Delta modulation techniques for transform domain interference suppression

Abstract

The purpose of this thesis is to present a new transform domain processing technique for suppressing narrow-band interference in a direct-sequence spread-spectrum communication system. The new filtering technique is based on delta modulation theory. Also presented are two new techniques for processing multiple bits in the transform domain. These techniques further increase the performance of the system in the presence of a narrow-band interference. Finally, an adaptive threshold technique is described with intentions of compensating for changes in signal and interference power. A spreadspectrum system using a binary phase shift keying (BPSK) structure and computer generated pseudorandom sequences for the spreading codes will be used to test all the techniques presented. The

main objectives of the delta modulation based processing technique are speed and performance. The proposed technique will not be restricted to convergence rates and will have the ability to suppress narrow-band interference regardless of previous frequency locations. Similar to median based filters the proposed filter will have two variations: the conditional delta modulation filter and the conditional delta modulation notch. The performance of each filter will be compared to the conditional median filter, the conditional median notch and the fixed notch filter. The popular least-mean-squared (LMS) filter will also be compared. Both the processing time and the bit error rates (BER) of individual filters will be presented and compared. This will provide a good

comparison between the adaptation rates of the LMS algorithm in the time domain and the speed of the proposed TDP filters and time domain filters. The performance of the DS-SS receiver employing the proposed filters will be tested in the presence of several types of interference. The types of interference to be tested will be stationary or non-stationary. Also tested will be the ability of the system to suppress multiple interference. The performance of the proposed filters when no interference is present will also be considered. The effects of the Sine and Hamming windows will be examined against the case of no windo\ving (a rectangular window with a magnitude of one). The Sine window may not be a familiar window but promising results have been reported in other TDP techniques so is being considered, and the full details will be given later. Two

TDP methods that use multiple bits in processing are presented. The proposed methods~ not to be confused with the overlap and save scheme which attempt to reduce intersymbol interference, which are referred to as the non-overlapping and the overlapping methods. These techniques are implemented in the transform domain processing block itself, therefore can be used in the overlap and sm.e schemes, as well with any TDP filter. The performance of these methods will be compared against the performance of a system that processes one bit at a time. The effect of the Sine and Hamming windo,v on this technique are also presented. The type of interference for this technique is limited to stationary narrow-band interference for reasons that will be described later. It will be shown that this technique significantly improves the performance of the proposed filter and the TDP median filters.

All • transform domain processing filters requtre some sort of threshold, so a technique that eliminates the need for manually determining an optimum threshold is proposed. The adaptive threshold attempts to compensate for changes in Fourier transform length, jammer power, signal power, noise power and processing gain. It will be shown that the described adaptive threshold technique provides equivalent BER performance and has the ability to adjust for several variations.

Notes

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Graduation Date

1995

Semester

Fall

Advisor

Kasparis, Takis

Degree

Master of Science (M.S.)

College

College of Engineering

Department

Electrical and Computer Engineering

Degree Program

Electrical Engineering

Format

PDF

Pages

85 p.

Language

English

Length of Campus-only Access

None

Access Status

Masters Thesis (Open Access)

Identifier

DP0029499

Subjects

Dissertations, Academic -- Engineering; Engineering -- Dissertations, Academic

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