Adaptive Processing for Improved Jamming Resistance against Narrowband Interference

A scheme to adaptively null narrowband interference of a PN direct-sequence signal is described. The nulling strategy is to minimize the mean-square output of a transversal filter with one tap weight constrained to be non-zero. The achievable improvement in processing gain is first established by analyzing the nulling performance of a Wiener filter for the hypothetical case of interference with known parameters. Next, the LMS algorithm is applied and simulation results of the adaptive interference nuller are described. When the interference is narrowband, the potential improvement in antijam processing gain is shown to be very large, typically 40 dB. When the interference is wideband, however, the nulling scheme becomes ineffective. Nevertheless, the theoretical processing gain is never less than the conventional processing gain of a PN system. Digital processing techniques for both the adaptation and PN correlation are proposed. Implementation considerations and the resulting quantization effects are discussed. The practical considerations of acquisition and tracking in the presence of interference nulling are also discussed. For example, correlation sidelobes are induced by the nulling scheme. This does not affect acquisition since the sidelobes are delayed in time, however, multiple delay-lock tracking points are created. A modification to eliminate this limitation is described.