# A Hybrid Handover Scheme for Vehicular VLC/RF Communication Networks
###### Abstract
An MDP model was proposed to manage user mobility and handovers among the cells in heterogeneous ultra-dense networks in 5G cellular systems, with the Stackelberg competition model introduced to increase the relay node selected and to guarantee communication quality and load balancing [20]. The deep reinforcement learning algorithm, as an advanced MDP-based model, has also been applied to cope with the handover problems of neural networks trained to replace unknown state transition rates or reward functions [21; 22; 23; 24]. An adaptive handover mechanism, including a seamless handover protocol and a selection algorithm, was optimized by a deep reinforcement learning (DRL) method in VLC/6G hybrid indoor networks with ultradense deployed VLC APs [21]. A multi-UAV cell association and their moving velocity decision problem in multi-UAV networks was formulated as an MDP, and a deep reinforcement learning algorithm was developed to improve both the transportation and communication performance with the branching duel (BDQ) and branching dueling double deep Q-network (Dueling DDQN) [22]. A complex inter-satellite beam handover that required balanced channel capacity was characterized as an MDP and solved using an improved deep reinforcement learning (DRL) algorithm to determine the handover satellite and handover beam [23]. In heterogeneous VLC/RF vehicular networks, persistent transmission via a VLC LoS link is always preferable, due to its better perceptible QoS. However, MTs suffer from severe QoS degradation when the LoS link is interrupted. VHO operation offers MTs wireless access by switching to RF links with reduced transmission rate. Once the VLC LoS link has been recovered, switching back to the VLC AP is preferred for the QoS reasons. Because the LoS blocking does not last long for a common case, switching back and forth can cause the ping-pong effect. As a result, waiting for recovery of the VLC LoS link may be a better choice [11]. For vehicular VLC/RF networks, switching horizontally to the next accessible VLC AP is even better, because HHO is meant to be executed as the traffic moves and thus can avoid the additional signaling cost and latency of VHO. Therefore, we present an efficient hybrid handover strategy that can make a choice between VHO, HHO, or waiting in the case of VLC link disruption. Our main contributions are summarized as follows:  
* With due consideration of the mobility of the vehicles, we put forward a hybrid handover strategy that makes proper handover decisions from one of VHO, early HHO, or waiting, so as to maintain persistent wireless access and improve the transmission performance in vehicular VLC/RF communication networks. * A Markov decision process (MDP) is formulated to describe the hybrid handover problem, with a cost function considering the handover consumption, delay, and the reliability. A value iteration algorithm was applied to solve the decision-making problem. * Simulation results demonstrated the performance of the proposed hybrid scheme comparison to benchmarks. The rest of this paper is organized as follows: In Section 2, the system model and the main assumptions for the vehicular VLC/RF communication networks are introduced and the hybrid handover decision problem is proposed. In Section 3, the hybrid handover problem is illustrated as an MDP and the value iteration algorithm is applied for the optimal handover decisions. Section 4 shows a performance comparison of the proposed optimal handover scheme and other benchmark handover schemes. Finally, Section 5 concludes this paper.