Interference cancellation for enhanced detection of frequency hopped signals

It is well known that a prewhitening filter, or interference canceller can significantly enhance the detectability of a wideband signal received in the presence of narrowband interference. For example, an adaptive prewhitening filter using the Widrow LMS algorithm has been shown to provide greatly improved performance in wideband direct-sequence (DS) spread-spectrum communications systems. Frequency-hopping is another useful spread-spectrum signaling format that is, however, very sensitive to narrowband jamming. Unlike DS signals, frequency-hopped (FH) signals are instantaneously narrowband, and thus would not seem to be good candidates for the use of prewhitening techniques for interference suppression. However, when observed over a time span encompassing multiple hops, the FH signal becomes wideband. In this paper, we show how prewhitening filters designed using linear least-squares estimation techniques can be applied to improve the detection performance of FH signals. Two specific interference suppression filters are presented, the first, with taps spaced at the hop duration Th, can reject interference with a bandwidth of up to π/Thradians/sec. The second filter uses "fractionally-spaced" taps at intervals of Th/L. This filter can reject interference with a bandwidth of up to Lπ/Thand provides improved detection performance when the FH signal is linearly combined over L hops. We present analytical results for signal-to-noise ratio improvement factors for the two types of FH interference suppression filters and discuss their implementation using the LMS algorithm.