# UVCS: Unit Virtual Coordinate System for UAV Intra-Swarm Routing in GPS-Denied Environment
## 2 Related Work
To be suitable for this task, virtual coordinates should reflect the position of nodes relative to each other in the virtual graphs similar to the physical world. To be applicable in practice, algorithms for deducing virtual coordinates should be distributed in nature, operating with minimal overhead by relying upon information about the closest proximity of nodes, for example, locations of neighbor nodes within the communications range [19]. Virtual coordinate systems (VCSs) can be classified into systems that rely upon the use of anchor nodes [20; 21; 22] and those that do not utilize this information [23; 24]. Table 1 summarizes the related algorithms according to the anchor involvement. Anchor nodes (or landmarks) are designed to serve as a guide for other nodes in determining their virtual coordinates. A set of anchor nodes can consist of nodes that have information about their geographic location or are assigned according to some rule. In general, the performance of VCSs systems relying upon the anchor nodes is better as compared to those, where this information is not utilized. In VCS algorithms that utilize anchor nodes, the location of the anchor nodes in the network is of special importance. In [25], the authors proposed two utilize the voting functions, whose values are assigned to network nodes and define the priority to be declared as anchor nodes. The authors propose two anchor node distribution patterns: (i) uniformly over the network at the maximum distance relative to each other and (ii) along the network boundary. The simulation results demonstrate that the proposed algorithm improves the efficiency of greedy forwarding, preserving the simplicity and high scalability of this routing technique. The authors in [27] utilized perimeter nodes as anchor ones and considered two cases: (i) when none of the nodes has knowledge about its position and (ii) when some of the nodes have location information. They proceed by proposing geographic routing protocols by replacing geographic coordinates with virtual coordinates. Their study was further continued in [28], where an approximate solution for the values of virtual coordinates in terms of fixed coordinates of peripheral anchor nodes is proposed. To find virtual coordinates, it is convenient to represent the network as a graph. Unit disk graph (UDG) models have proven useful in simulating a variety of real-world physics problems. In [29], UDGs are defined as follows: given \(n\) circles with the same radius in the plane, construct a graph with \(n\) vertices corresponding to \(n\) circles and edges between two vertices if one of the corresponding circles contains the center of the other. The UDG model has been repeatedly used in routing problems [26; 30]. Summarizing the related work, we note that the anchor-based algorithms proposed to date so far are a good fit for static networks. Although the complexity and execution time of the anchor node assignment algorithm is greatly reduced, this process requires both time and computational resources. In the cited works, anchor nodes are usually distributed along the edge of the network, but due to the nodes' mobility the edge nodes change, which will require the dynamical reassignment of anchor nodes. The choice of the number of anchor nodes is also not unambiguous. In our paper, we design an algorithm that efficiently utilizes only the information available on the neighboring nodes which solves the described above \begin{table}
\begin{tabular}{c c} \hline \hline
**Anchor-Based** & **Anchorless** \\ \hline ENS [20] & VCP [23] \\ Iterative Anchor Selection Algorithm [21] & GPSR [24] \\ Distributed EATL Anchor Node Selection Algorithm [22] & GAR [19] \\ Selforganized Landmark Algorithm [25] & UDG [26] \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Examples of VCS algorithms Classification. issue. Specifically, instead of choosing anchor nodes, we employ locally available information and received signal strength (RSS) to calculate the virtual coordinates of the nodes.