determine the efficacy of a false-alarm detection system.     

Word Error Rates in Cryptographic Ensembles

The word error rate of an ensemble of cryptographic systems is determined. The word error rate is specified as a function of the corresponding plain-text bit error rate. Degradation is defined and computed for the case of phase-shift keying and white Gaussian noise. Finally, the effect of differential encoding on a cryptographic system is investigated.     

Adaptive Thresholding Systems

The conditions under which adaptive thesholding should be employed are discussed. The three most important systems for accomplishing adaptive thresholding are the level adjuster, the constant-fraction discriminator, and the double differentiator. Practical implementations of these systems, including false alarm protection, are described in terms of subsystems. Approximate position variances associated with the three systems are derived. Relative advantages and disadvantages are discussed.     

Frequency Compensation in an Adaptive Antenna System for Frequency-Hopping Communications

For the full exploitation of an adaptive antenna system forwideband frequency-hopping communications, some form offrequency compensation is necessary. Parameter-dependent,spectral, and anticipative processing are the three methods offrequency compensation that are examined; their strengths,limitations, and variations are discussed. The underlying adaptivealgorithm is the Maximin algorithm, which has major advantagescompared with other algorithms.     

Statistical Theory of Passive Location Systems

A derivation of the principal algorithms and an analysis of the performance of the two most important passive location systems for stationary transmitters, hyperbolic location systems and directionfinding location systems, are presented. The concentration ellipse, the circular error probability, and the geometric dilution of precision are defined and related to the location-system and received-signal characteristics. Doppler and other passive location systems are briefly discussed.     

Arrival Time Estimation by Adaptive Thresholding

Arrival time estimation by adaptive thresholding is described. The probability density of arrival time is derived for differentiable Markov processes. The special case of additive, stationary noise is given particular attention. A direct derivation of the probability density of arrival time for pulses with sharply rising edges is given for arbitrary noise. The results are applied to the Gaussian and Rice distributions. Comparison with the Cramer-Rao bound of estimation theory indicates the asymptotic optimality of adaptive thresholding in the latter two cases.