# Distributed Localization with Complemented RSS and AOA Measurements: Theory and Methods
## 2 RSS-AOA Distributed Localization Problem  
Future networks are expected to be composed of heterogeneous devices. Here, we use a generic term, _nodes_, to denote (any kind of) these devices. Note that wireless localization systems typically assume the presence of some reference nodes (usually called anchors or landmarks) deployed at fixed known locations (or equipped with GNSS receivers to determine their locations) and one or more nodes with a location one desires to determine (often referred to as targets or agents). Across the literature, the used terminology is not universal, and it frequently depends on the deployed technology. For instance, in cellular networks, the term base station is employed for anchors, whereas the term mobile station is utilized for a target node (the term user equipment is frequently used in the literature, as well) [33].  
In general, the solution to localization problem comprises two phases: collecting measurements and exploiting them together with some known reference points to estimate target locations [34]. In the former phase, nodes communicate between themselves in order to acquire (radio) measurements. Based on the type of communication, we here distinguish two types of networks: non-cooperative (targets communicate with anchors exclusively) and cooperative (targets communicate with any nodes within their communication range), as illustrated in Figure 2. While in non-cooperative networks one takes advantage of known locations of anchors, which minimizes the error propagation due to localization inaccuracy, in many applications the number of anchors is scarce, and due to limited resources of nodes (e.g., battery life), only some of them can establish direct communication with anchors; hence, node cooperation is required in order to determine locations of all targets. It is well known that node cooperation can bring advantages and lead to _better_ decisions, especially when connectivity in the network is limited [35; 36; 37].  
The most frequent radio measurement techniques utilized in practice are time (difference) of arrival, round trip time, phase of arrival, received signal strength (RSS) and angle of arrival (AOA). Moreover, the notion of hybrid system in which two (or more) measurements are integrated has gained much popularity lately [4; 7; 12; 38]. Even though it is intuitive that more information should lead to better localization accuracy, one should note that joining any two measurements does not necessarily bring advantage in every scenario [12; 39]. In the latter phase, the measured quantities are converted into distance or direction estimates, which combined with some known reference locations are processed to determine locations of interest.  
Depending on where the information processing is occurring, we discern two types of networks: centralized (assumes existence of a sink or a central node (CN) in the network which collects all measurements gathered in the network and executes all information processing) and decentralized or distributed (each target node collects measurements from its direct neighbors and executes all information processing by itself, usually in an iterative fashion), as illustrated in Figure 3. Both types of networks have their pros and cons: the centralized ones are more stable, but their computational burden becomes too costly as the number of nodes in the network increases, whereas the computational cost in distributed networks depends only on the size of local neighborhood of a target, but distributed processing often requires iterations which might disseminate localization errors across the entire network [40]. Here, we will concentrate on the distributed cooperative networks exclusively, both because in 5G networks we are expected to deal with networks with a large number of nodes, and because some of them might not wish to reveal their private content (e.g., their objective functions) to other nodes.  
Figure 2: Illustration of different network types based on the type of communication.