# Deep-Learning-Based Stream-Sensing Method for Detecting Asynchronous Multiple Signals
## 3 Preliminary
### The Need for an Asynchronous Stream-Sensing Technique  
In this subsection, we describe the need for an asynchronous stream-sensing technique by introducing the UAVs' limitations in synchronous transmission and by analyzing the asynchronicity in transmission over Wi-Fi depending on the placement of users and various pathloss models. **Limitations on the use of synchronous transmission for UAVs utilized for a survivor discovery purpose**: To ensure synchronization between devices, a rescue UAV should serve as a base station in LTE that is a representative synchronous communication and utilizes a centralized synchronization process with several uplink channels [32; 33]. That is, a rescue UAV should carry all the equipment required for the synchronization procedure. The equipment increases the overall load, which leads to an increase in the amount of power the UAV consumes for hovering and moving. UAVs, unfortunately, have battery limitations, and thus increasing power consumption will inevitably shorten the UAV's operating time. Moreover, the size of UAVs has been decreasing recently, which makes it infeasible to mount a lot of equipment on small UAVs. Despite the importance of uptime in a disaster situation, due to the heavy equipment requirement, synchronous transmission can shorten the uptime of rescue UAVs that should search the entire disaster area. Moreover, with the variety of UAV sizes, it is necessary to develop a light survivor discovery technology that can be used regardless of the size of rescue UAVs. **Asynchronicity in Wi-Fi transmission**: This work considers a scenario in which a rescue UAV and mobile devices transmit their signals using Wi-Fi, a lightweight wireless technology compared to LTE (Note that commercial UAVs such as the DJI Phantom series already use Wi-Fi). Figure 1: Example of CPs overlapping and not overlapping. To determine the number of survivors, a rescue UAV sends a trigger message to collect rescue signals from mobile devices. As soon as mobile devices receive the message, they immediately begin sending rescue signals. The devices appear to be synchronized, but in reality they cannot be synchronized. Depending on the distance between an UAV and each mobile device, the arrival time of the trigger message on each mobile device is different, and vice versa. If the difference between the arrival times of the trigger message in two mobile devices is \(T_{\text{diff}}\), at the UAV, the difference between the arrival times of rescue signals is \(2\times T_{\text{diff}}\). That is, if \(2\times T_{\text{diff}}<T_{\text{CP}}\) is satisfied, where \(T_{\text{CP}}\) is the time duration of a CP, the CPs of two rescue signals can overlap. In Wi-Fi [34], \(T_{\text{CP}}\) can be set to 0.2 \(\upmu\)s, 0.4 \(\upmu\)s or 0.8 \(\upmu\)s, and thus \(T_{\text{diff}}\) should be less than 0.1 \(\upmu\)s, 0.2 \(\upmu\)s, or 0.4 \(\upmu\)s for the synchronization. In other words, the difference between the distances from two mobile devices to a rescue UAV must be less than \(c\times T_{\text{diff}}\) where \(c\) is the speed of light. Hence, for each case, the maximum synchronization distance is 30 m, 60 m, or 120 m, respectively. To investigate whether asynchronous transmission can occur, we calculated the maximum distance that a trigger message can be delivered to a mobile device considering various pathloss models: (1) Urban Micro (UMi), (2) Urban Macro (UMa), and (3) Rural Macro (RMa) [35]. The maximum distance is considered for both line of sight (LOS) and non-line-of-sight (NLOS). In this analysis, a trigger message is transmitted with the lowest modulation and coding scheme, the minimum receiver sensitivity of which is \(-82\) dBm on Wi-Fi [31]. This analysis considers the transmission of a trigger message to be successful if the signal strength at the time of receiving the trigger message is greater than \(-82\) dBm. Figure 1(a) presents the maximum distance for various hovering heights of UAVs with a transmit power of 23 dBm.