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Wireless communication systems are increasingly being deployed in energy-constrained environments such as inside the human body, dense forests, and deep interstellar space. However, these environments pose significant challenges for communication systems due to the scarcity of energy resources, often requiring reliance on harvested energy. Traditional wireless systems require substantial amounts of energy to operate and perform fundamental functions, including sensing, maintaining security and privacy, and ensuring reliable data transmission. When the energy to operate is in short supply, the connectivity becomes unreliable, security gets compromised, and overall communication performance degrades. This challenge has stimulated the development of numerous innovative solutions, though often at the cost of several trade-offs and limitations. Nevertheless, such enabling technologies are essential to provide functionality that would otherwise be unattainable in energy-constrained environments.

This dissertation investigates key challenges in achieving reliable and efficient wireless communication in energy-constrained environments and proposes a set of novel solutions to strengthen the performance of wireless systems in such settings. We explore how passive devices can be leveraged for sensing in energy-limited settings, introduce a low-power channel estimation technique and present a desynchronized querying method for analog backscatter communication systems. Further, we demonstrate how analog backscatter can be employed for localization and tracking of ambient RF sources. Beyond backscatter communication, we introduce a novel low-power transmission technique using tunnel diodes, to enable energy-efficient sensing and communication in such constrained settings. Furthermore, we address the critical aspects of security and privacy by analyzing signal leakage from in-body communication systems, and proposing covert communication as a means to enhance protection by leveraging inherent properties of the communication medium. Overall, this dissertation contributes new insights and practical approaches toward enabling sustainable, secure, and efficient wireless communication mechanisms in environments where conventional solutions struggle.

Brain–computer interfaces hold significant promise for restoring function to individuals with neurological disorders and spinal cord injuries. However, the technical evolution of these systems is increasingly challenged by the substantial data throughput requirements of high-density neural recording. Invasive electrode arrays can generate vast amounts of neural data that exceed the capacity of current wireless links, while wired connections introduce infection risks and lifestyle burdens. This thesis explores fat-intrabody communication as an alternative, which utilizes the low-loss dielectric properties of subcutaneous fat. Bounded by skin and muscle, fat tissue acts as a waveguide for microwave signals at gigahertz frequencies. This approach aims to provide the high data capacity necessary for real-time, uncompressed neural data transmission in brain–computer interface applications. Using a three-layer tissue-mimicking phantom (skin–fat–muscle), link performance is characterized with antennas placed against the fat and on the skin at 2.45 GHz. Results show that fat-intrabody communication can support high-order modulation schemes, and a practical transceiver platform, based on commercial off-the-shelf hardware, is developed to quantify data throughput. In phantom experiments with in-body and on-body links, data rates of up to 120 Mb/s are achieved, more than two orders of magnitude higher than previously reported. Beyond performance, security and privacy implications are examined, including signal leakage to external eavesdroppers and the feasibility of in-body covert communication enabled by skin attenuation and friendly jamming. Finally, fat-intrabody communication with on-skin antennas is validated in vivo. This includes an end-to-end closed-loop demonstration of a brain-implanted macaque monkey controlling a prosthetic hand, and measurements on twelve human participants to quantify received signal strength and throughput across multiple body locations, postures, and environmental conditions. Overall, the results show that fat-intrabody communication can provide high-throughput links in realistic scenarios, supporting the case for fat tissue as a communication medium for future implant networks and brain–computer interfaces.