# Distributed Localization with Complemented RSS and AOA Measurements: Theory and Methods
## 1 Introduction  
Real-time localization systems have gained much attention in the research community in the past several years [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11; 12; 13; 14; 15; 16; 17; 18; 19; 20; 21]. In addition, works that highlight the potential of exploiting mobility of nodes in the localization process have emerged recently [22; 23; 24; 25]. This is mainly owed to extensive technological advances and the requirement for seamless solutions in location-based services. It is expected that fifth generation (5G) network will have the potential to interconnect an extensivenumber of heterogeneous (mobile or stationary) devices (objects, users, vehicles) into a single network (of nodes), called Internet of things (IoT) [26]. In large-scale networks with thousands of nodes, human - moderation is almost impossible, and autonomous network configuration becomes a very desirable feature. Therefore, the topic of localization is expected to attract even more interest in the future [27; 28; 29]. In fact, location-awareness is the key component in various other applications, such as emergency and health care services, intruder detection, precision agriculture, ocean data acquisition, or asset tracking, to name a few. For instance, location information is crucial for detecting and preventing fire hazards in forests, as illustrated in Figure 1, where a sensor network is employed to detect fire and, in cooperation with a fleet of autonomous aircraft, extinguish the danger [30]. A possible way to obtain a more accurate and granular information from the field is to employ a low cost, power-efficient, flexible, reconfigurable, and multifunctional smart sensing system (e.g., with programmable filters and power management units [31] or sensor-to-digital interface [32]). Such a system combines fixed and mobile sensors in a flexible wireless sensor network, which can acquire weather data, together with visible and infrared images for local classification of the fire front. These sensors could be installed on vehicles/aircraft involved in fire suppression, to reduce subjectivity of human-dependent analysis and improve operational coordination. This could be done by sending the location information of the target that sensed the fire danger (after processing it on its own) and fire intensity to the decision support systems to alert the system, in a similar manner as illustrated in Figure 1.  
In terms of outdoor environments, global navigation satellite system (GNSS) is considered to be a universally accepted solution, since it offers global coverage and excellent localization accuracy. Nonetheless, its service is limited or even unavailable in indoor or obstructed environments where line of sight to satellites is obstructed, such as dense urban areas, forests, or canyons. Instead, alternative solutions for accurate localization based on already deployed terrestrial technologies is encouraged (which can possibly complement the ones based on satellites). Such technologies include wireless fidelity, Bluetooth, narrow-band IoT, ultra-wide band, near-field communication, radio-frequency identification, signals of opportunity, third generation partnership project/long-term evolution, and inertial measurement units. On the other hand, in indoor environments (or other ones where satellite reception is not feasible), there is no convergence towards a single solution, and it is an open question if there will ever be, due to the high degree of difficulty of the problem in such harsh environments (multipath propagation, obstruction of a direct path, etc.) [7]. Figure 1: Application of a wireless sensor network in forest firefighting. In this survey, we study the problem of distributed localization based on an integration of a specific set of radio measurements. The problem is studied in detail from a signal processing point of view in its most general and widely used statistical form. A set of solutions to the problem, considered as the state-of-the-art, are presented and their main ideas are explained in a clear although informal fashion.