📝 SummarXiv

Abstract

Optical fibers have long been employed as sensors in a wide range of commercial systems. Distributed Acoustic Sensing (DAS) extends this concept by enabling the detection and localization of acoustic sources along the fiber, using backscattered light from small segments to achieve spatial resolution on the order of meters. Recently, DAS has also been explored as a component in underwater acoustic communication systems. Emerging interest in bidirectional configurations where both transmitter and receiver are placed at opposite ends of the fiber has opened new possibilities. However, in such setups, source localization is not inherently integrated into the signal decoding process. For scenarios where source positioning is valuable, we propose an approach inspired by bi-static radar principles. This configuration utilizes acoustic signals received at both ends of the fiber to estimate source position based on propagation delay differences. Although the localization accuracy is lower than that of DAS due to reduced sampling rates, the method offers a viable alternative for integrated communication and positioning. We present the system topology and configuration for a dual-fiber layout, each end equipped with optical transmitters and receivers. The position estimation is derived from the time difference of arrival (TDOA) between the two receivers. The Cram\'er-Rao Bound is derived to characterize the theoretical limits of localization accuracy, highlighting dependencies on system parameters such as optical power loss. Our analysis shows that increased acoustic bandwidth and higher carrier frequencies enhance spatial resolution. We formulate the Cross Ambiguity Function as a maximum likelihood estimator for TDOA and provide simulation results illustrating its performance under varying system conditions. Finally, we discuss key challenges that must be addressed for practical implementation.