Nonlinear Detection System Research Articles

Utilizing Passive Intermodulation Response of Frequency-Modulated Continuous-Wave Signal for Target Identification and Mapping

In this paper, a passive-intermodulation-based tracking system working with a frequency-modulated continuous-wave (FMCW) is proposed for target identification and localization. Signals in the microwave range are chosen to realize ranging purposes for the benefits of small device size, lightweight, and wide available bandwidth. Depending on whether the nonlinear properties of electrical components, such as diodes, transistors, and mixers, are exploited, the tracking systems can be categorized as linear and nonlinear systems. By nature, the linear tracking methods are vulnerable to surrounding clutter, such as furniture, walls, and other objects, since the reflection signals from clutter are of the same nature as the target. In comparison, the nonlinear detection methods are attractive owning to the fact that the majority of the clutter does not possess nonlinear properties. Hence, the nonlinear detection system does not “see” the linear clutter and only senses the target exhibiting nonlinear responses. Among all the nonlinear technologies, the intermodulation-based tracking and mapping approach has less path loss than the harmonic-based, smaller circuit size than the subharmonic-based, and simpler tag design and fewer signal licenses required than both of its counterparts. Therefore, in this work, the intermodulation-based technology is adopted, and a new nonlinear tracking system is designed to track the distance of the target in a complex indoor scenario crowded with unwanted objects.

Spatially coincident vibrotactile noise improves subthreshold stimulus detection.

Stochastic Resonance (SR) is a phenomenon, mainly present in nonlinear detection systems, in which the addition of certain amount of noise, called optimal noise, has proven to enhance detection performance of subthreshold stimuli. When added noise is present only during the stimulus, an additional enhancement can be reached. This phenomenon was called time Coincidence Enhanced Stochastic Resonance (CESR). The aim of this study was to study the effect of spatially distributed vibrotactile noise in subthreshold stimuli detection. The correct response rates from two different stimuli conditions were compared, using four tactile stimulator systems to excite four different spatial locations on the fingertip. Under two different conditions, the stimuli were present in only one randomly chosen stimulator. For the first condition, all stimulators contain optimal noise level. In the second condition, the optimal noise was present only at the stimulator with the stimulus. SR threshold principle should not produce different correct response rates between the two conditions, since in both cases the noise enables the subthreshold stimulus to go above threshold. The stimulus signal used was a rectangular displacement controlled pulse that lasted 300ms within a 1.5s attention interval, applied to the exploratory zone of the index finger of 13 human subjects. For all subjects it was found that detection rates were better (p<0.0003) when noise was spatially coincident with the stimulus, compared to the condition in which noise was present simultaneously in all the stimulators. According to our literature review this is the first report of SR being influenced by the spatial location of the noise. These results were not found previously reported, so represent the discovery of a new phenomenon. We call this phenomenon Spatial-Coincidence-Enhanced Stochastic Resonance (SCESR). As results show, the optimal noise level is dependent on the relative position between stimulus and noise.

Recents Developpements en Spectroscopie Instrumentale

Abstract The three main functions of spectroscopic instruments are considered in relation to certain important improvements which are discussed. (i) In the field of monochromators and dispersive systems, improvements due to the use of holographic gratings have their origin mainly in the shape of the grating, rather than in its size or regularity of ruling. (ii) The second field is that of new light sources. The use of narrowly monochromatic and tunable lasers may suppress all dispersive system. However, this raises the problem of the accurate measurement of the source frequency. Here, standard lasers and non-linear detection systems have an important role to play. (iii) Finally, an improved treatment of the electric output allows us to obtain better sensitivities. Besides the well-known method of the sinusoidal modulation followed by a synchronous detection, we now have techniques of fast Fourier transforms following rectangular pulse modulation, stochastic modulation or chirp pulse modulation.

Comparison of Nonlinear and Linear Multireceiver Detection Systems

Detection of a binary transmission by both optimum and suboptimum nonlinear and linear multireceivers is considered by comparing their asymptotic performance characteristics. The multichannel model is presumed to be of the Rician type. Particularly, we consider Turin's nonlinear specular-coherent multi-receiver and the nonlinear noncoherent Pierce-Stein multireceiver. These two termination error rate characteristics are graphically compared for low and high output signal-to-noise ratios. The performance characteristics of two other coherent linear multireceivers, one optimum and one easier implemented suboptimum, are derived and compared with the above-mentioned nonlinear multireceivers. The numerical results indicate system design trends and provide information on the degradation or improvement afforded by employing nonlinear detection systems as compared with linear detection systems. In particular, the optimum nonlinear coherent multireceiver is difficult to implement. It is shown that, for multichannels which are largely specular in nature, a more easily implemented linear coherent unit behaves optimally for all practical purposes. For channels which are largely scatter in nature it is shown that the linearized suboptimum system performance is highly inferior to the corresponding optimum coherent unit. In these situations, the noncoherent "square-law combining" system would be more reliable than the suboptimum coherent unit. In fact, for large scatter components we find that the noncoherent unit performs almost identically to the nonlinear coherent unit. This is due to the signal suppression effects known to occur in all nonlinear detectors throughout the field of statistical detection theory.