Flaw signals measured in ultrasonic testing include the effects of the measurements system and are corrupted by noise. The measurement system response is both bandlimited and frequency dependent within the bandwidth, resulting in measured signals which are blurred and distorted estimates of actual flaw signatures. The Wiener filter can be used to estimate the flaw's scattering amplitude by removing the effect of the measurement system in the presence of noise. A method is presented for implementing an optimal form of the Wiener filter that requires only estimates of the noise distribution parameters. The theoretical error for scattering amplitude estimation, assuming various levels of available prior information, is analyzed. Three estimation techniques, one a maximum-likelihood based method and the other two residual-sum-of-squares methods, are formulated and tested. The results demonstrate that any of the three approaches could be used to optimally implement the alternative form of the Wiener filter with limited prior information.
With the advent of advanced wireless standards has come the ability to address more sophisticated problems. One such issue is to provide robust navigation in challenging environments. Our approach to develop a signals of opportunity (SoOP) based cooperative navigation network that has the potential to provide improved performance and extend coverage under a variety of difficult conditions from jamming to multipath fading. Under the premise that GPS is not always available, we employ SoOP as the core of this radionavigation system. A critical advantage of signals of opportunity is that they are more abundant and of significantly higher received signal power than other GNSS signals. There are many signals that are available but not all meet the requirements of robust navigation. Of particular importance are digital television (DTV), AM radio, and 3G cellular signals. Our basic premise is to implement these signals to provide geolocation capability using time difference of arrival (TDOA) which requires cooperation among users. We extend these concepts further to the development of a cooperative network that can be used to provide coverage to users outside the range of the navigation signals. We also demonstrate that cooperation can be used to mitigate the effects of multipath fading and provide simulation results.
Navigation in low signal-to-noise ratio (SNR) environments continues to be an extremely challenging problem for GNSS. Effects such as multipath fading, shadowing, jamming of the waveform not only greatly limit the navigation accuracy, but also increase the outage probability that occurs when the received signal level falls below the minimum threshold. Newer navigation approaches have focused on employing terrestrial signals that maintain higher signal power such as digital television (DTV), cellular, etc. These are referred to as signals of opportunity (SoOP). The focus of this research is using these signals in a time difference of arrival (TDOA) cooperative navigation network. A major problem arises with relation to convergence of the positioning algorithm for this type of network. To address this problem, our research studies this problem from a different perspective. Here we will pose the problem as a convex optimization problem and address it as such. The main idea being that if the problem is structured in this form, we can use this well understood area to develop algorithms for faster convergence with respect to cooperative navigation. As such, this research is a combination of analytical analysis, algorithm development, and simulation results that describe performance under certain conditions.
A method to reliably transmit multimedia data over wireless fading channels by using a generalized OFDM technique is presented in this research. Multimedia delivery over wireless links is sensitive to fading effects. It is furthermore characterized by a number of different properties, including unequal error protection needs, time-varying bandwidth requirements, etc. Layered multimedia representation, such as MPEG-4 FGS, has been widely studied to meet these requirements. However, only this component is not sufficient to provide the desired quality of service (QoS). To further improve the overall system performance, we focus on the design of an adaptive modulation scheme, which maintains the ability to adapt to time-varying channel conditions as well as the layered multimedia representation. In particular, we study and analyze the performance of a technique known as Wavelet Packet Modulation (WPM) from the viewpoint of Orthogonal Frequency Division Multiplexing (OFDM), and demonstrate its application to wireless multimedia communication. In our framework, WPM is viewed as a general case of OFDM, which allows flexibility in terms of channel allocation and adaptability with respect to channel conditions. The algorithm demonstrates the flexibility of the new proposed method in transmitting multimedia data over interference-laden channels.
No abstract
In ultrasonic nondestructive evaluation (NDE), flaw characterization is inhibited by the effects of the measurement system and by acoustic noise due to non-flaw related scattering of the sound.The Wiener filter can be formulated to optimally remove the effects of the measurement system and suppress the noise; however, prior information must be available abeut the neise and flaw distributions, respectively. The objective of this research is to develop an approach for the optimal implementation of the wiener filter given prior noise information but no prior flaw information. This paper beg ins with a background section in which necessary models are formulated, previous work is discussed, and the objective of the research is restated.The estimation error is then stated and analyzed. Finally, three techniques for the optimal application of the Wiener filter with limited prior information are formulated and tested. BACKGROUND
This work describes a method to optimally transmit multimedia data over wireless fading channels via Orthogonal Frequency Division Multiplexing (OFDM). It is well known that multimedia data is sensitive to fading effects because each data bit varies in importance. Hence, reliable broadband wireless media communication systems maintain a number of very specific needs such as unequal error protection and time-varying bandwidth requirements, etc. Up to now, most OFDM systems allocate bits in a uniform manner across the subcarrier distribution. This type of methodology is fine for text data transmission with a performance measure in terms of the bit error rate (BER), but not sufficient to maintain high end-to-end visual quality for multimedia data transmission. To improve the overall system performance, we focus on optimal bit allocation in a rate-distortion sense. Our algorithm examines the data content and its associated performance and allocates the most important bits to the best channels in order to minimize the overall distortion. The proposed scheme demands channel state information that specifies estimates on the subcarrier quality distribution. It is demonstrated by experimental results that a significant gain in multimedia QoS is achieved with our approach with a layered video codec i.e. Motion JPEG2000.
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