# Structured Urban Airspace Capacity Analysis: Four Drone Delivery Cases
## 2 Urban Airspace Concept: Route Network over Roads  
Many studies have focused on the sUAS path planning problem [24; 34; 35; 36; 37; 38; 39; 40; 41; 42; 43; 44; 45; 46]. The majority of them solve the problem with the unstructured airspace assumption. For urban airspace users in high-density VLL urban airspaces, however, many difficulties are expected with sUAS studies, which are based on free routing and free flight operations, such as the risk of unforeseen crashes, the invasion of privacy, etc. On the other hand, structured and controlled airspace can predict crashes more easily than the unstructured airspace, and privacy can be considered in the airspace structural design stage.  
In this section, we suggest a route network-based urban airspace structure, which can be a potential candidate for the high-density VLL airspace structure. The proposed urban airspace structure concept resembles the airspace structure used in en-route phases of the ATM system today that can be considered as a combination of the Layers and Tubes concepts amongst four concepts Full Mix, Layers, Zones, and Tubes presented in [31]. The characteristics of the suggested airspace structure concept are as follows: consist of waypoints (nodes) and segments (edges) connecting some pair of waypoints; flight direction is strictly defined for all segments; at least two layers are stacked; separate traffic based on a time-based separation concept within each layer. As shown in Figure 1, for example, there is an eastbound layer and a westbound layer, and each layer consists of parallel-aligned northbound segments and southbound segments. Such a route network over roads with directions in urban airspace will increase the predictability of traffic by using flight plans. In this concept, the urban airspace structure can be thought of as a directed graph (or digraph) \(\mathcal{G}=(\mathcal{E},\mathcal{V})\), called an airspace graph to highlight the aerospace context. In the airspace graph \(\mathcal{G}\), node \(v\in\mathcal{V}\) is a point in Euclidean space of dimension two or three representing a waypoint candidate to be traversed. Edge \(e\in\mathcal{E}\), corresponding to a rectifiable curve, is a segment between some pair of waypoints in the airspace graph \(\mathcal{G}\).  
The main assumptions we make for the route network-based operations in this study are as follows: each flight route is composed of a series of linear segments as shown in Figure 2; each sUAS flies along each segment at a constant speed while it changes speed in  
Figure 1: An example of the proposed route network-based urban airspace structure concept.  
between segments; each sUAS is able to exactly follow the flight route without deviation; a time-based separation concept is utilised for collision avoidance; uncertainties produced by external sources are neglected such as adverse weather (i.e., true airspeed is considered); a control tower exists (i.e., flight plans are shared between sUASs and a control tower); the hovering and vertical manoeuvres are not considered; sUASs have limited payload and flight range; we only consider flights within each layer (e.g., no transitions between layers), but the transitions could be adopted by establishing the vertical segments in reality; homogeneous sUASs.