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Indoor localization systems are poised to revolutionize mobile applications, enabling precise navigation, immersive augmented reality, and context-aware pervasive computing. Emerging 5G/6G Joint Communication and Sensing (JCAS) architectures offer a promising pathway for achieving accurate and ubiquitous indoor positioning driven by rapid technological advancements and standardization efforts (e.g., 3GPP). This paper presents Echoes, a novel system within the Open RAN (O-RAN) architecture that leverages smart surfaces for precise user positioning. Echoes improves current smart surfaces sensing systems inferring the received signal phase solely from power measurements of reference signals used to measure the uplink channel. Echoes achieves this by exploiting an inherent property of smart surfaces: steering configurations are not singular, and introducing a constant phase offset to all elements achieves the same intended reflection direction but with a different reflected phase. Echoes combines the sensed direction of a user with the angle of arrival estimation to estimate the position of a user in a sensing area. Our prototype, evaluated in a real-world indoor setting, exhibits a mean absolute user localization error (MAE) of 0.9 meters. These results highlight the potential of Echoes to provide robust and accurate indoor positioning for next-generation mobile applications.

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