Covert Wireless Communications in a Dynamic Environment

This dissertation investigates covert communication in dynamic wireless communication environments. A key goal is to provide insight about the capabilities of a transmitter desiring to remain covert and analogously, the capabilities of the party attempting to detect covert communications. The first chapter provides background on covert communications prior to this work. The second chapter studies the theoretical limits of covert communication and proves that positive rate is achievable when a jammer is added to the classical Alice/Bob/Warden Willie model. The third chapter expands on the second chapter by considering more generally the impact of the dynamics of the environment on the Alice/Bob/Warden Willie model. The dynamics of the environment generate uncertainty at Willie even if the jammer does not vary his/her power or even if Willie employs an antenna array to mitigate the jamming. The fourth and fifth chapters investigate the impact of considering the exact continuous-time model rather than a discrete-time model approximation. In particular, detectors at Willie which leverage information in the continuous-time domain outperform detectors based on the discrete-time model approximation. The fourth and fifth chapters consider the continuous-time model of the Alice/Bob/Willie scenario and the Alice/Bob/Willie/Jammer scenarios respectively. The fourth and fifth chapters may appear to question the results of Chapter 2, Chapter 3 and prior wireless covert communication related research. However, these final chapters provide insight about different detectors available to Willie and the importance of Alice implementing communication schemes which do not contain features that significantly differ from Willie\u27s observation under the null hypothesis. Our work has demonstrated how the covert throughput critically depends on Willie\u27s knowledge of the environment and how the covert transmitter, allies in the area, or the dynamics of the environment itself might impact that knowledge. Future work will continue to move covert communications closer to practice by integrating further aspects of practical communication system design

The Measurement of Internal Temperature Anomalies in the Body Using Microwave Radiometry and Anatomical Information: Inference Methods and Error Models

The ability to observe temperature variations inside the human body may help in detecting the presence of medical anomalies. Abnormal changes in physiological parameters (such as metabolic and blood perfusion rates) cause localized tissue temperature change. If the anatomical information of an observed tissue region is known, then a nominal temperature profile can be created using the nominal physiological parameters. Temperature-varying radiation emitted from the human body can be captured using microwave radiometry and compared to the expected radiation from nominal temperature profiles to detect anomalies. Microwave radiometry is a passive system with the ability to capture radiation from tissue up to several centimeters deep into the body. Our proposed method is to use microwave radiometry in conjunction with another imaging modality (such as ultrasound) that can provide the anatomical information needed to generate nominal profiles and improve detection of temperature anomalies. An inference framework is developed for using the nominal temperature profiles and radiometric weighting functions obtained from electromagnetic simulation software, to detect and estimate the location of temperature anomalies. The effects on inference performance of random and systematic deviations from nominal tissue parameter values in normal tissue are discussed and analyzed