# OTFS-Based Handover Triggering in UAV Networks
## 1 Introduction
By transforming the signal into a 2D representation, OTFS ensures that symbols remain well separated in the presence of high mobility or multipath propagation, thereby mitigating ICI in addition to inter-symbol interference (ISI). Thus, in OTFS, the DD channel gain becomes constant over the whole OTFS symbol duration regardless of the Doppler frequency shift coming from different velocities of the transmitter (Tx), receiver (Rx), or surrounding reflectors. Since its invention, a lot of research efforts have been made to deeply understand and investigate the operation of OTFS, including the design of efficient OTFS equalizers and channel estimators [14; 15]. Also, it was applied in various communication systems as given in [16; 17]. In this paper, leveraging the estimated DD channel using OTFS modulation, we suggest initiating handovers between UAV and ground entities based on it rather than relying on the time-varying RSS or FT channel gains. Although we used OTFS in this paper, other DDMC alternatives like orthogonal delay Doppler modulation (ODDM) [18] or orthogonal delay scale space (ODSS) modulation [19] can be applied to the proposed scheme without any necessary modifications. Thus, the main contributions of this paper can be summarized as follows:  
* We will investigate the issue of handover triggering within an integrated UAV-to-ground network, employing the time-varying DD channel modeling given in [8]. Subsequently, we will demonstrate that both the conventional RSS and the FT channel gain-based handover triggering are significantly time-variant, particularly in this highly dynamic environment characterized by extreme UAV velocities. * We will propose a handover-triggering framework based on OTFS modulation, where the estimated DD channel will be used for initiating the handover routines. In this scenario, the DD channel gains from the serving and target cells are compared, upon which a handover decision will be made. In this paper, we will mathematically prove that the DD channel power is constant over the whole OTFS symbol duration even in time-varying DD channel conditions. Through this stable handover-triggering strategy, a low number of handover decisions are made, and high throughput will be achieved coming from the lower handover-signaling overhead and the lower unnecessary handovers. * As the proposed approach relies on the estimated DD channel, it is vulnerable to channel estimation errors. To bind this effect on the performance of the proposed scheme, we mathematically study the channel estimation errors in the most dominant DD channel estimation strategies, which are least square (LS)- and minimum mean square error (MMSE)-based channel estimators. In both cases, we showed that the variance of the channel estimation error is lower than that introduced by additive white Gaussian noise (AWGN) at Rx, which proves the efficiency of the proposed scheme even under channel estimation errors. * Numerical analyses are conducted to prove the potency of the proposed DD handover strategy over the conventional RSS and FT channel gain-based ones in terms of the handover overhead, the achievable throughput, and the ping-pong ratio in different scenarios. Also, the performance of the proposed scheme under the effect of channel estimation error is bounded using numerical simulations. The reminder of this paper is structured as follows: Section 2 gives the works related to that presented in this paper. Section 3 presents the UAV-to-ground handover scenarios and channel modeling. Section 4 explores the OTFS modulation, including its mathematical foundation. Section 5 gives the proposed DD channel-based handover triggering. Section 6 gives the mathematical analysis of the channel estimation error effect on the performance of the proposed scheme. Section 7 gives the conducted numerical analysis followed by the concluding remarks in Section 8. The definitions of mathematical symbols used in this paper are given in Table 1.