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New two-dimensional (2D) position sensitive solid state pixel detectors with dedicated multi-channel mixed-signal front-end readout integrated circuits (ICs) have been developed for x-ray and gamma ray imaging. These pixel detectors were designed to be versatile and accommodate many types of position-sensitive solid-state sensors. These 2D pixel detectors are designed to address the needs of a wide range of photon counting applications with high energy resolution imaging, high spatial resolution and for fast photon counting with simultaneous multiple energy binning capability. The first family of these pixel detectors is intended for spectroscopy applications with sensors such as Si, Ge, GaAs, HgI2, PbI2, Se, CdTe and CdZnTe in a variety of configurations to detect and image x-rays and gamma-rays of energies up to 1.3 MeV. The second family is developed for fast photon counting with simultaneous energy binning. They can be used with similar solid state sensors. The third family is for high spatial resolution imaging with resolution down to 50 times 50 micron.

Utilization of chaotic signals for covert communications remains a very promising practical application. Multiple studies indicate that the major shortcoming of recently proposed chaos-based communication schemes is their susceptibility to noise and distortions in communication channels. In this paper, we review a new approach to communication with chaotic signals, which demonstrates good performance in the presence of channel distortions. This communication scheme is based upon chaotic signals in the form of pulse trains where intervals between the pulses are determined by chaotic dynamics of a pulse generator. The pulse train with chaotic inter-pulse intervals is used as a carrier. Binary information is modulated onto this carrier by the pulse position modulation method, such that each pulse is either left unchanged or delayed by a certain time, depending on whether "0" or "1" is transmitted. By synchronizing the receiver to the chaotic-pulse train we can anticipate the timing of pulses corresponding to "0" and "1" and thus can decode the transmitted information. Based on the results of theoretical and experimental studies we discuss the basic design principles for the chaotic-pulse generator, its synchronization, and the performance of the chaotic-pulse communication scheme in the presence of channel noise and filtering. We also discuss the possibilities of multiuser communication using CPPM and its advantages over standard communication techniques.

In this paper, we discuss fundamentals for the design of a source of chaotic signals based on a blocking oscillator circuit. We study a modification of a well-known circuit of blocking oscillator which leads to the onset of chaotic oscillations. The output signal of such chaotic oscillator is a series of short term pulses characterized by chaotic fluctuations of time intervals between pulses. Such chaotic pulse signals possess a broad-band continuous power spectrum and short correlation length. We discuss the results of theoretical and experimental studies of nonlinear dynamics of the chaotic blocking oscillator.

Using chaotic signals in spread-spectrum communications has a few clear advantages over traditional approaches. Chaotic signals are nonperiodic, wideband, and more difficult to predict, reconstruct, and characterize than periodic carriers. These properties of chaotic signals make it more difficult to intercept and decode the information modulated upon them. However, many suggested chaos-based communication schemes do not provide processing gain, a feature highly desirable in spread-spectrum communication schemes. In this paper, we suggest two communication schemes that provide a processing gain. The performance of these and of the earlier proposed differential chaos shift keying is studied analytically and numerically for discrete time implementations. It is shown that, when performance is characterized by the dependence of bit error rate on E/sub b//N/sub 0/, the increase of the spreading sequence length beyond a certain point degrades the performance. For a given E/sub b//N/sub 0/, there is a length of the spreading sequence that minimizes the bit error rate.

Synchronization of chaotic systems by control using a phase feedback loop is investigated. Suggested algorithm of synchronization can be used both for discrete time and for continuous time systems. Due to similarity to the usual phase lock loop, this method allows generalization of the majority of applications of phase lock-in for chaotic systems.

In this letter we investigate a communication strategy for digital ultra-wide bandwidth impulse radio, where the separation between the adjacent pulses is chaotic arising from a dynamical system with irregular behavior. A pulse position method is used to modulate binary information onto the carrier. The receiver is synchronized to the chaotic pulse train, thus providing the time reference for information extraction. We characterize the performance of this scheme in terms of error probability versus E/sub b//N/sub 0/ by numerically simulating its operation in the presence of noise and filtering.