# OTFS-Based Handover Triggering in UAV Networks
## 1 Introduction  
Recently, many research efforts have been performed in the integration of the air segment, containing aerials such as fixed-wing aircrafts, unmanned aerial vehicles (UAVs), or drones, with the ground segment, containing mobile base stations (BSs) and user equipment (UE) [1, 2], especially in space-air-ground integrated networks (SAGINs) [3]. The UAVs' ability to fly, hover, and maneuver enable them to be utilized in a wide range of wireless communication applications [4, 5, 6, 7]. They can be used in rescue operations after natural disasters, where they can quickly establish temporary communication networks for successful relief services [4]. Also, they can be used for providing wireless connectivity like flying BSs or relays for connecting users in remote and rural areas as well as in advanced technologies like metaverse [5]. By utilizing reconfigurable intelligent surface (RIS), UAV-mounted RIS can strengthen the coverage in crowdy hotspots [6]. Furthermore, collecting data from the internet of things (IoT) devices can be easily performed using flying UAVs [7]. Although the integration of UAVs with ground networks has a significant impact on future communications, mobility management poses a significant challenge due to the high speeds of the UAVs or ground entities like highspeed trains (HSTs). These speeds vary, ranging from 5 Km/Hr for UAVs to over 1000 Km/Hr for fixed-wing aircrafts or HSTs. This high speed introduces a substantial Doppler shift in the communication channel, rendering it exceedingly time varying [8]. This results in highly time-varying channels between UAVs and ground entities in the time domain. In both pre-planned aircraft routes and spontaneous UAV trajectories, it is crucial to seamlessly transfer the UAV connection from one ground BS (GBS) to another to maintain un-interrupted connectivity. Likewise, when UAVs function as airborne BS (ABS), smooth handovers of ground entities, e.g., HST, should occur between different coverage ABSs while they are in motion [9]. The current technology of using received signal strength (RSS)-based handover triggering or the estimated channel gains in frequency time (FT) domain using OFDM modulation will be inefficient in these scenarios [10]. This is because, in RSS, the handover decision is taken based on comparing the received powers in the time domain from the serving and the target cells, which are highly fluctuated. Also, the FT channel suffers from this time dependency due to the inter-carrier interference (ICI) coming from the high Doppler shifts. This results in highly increasing the handover overheads and decreasing the achievable throughput in consequence. Moreover, unnecessary handovers occur, which increases the ping-pong ratio. Sometimes, when the handover rate is ultimately high, the communication links between UAVs and ground entities cannot be efficiently established, which ends up with frequent communication failures [11]. Thus, designing a stable handover triggering mechanism is of the most importance in this highly time-varying environment. In this paper, an efficient handover-triggering scheme is proposed based on the delay Doppler (DD) representation of the channel [12; 13; 14; 15]. In this context, DD multi-carrier (DDMC), like orthogonal time frequency space modulation (OTFS), can be used for estimating the DD channel to trigger the handover decisions. OTFS is a new modulation scheme presented in 2017 by [PERSON] as an alternative to the conventional OFDM modulation [12]. The main idea of OTFS is to combat the ICI occurring in OFDM due to high Doppler shifts [13]. This can be accomplished by spreading the transmitted symbols over the whole channel in the DD domain instead of the FT domain used in OFDM. This approach allows OTFS to maintain orthogonality across both time and frequency axes simultaneously, resulting in superior resilience to time-varying channels and Doppler shifts [12; 13].