Multipath detection and compensation in reflections from distributed targets
Steiner, Gerard Royal
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This work is motivated by the use of the power spectra of target signatures for target classification and the effect multipath has on these signatures.The differences between the effects of the multiple reflector target and those of multiple transmission paths are examined in the context of the resultant radar or sonar target signature. In particular, the manifestation of these effects on the power cepstrum of the received signal are investigated. The behavior of the power cepstrum is reviewed to show its usefulness in recognizing multiple arrivals of the same signal. It is proven that the expected height of peaks in the cepstrum decreases with the number of replicas. On this basis, the expected peak value of the cepstrum is higher with the presence of a small number of multipaths than it is from a target with many or distributed reflectors.The performance of the power cepstrum in detecting the presence of multipath in the echo from an unknown target is investigated through a computer simulation. A variety of modeled targets are used with five waveforms: continuous wave pulse, linear frequency modulated pulse, linear period modulated pulse, a bionic waveform, and stepped frequency pulses. The multipath detection capability is shown with each wave form. Detection probabilities and false alarm rates are presented for continuous wave, frequency modulated and bionic signals based on the limited number of simulations carried out. The detection performance of the cepstrum, with each of these signals, is presented as it varies with signal-to-noise ratio and multipath strength.The effects of noise on multipath detection with the cepstrum are discussed and earlier work is extended to include the case under study. Target motion is shown to modify the cepstrum behavior in a manner that allows straightforward compensation. The existence of three or more paths is found to reduce detectibility below that in the two path case.Detectibility is seen to be highly dependent on relative path strengths and signal-to-noise ratio, and to a lesser extent on the signal used and the variations from target-to-target in the cepstrum of the basic target signatures.The usefulness of cepstral processing in removing the effects of multipath from the measured spectrum is shown theoretically and is demonstrated for several examples from the simulation study. Further measures to remove the multipath corruption of the spectrum of the detected target signature are suggested.Suggested areas for further study include examination of actual radar or sonar data and methods to further separate the cepstrum contributions of multipath and that of the target.
- Electrical engineering