## 4 Performance Evaluation and Discussion
### LFU Communication Performance  
In this subsection, two different experiments were performed to measure the LFU transceiver performance in the proposed audio marker system. The first experiment was performed to investigate acoustic noise effects on the communication performance by measuring the STR under noise conditions. The second one was conducted to compare the transmission accuracy between the conventional LFU-based audio marker [14] and the proposed one. For the first experiment, a laptop computer with an embedded microphone (receiver) and a COTS speaker (transmitter) were prepared in a meeting room with a conditioner on, as shown in Figure 7a. They were actually placed at 1, 2, and 4 m distance and 0\({}^{\circ}\), \(\pm 40^{\circ}\), and \(\pm 80^{\circ}\) azimuth, as shown in Figure 7b. One hundred audio markers, with an average length of 13 characters, were sent from the speaker to the microphone equipped in the laptop at each distance, using the proposed audio marker transmitter, and the number of successful transmissions was counted. Here, the averaged sound pressure levels (SPLs) for the LFU transmission signal and background air conditioning noise were set to 80 and 70 dBSPL, respectively, resulting in a 10 dB SNR. Table 1 shows the STR of the proposed audio marker system at the various positions of the laptop. As shown in the table, the proposed system perfectly transmitted all audio markers from 1 to 4 m at 0\({}^{\circ}\). However, the STR decreased as either the distance or azimuth between the laptop and the speaker increased. This was due to the attenuation loss of the LFU signals through the aerial channel according to the distance, as well as to less sensitivity according to the degree of directivity of the speaker. For the second experiment, it was assumed that a user was sitting 2 m away from the transmitter; thus, a 2-meter radius circle centered on the transmitter became the maximum transmitting zone (MTZ). As shown in Figure 8a, two laptop computers were located 2 m and 4 m apart from the COTS speaker, which corresponded to the inside and outside of the MTZ, respectively. In addition, five different azimuths, at 0\({}^{\circ}\), \(\pm 40^{\circ}\), and \(\pm 80^{\circ}\), were considered, as defined in Figure 8b. Hence, there were 25 different azimuth and distance combinations. As performed in the first experiment, one hundred  
Figure 6: Receiver operating characteristic (ROC) curve of the object detection methods: proposed LFU-based vs PIR-based audio marker system, where the star-shaped point is a candidate for the threshold of the log-likelihood test for the decent service shown in Figure 1.  
audio markers were transmitted for each azimuth and distance combination. Finally, a false positive rate (FPR) outside of the MTZ and a false negative rate (FNR) inside of the MTZ were measured for both the conventional and proposed audio marker systems. Table 2 compares the FNR inside the MTZ and the FPR outside the MTZ between the conventional and proposed audio marker systems. As shown in the table, the conventional audio marker system had a markedly increased FPR outside the MTZ because it always transmitted LFU signals with a maximum scale (see Equation (3)). On the other hand, the proposed system controlled the transmission range up to the MTZ, i.e., 2 m for this experiment, by adaptively scaling the modulated signal produced by the LFU-based objective detection (see Equation (15)). Thus, the proposed system is suitable to provide marker information only to the people of interest in each location, rather than to all the people in the room, as would be the case in a conventional audio marker system. \begin{table}
\begin{tabular}{c c c c} \hline \hline
**Azimuth Distance** & **1 m** & **2 m** & **4 m** \\ \hline \(80^{\circ}\) & 100.0 & 96.6 & 86.6 \\ \(40^{\circ}\) & 100.0 & 100.0 & 93.3 \\ \(0^{\circ}\) & 100.0 & 100.0 & 100.0 \\ \(-40^{\circ}\) & 100.0 & 100.0 & 93.3 \\ \(-80^{\circ}\) & 100.0 & 93.3 & 80.0 \\ \hline \hline \end{tabular}
\end{table}