Partial-band detection of frequency-hopped signals

Abstract

Channelized intercept receivers with fast Fourier transform (FFT) front-end preprocessors are often used to detect frequency-hopped (FH) signals. Ideally, the FFT frequency band should coincide with the entire FH spread-spectrum bandwidth. However, in some practical implementations, hardware and/or real-time computational signal processing capabilities limit the FFT to a fraction of the total band of interest. In this case, the preprocessor will only intercept some of the received hops, but the FH signal can still be successfully detected assuming that enough hops are observed.If the receiver front end is tunable, an alternative approach is to step the FFT band within each hop such that more or all of the FH signal bandwidth is covered. The tradeoff is that more of the hops will be intercepted but the receiver will detect only part of each hop. This paper analyzes this partial-band FH detection scheme and determines the optimum performance strategy.

Performance comparison of multichannel energy detector output processing methods

“…[7]) channelized radiometer calculates where Cij is the radiometer output thresholded with 7 in channel j at time interval i. The final decision is made by comparing Dsr;nr to a second threshold T,.…”

“…However many practical communications signals have larger time-handwidth product. For example the slow frequency hopping SINGCARS (single channel ground and airborne radio system) combat radio can use time-frequency product 250 [7]. Previously [6] has proposed summing contiguous FFT bins to increase time-frequency product in the context of detecting transient signals.…”

Detection of frequency hopping signals with a sweeping channelized radiometer

“…Final decision is made by comparing the sum value against another threshold k M . It is also possible to use a sum instead of a logical-OR function to combine different radiometer outputs [6]. This is good choice in a situation where multiple signals are excepted to be present.…”

“…In this case, the probability of intercept is limited but when a signal is intercepted its whole hop energy is integrated. Another possibility is to change the center frequency of the receiver, i. e, to perform frequency sweeping [4], [6]. In this paper we analyze a channelized radiometer that steps the received frequency band K times within each hop, i. e., the center frequency changes faster the signal hops hopefully increasing the probability detection.…”

“…The situation K = 1 corresponds to no scanning (or slower scanning in the hop level, which does not change the results). This type of sweeping has been proposed in [6], where the performance of sweeping sum based envelope receiver implemented with the Fast Fourier Transform (FFT) has been analyzed. Miller et al [2] have analyzed a system where the bandwidth of each radiometer is increased so that the intercept probability is increased.…”