Broadband signal processing for AJ and RFI reduction in spread spectrum systems

This paper describes recent advances in broadband radio-frequency interference (RFI) reduction and anti-jam (AJ) signal processing. The primary use for this technology is in communication systems like spread spectrum which are planned to have some built-in immunity to inband RFI or intermodulation (IM) distortion but which need help in lowering the effective interference levels ahead of despreading or demodulation. The intent is to reduce the amount of processing gain needed, thereby allowing for improved communications capability. An active, voltage-variable nonlinear device called the biased inverting limiter (BIL) provides a practical example of this technology. Laboratory testing and measurements with modulated signals show significant end-to-end improvements (as much as 35 dB) in interference-to-wanted-carrier ratio (I/C). When the RFI envelope is predicted perfectly, the BIL effectively eliminates RFI, leaving the wanted signal unaffected and adding an IM component of power equal to the wanted signal. Even if the RFI envelope is not predicted perfectly, a significant reduction in I/C may still be obtained. Cascading two or more active nonlinear devices has been found to be successful in cumulatively increasing the amount of RFI suppression. Evidence exists that overall performance sensitivity to interference tracking error will also be reduced in a cascade. When nonlinearities are cascaded, conventional wisdom holds that significant new IM products, which would degrade communication performance, are generated at the output of each cascaded stage