###### Abstract
In FANETs, there are five different types of communication links: UAV-to-ground base station link (UAV/BS), ground base station-to-ground base station link (G/G), UAV-to-UAV link (UAV/UAV), UAV-to-satellite links (UAV/Satellite), and UAV-to-sensor device links (UAV/X). UAV/BS communication connections send data from a UAV in the air to a ground base station, such as real-time video or pictures. G/G connections allow several ground base stations to communicate and share information with each other. UAVs can function in ad hoc communication in UAV/UAV, where they must interact with one another to reach a consensus and share data. UAV/Satellite provides a communication link between UAV and satellite at high altitudes. Finally, the UAV/X link gathers information from sensors or mobile devices on the ground. A routing protocol is necessary for data transfer between UAV nodes. Traditional ad hoc network routing protocols designed for VANETs and MANETs are often inadequate to suit the demands of FANETs [21]. FANETs have unique characteristics that make developing reliable routing protocols challenging, such as flying in three dimensions, high mobility, low node density, rapid topology changes, often-severed links, network segmentation, and limited resources [22]. However, UAV can cooperate with VANETs to assist the process of routing data packets to meet constraints of delay and minimum overhead [23] and also detection of malicious vehicles attacks [24]. Additionally, several FANETs applications have various quality of service (QoS) requirements that need to be adjusted. Although several applications such as data collecting and mapping can tolerate  
Figure 2: (**a**) Single-UAV network; (**b**) multi-UAV network. delays, others, such as monitoring and tracking, and search and rescue (SAR), need real-time data flow with minimal delays. Consequently, various studies have been conducted to design routing protocols that consider application requirements and the specific properties of FANETs. These are either novel routing protocols [25] or updates to existing ad hoc routing protocols [26]. Several reviews have discussed the different FANETs routing techniques [27; 28; 29; 30; 31; 32; 33; 34; 35]. Table 1 summarizes the key contributions and limitations of these reviews. The difference in this study is that it covers modern topology-based routing protocols designed specifically for FANETS, which are not covered yet in the literature. In this study, the latest state-of-the-art developments in relation to the primary features, limitations, routing method, and application scenario for the routing protocols are covered. To pursue these objectives, an attempt was made to integrate many thoroughly examined and thought-provoking solutions of FANETs routing techniques to achieve precise and concrete deduction for interested researchers. The major contributions to the knowledge of this study are summarized as follows:  
1. An in-depth look into existing topology-based routing protocols in FANETs. A review and comparison of topology-aware routing protocols explicitly designed for FANETs with other studies considering classical rioting protocols is presented. 2. Topology-based routing protocols classification for FANETs using the fundamental routing mechanisms. There are 22 topology-based routing protocols studied and described, both existing and recent. 3. The reviewed topology-based routing protocols are compared qualitatively on the main features, routing mechanism, limitations, mobility models, simulation tools, performance parameters, and application scenarios. Existing studies do not consider all these parameters in comparative analysis. Moreover, engineers and researchers may find this comparison useful in deciding which topology-based routing protocol is appropriate for their needs. 4. The most critical research challenges and issues in developing a topology-based routing technique for FANETs are updated based on this field's current active research progress.