A direct-sequence spread-spectrum (DSSS) receiver using both decision feedback (DFB) and two-sided transversal filters for combatting narrowband interference (NBI) is proposed. The receiver is made up of two branches. In the first branch, the conventional demodulator is followed by a DFB filter, while in the second, auxiliary branch, a demodulator with the carrier in quadrature is followed by a two-sided adaptive transversal (AT) filter. Performance of this receiver was analyzed on the basis of the calculated mean-square error and the probability of error at the output of the receiver. Special attention was paid to the effects caused by the propagation of errors in the DFB filter. The results obtained show that NBI rejection is fairly high and practically does not depend upon the difference of frequencies of the desired and interfering carriers or upon the interfering carrier level.>

A new proposed robust FM demodulator (RFMD) is a simplified version of the recently reported demodulator (FMD) [1] which improves FM system interference immunity. The RFMD is applied to combat the adjacent channel interference and its improvement factor is only negligibly smaller than that of the FMD.

Representing a satellite transponder as a nonlinear, memoryless, band-limited device, the error probability variation due to on-off transitions of the TDMA signal envelopes in TDMA/PSK multicarrier operation is analyzed. The effects of AM/AM and AM/PM conversion are also taken into account. The results of this study are applied to4\phiPSKand8\phiPSKsystems, for which the parameters describing the transponder nonlinearity have been taken from the real characteristics obtained by measurements. The numerical calculations show that on-off transitions of the TDMA signal envelopes could cause significant variation of the error probability.

As a result of investigation of interference into FM systems, a new algorithm for the process of demodulation is proposed. When compared to the method using the conventional limiter-discriminator, it offers better immunity against the baseband interference noise. Desired signal processing is performed by the functional devices added to the conventional limiter-discriminator in such a way that this new demodulator can be optimized in the sense of the minimum baseband interference noise. The statistics of the wanted and interfering signals must be known. Several examples involving interference problems in FDM-FM radio-relay systems carrying multichannel telephone signals are elaborated to illustrate the performances of the proposed demodulator. FDM-FM, PSK, or FSK systems are considered the cause of the interference. The noise power ratio (NPR) at the output of the conventional limiter-discriminator and the improvement factor offered by the new demodulator, obtained on digital computer, are presented versus baseband frequency in the form of diagrams. Different IF filters in FDM-FM receiver and transmit filters in the interfering systems are taken into account. The improvement offered by the proposed demodulator is of such a degree that some of the important restrictions in planning different radio systems could be relaxed.

A general method for the accurate analysis of the response of a frequency demodulator using feedback (FMFB) to an arbitrary frequency modulated (FM) signal is developed. This approach is employed to determine the FMFB outputs for two specific modulating signals, Gaussian noise, and a single sine wave. The proposed method is then applied to three different FMFB configurations differing in their low-pass filter structure. As a result, generalized curves enabling easy evaluation of the intermodulation (IM) distortion and the harmonic distortion are obtained. When designing the FMFB on the basis of these results, the influence of the relevant system parameters, such as IF filter bandwidth, feedback factor, position of the channel in the baseband, structure of the low-pass filter, etc., on the nonlinear properties of the FMFB can be controlled, simply and in a straightforward manner. The accuracy of the method has been verified in those rare cases for which solutions are already known.