Fast Frequency Hopping Research Articles

Error probabilities of fast frequency-hopped MFSK with noise-normalization combining in a fading channel with partial-band interference

An error probability analysis performed for an M-ary orthogonal frequency-shift keying (MFSK) communication system employing fast frequency-hopped (FFH) spread-spectrum waveforms transmitted over a frequency-nonselective, slowly Rician fading channel with partial band interference is discussed. Diversity is obtained using multiple hops per data bit. Noise-normalization combining is employed by the system receiver to minimize partial-band interference effects. The partial-band interference is modeled as a Gaussian process. Thermal noise is also included in the analysis. Forward error correction coding is applied using convolutional codes and Reed-Solomon codes. Diversity is found to dramatically reduce the degradation of the noise-normalization receiver caused by partial-band interference regardless of the strength of the direct signal component. Diversity offers significant performance improvement when channel fading is strong, and performance improvement is obtained for high modulation orders (M>2). Receiver performance is improved when diversity, higher modulation orders, and coding are combined. >

Interference rejection in FFH systems using least squares estimation techniques

A fast frequency-hopped (FFH) receiver which uses a prewhitening filter to reject narrowband interference is described. By using an appropriate fractional tap spacing, it is shown that interference can be estimated independently of the desired signal. Bit error rate results are presented for the receiver for linear square-law combining. The results compare favorably to those obtained by a near-optimal automatic gain-control (AGC) combining technique. The performance of the prewhitening filter interference rejection method is shown to be superior to that of a nonparametric self-normalizing receiver. Finally, simulation results for an FFH receiver using the complex least mean-square (LMS) algorithm to update the prewhitening filter coefficients are presented.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>

On the performance analysis of asynchronous FH-SSMA communications

A method for calculating the asynchronous frequency-hopping (FH) spread-spectrum multiple-access performance is presented. The method is based on a Gauss quadrature rule, and for this purpose a moment-generation algorithm of the random variables involved is introduced. False alarm and miss probabilities in a basic FH search acquisition configuration and bit error rate in both slow and fast FH binary frequency-shift keying are obtained. Different power levels of the multiuser interference, user activity modeled as a Poisson point process, and nonfading as well as nonselective Rayleigh fading channels are considered. >

Antijam Performance of Fast Frequency-Hopped M-ary NCFSK--An Overview

The antijam performance of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> -ary noncoherent FSK with fast frequency hopping (one or more hops per symbol) is studied. A summary of terminology, definitions, and an overview of the literature is given. A general analysis method based upon circularly symmetric signal theory is presented. It provides a means for exact analysis of performance for <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">M</tex> -ary operation with any combination of signal tone distribution (frequency bins contiguously or noncontiguously spaced), jamming distribution, jamming form (partial-band noise or multitone), and receiver system noise. Some previous results are derived as subcases of the general theory. A simple robust diversity combining technique of transmitting a symbol on <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">L</tex> hops followed by a majority vote decision is found to provide significant performance improvements. This improvement is comparable to that obtained by most error-correction coding methods. Illustrative calculations of performance were chosen to indicate trends rather than to be exhaustive. Performance is usually presented as probability of bit error versus fraction of the band jammed to emphasize worst-case operation.

Programmable Frequency-Hop Synthesizers Based on Chirp Mixing

Frequency-hopped communication equipment require synthesizers capable of providing a large number (N) of discrete frequencies over a wide band. In typical systems N lies in the range of 50 to 10000 and the required bandwidth ranges from 10 to 500 MHz. One technique for implementing a synthesizer is based on mixing chirp signals generated by impulsing SAW filters. Potentially, this method allows fast frequency hop generation over wide bandwidth (<500 MHz) with large numbers of selectable hop frequencies (N< 4000). Furthermore, the hardware can occupy a small volume and dissipate low power compared with conventional synthesizers. This paper examines the techniques and establishes likely parameter and performance bounds. Deleterious mechanisms are identified and their effects on CW spectral purity and fast frequency-hopped link error rate performance is discussed. Experimental results are presented for both a high performance modem, with N equal to 480 across a 96-MHz band and a recent development comprising the basic chirp synthesizer plus phased locked loop (PLL) to provide enhanced slow frequency hop and continuous-wave (CW) spectral purity.

A Spread Spectrum System with Frequency Hopping and Sequentially Balanced Modulation--Part I: Basic Operation in Broadband Noise

A spread spectrum system incorporating sequentially balanced modulation, fast frequency hopping, phase-comparison tone ranging, and pseudoorthogonal block coding of BPSK data is analyzed from the standpoints of bit error performance and ranging accuracy. A tight upper bound analysis incorporates the effect of the limiter present in the system and accounts for nonzero inner products of the signaling alphabet code words. Somewhat simpler approximate expressions are obtained by a novel application of the central limit theorem. Both the upper bound and the approximate analyses are compared with the results of Monte Carlo simulations and experimental data.

Studies on regulatory functions of malic enzymes. VII. Structural and functional characteristics of sulfhydryl groups in NADP-linked malic enzyme from Escherichia coli W.

NADP-linked malic enzyme from Escherichia coli W contains 7 cysteinyl residues per enzyme subunit. The reactivity of sulfhydryl (SH) groups of the enzyme was examined using several SH reagents, including 5,5'-dithiobis(2-nitrobenzoic acid) (DTNB) and N-ethylmaleimide (NEM). 1. Two SH groups in the native enzyme subunit reacted with DTNB (or NEM) with different reaction rates, accompanied by a complete loss of the enzyme activity. The second-order modification rate constant of the "fast SH group" with DTNB coincided with the second-order inactivation rate constant of the enzyme by the reagent, suggesting that modification of the "fast SH group" is responsible for the inactivation. When the enzyme was denatured in 4 M guanidine HCl, all the SH groups reacted with the two reagents. 2. Althoug the inactivation rate constant was increased by the addition of Mg2+, an essential cofactor in the enzyme reaction, the modification rate constant of the "fast SH group" was unaffected. The relationship between the number of SH groups modified with DTNB or NEM and the residual enzyme activity in the absence of Mg2+ was linear, whereas that in the presence of Mg2+ was concave-upwards. These results suggest that the Mg2+-dependent increase in the inactivation rate constant is not the result of an increase in the rate constant of the "fast FH group" modification. 3. The absorption spectrum of the enzyme in the ultraviolet region was changed by addition of Mg2+. The dissociation constant of the Mg2+-enzyme complex obtained from the Mg2+- dependent increment of the difference absorption coincided with that obtained from the Mg2+- dependent enhancement of NEM inactivation. 4. Both the inactivation rate constant and the modification rate constant of the "fast SH group" were decreased by the addition of NADP+. The protective effect of NADP+ was increased by the addition of Mg2+. Based on the above results, the effects of Mg2+ on the SH-group modification are discussed from the viewpoint of conformational alteration of the enzyme.