## 2 Related Method and Work
### Dual-Receiving Acoustic Signal Processing
To reduce the impact of NLOS scenarios in the system, it is necessary to adjust the indoor localization process according to the NLOS identification results to boost the final positioning accuracy. In combination with [9; 10; 11; 21], this paper makes an external circuit board connected to a smartphone and constructs an acoustic anchor, so that it can complete the dual-receiving process with the main anchor. The system uses six smartphones to build up the anchors. Additionally, one PC is used as the server. The reference anchor will help the system avoid the clock synchronization module. Meanwhile, the reference anchor can also share a smartphone with an acoustic anchor if necessary, thus reducing the development cost. In the main process, the chirp signal (linear frequency modulation signal) in this indoor localization system is considered as the main research object, and its frequency band is 17.5-20.5 kHz. After these actions, a novel dual-receiving and dual-transmitting relationship is arranged, whose fundamental purpose is to coordinate the processes of recording or receiving instructions at the main anchor. Then, according to the generalized cross-correlation (GCC-PHAT) method, the time delay difference of the transmitted acoustic signal should be calculated. At this time, the time at the main anchor is taken as the core reference and combined with the time axes of receiving or transmitting process at other anchors in each period. The protection interval between the selection process based on the RSS preservation threshold and the protection interval before the next sending process is set to 0.4 s. If the selection cannot be finished according to RSS, the latest instruction shall prevail by default. As for the anchors 1, 2, 3, and 4 in sequence, it is essential to distinguish the sending acoustic signal process twice at the main anchor. The following process and the feedback solutions are determined according to RSS at each acoustic anchor. If all of the localization loops can be carried out within the whole indoor localization period, the transmitting and receiving relationship can be adjusted and shown as in Figure 1 below [11; 21; 22]. Figure 1: Dual-receiving signal processing diagram. In the indoor localization system, the single receiving acoustic anchors are subject to more restrictions when deployed, including the surrounding placement and the effective orientations. If the dual-receiving anchors are used, the application scope of the indoor localization system can be expanded immediately. For each acoustic anchor, if only one horn or piezoelectric tweeter is used to construct acoustic anchors, it is difficult to ensure sufficient signal quality, which is not conducive to the NLOS acoustic signal identification process. It is more flexible for the processing flow of a dual-receiving structure. In each localization period, the system should send multiple instructions from the main anchor first, and other acoustic anchors receive them on time based on their microphones in sequence, then reply to those instructions only for chirp signals with higher RSS intensity. There is a stable mapping correspondence relationship between the piezoelectric tweeter and the transmitted chirp signal. It guarantees that each two piezoelectric tweeters are linked to the corresponding acoustic anchor. It can also be seen that the received acoustic signal is an LOS acoustic signal with a greater probability according to the multiple receiving processes and mapping transmission processes. Compared with the acoustic anchor in [20; 22], the dual-receiving structure will also further avoid the generation of NLOS scenarios after the RSS selecting mechanism, especially for short-duration NLOS environments. According to the NLOS situations coping strategies in [22], the system will abandon all of the NLOS acoustic signals and only finish all of the indoor localization processes when the system has no more than one segment of NLOS acoustic signals in each localization loop.