Designing Spreading/Despreading Signals to Enhance Interference Rejection Capability in Direct Sequence Spread Spectrum System

Abstract

The process gain advantage of a DS system may be compromised when operating in a hostile environment. A determined enemy may render the system inoperative through the use of single tone, multiple tones or swept (chirp) jamming, particularly if the interference frequency is close to the DS carrier frequency. Therefore, some measures should be taken to enhance the inherent interference resistance of the DS system. Currently, such measures might imply either increasing the processing gain of the system or the use of some type of preprocessing technique to suppress the interfering signals prior to the spectrum despreading process. This preprocessor can take the form of a transform domain system which performs a Fourier transform of the signal, notches out the jammer, and then returns the signal to the time domain. Another approach uses a least-mean-square (LMS) adaptive filter to create a notch at the jammer frequency with out leaving the time domain. Other techniques such as chip code manipulation, noise canceling, and spatial domain (using adaptive antenna) processing were reported in the literature, though not as common as the former ones. In this work, a totally different approach is proposed. The binary or discrete sequences, used to spread the spectrum of the transmitted signal at the transmitter and remove the spreading at the receiver are replaced with non-binary noise-like functions. These functions are designed to have flat amplitudes and random phase spectra which make them robust to the types of jamming mentioned above. Their spectrum may be modified in response to channel perturbations in order to reduce the effect of the interference. Low probability of intercept (LPI) performance against delay and multiply receivers as well as the robustness of the system to frequency selective fading which is typical of mobile radio channels, have improved by the use of such wide-band signals. An experimental baseband system, and hence perfect carrier synchronization is assumed, has been simulated which uses these types of spread spectrum signals and its bit error rate performance against different types of interferers is presented. Results obtained for this new type of spreading functions are compared against those obtained when using conventional binary code sequences accompanied with current interference excision techniques. The proposed system, as indicated by the obtained results, has shown to be very promising in terms of its performance when compared with existing systems. v