However, the conventional LFU-based audio marker in [14] simply broadcast a rendering queue to anonymous devices inside the room, making it difficult to provide specific virtual objects of interest to a given user. Thus, an object detection method from the reflected LFU signal is required to provide user-awareness on the LFU-based AR marker, in order to overcome this limitation without relying on other types of additional sensors such as cameras [16] or PIR sensors [17]. Therefore, we propose a user-aware audio marker system using low frequency ultrasonic signal processing. Figure 1 shows an example application of a decent service in a museum. The proposed audio marker system only sends LFU signals to users detected in advance as being close to the object of interest, and thus it becomes suitable for a number of indoor AR applications. The proposed audio marker system consists of an object detection module using received LFU images as well as an LFU communication module providing the audio marker. For the LFU-based object detection, a COTS speaker repeatedly plays a low frequency pulsed ultrasonic chirp signal, and a COTS microphone simultaneously records audio including ambient and chirp signals around the audio marker. The signal will be reflected back to the marker when an object physically blocks the chirp signal propagation, which results in amplifying the LFU frequency band with a distinct pattern. Therefore, the anomaly in the reflected signal, resulting from the object's presence, can be detected by a time-frequency analysis. In other words, the reflected ultrasound is converted into a time-frequency representation, and then a background subtraction algorithm [18] is applied to segregate reflected chirp spectral components from the ambient audio. After that, the object presence or absence is then determined based on the likelihood ratio calculated from a _priori_ and _posteriori_ signal-to-noise ratios (SNRs) among the LFU and ambient noise. Next, the proposed audio marker operation switches from object detection to LFU communication when the object is considered to be in front of the proposed audio marker. Here, the LFU communication broadcasts a binary text message embedded in the audio marker. In detail, the binary text message is broadcast near the audio marker using a differential phase shift keying (DPSK) scheme, and consequently an AR device capable of demodulating DPSK can correctly receive the message when located near the audio marker. While the conventional audio marker system in [14] broadcasts the marker information to all the users inside the room, the proposed audio marker system first only utilizes low-frequency ultrasound to recognize users around the marker, and then sends the marker information to the detected user. In particular, when the visual sight is considerably distracted by ill-visual conditions affected by illumination or occlusion, the conventional image-based or location-based markers suffer greatly; however, the proposed system can still generate virtual objects and an environmental effect. The remainder of this paper is organized as follows. Section 2 reviews the conventional LFU communication-based audio marker system [14; 15], after which Section 3 proposes a user-aware audio  
Figure 1: Application of the proposed user-aware audio marker system for an augmented reality (AR) service in a museum. The dotted color circles indicate the proposed audio marker communication range with nearby users. marker system based on object detection using reflected LFU images and LFU communication for sending audio maker information. Section 4 evaluates the objective performance of the proposed audio marker system in terms of object detection accuracy and communication robustness. In addition, a comparison of the proposed system with other positioning systems for AR applications is given. Finally, Section 5 summarizes and concludes this paper.