## 1 Introduction
### Connectivity-Based Proposals  
Despite its well-known inefficiencies, there exist some few approaches without any connection requirements where robots meet each other by chance. Nevertheless, this section only surveys the proposals that depend on connectivity in one way or another. In [21] a behaviour-based approach is presented. The architecture is designed to guide the exploration constraining the fleet to keep within the communication range, establishing a mobile network. The well-known disk model and a graph structure are used to model the network connectivity and identify possible disconnections. Frontier cells are evaluated regarding costs (computed utilising a flooding algorithm) and information utility (based on the ideas proposed in [3]). Behaviours are selected according to the network topology conditions. In [22] a centralised communicative exploration algorithm is proposed. Communicative exploration implies that the team of robots have to maintain connections between each other at all times. The target selection is based on a utility function that weights the benefits of exploring new regions versus the goal of keeping connected. While connectivity is valued using the classic _disc_ model, the costs of the shortest paths are computed from the Manhattan distance notion. Due to spatial and movement restrictions, specific behaviours are defined to deal with deadlocks. Also following a centralised approach, [23] presents four fully reactive exploration strategies. They consist in translating the distance to tasks and disconnection situations into artificial forces that pull and push the robot to reach new positions smoothly, avoiding them to lose connectivity. The radio signal quality is modelled considering both the communication range and the distance attenuation effect. Deadlocks are avoided by assigning tasks to a cluster of robots. This allocation guarantees that robots belonging to the same cluster do not exert conflicting forces upon each other towards different directions. In [24], the authors propose a decentralised version of the strategy proposed in [22] based on message exchanging and a graph structure where the group always tries to keep a biconnected network efficiently. Communication model is based on the classic _disc_ model. In consequence, robot mobility is restricted by the communication range. Using the same graph theory, in [25] the experimental validation of a distributed algorithm that preserves connectivity is also discussed. Nevertheless, a different coordination mechanism--supported by a market-based negotiation algorithm--is adopted. Unfortunately, only results on connectivity maintenance are shown, lacking exploration metrics reports. The proposal of [26] aims to maintain and repair the underlying wireless mesh network while the coverage task is being performed, all at once. The system works in a fully asynchronous and distributed way. Differently from previous works, the authors propose a network disconnection detection by checking the real state of connections without assumptions on communication range or propagation model. On the contrary, all nodes require knowledge about the area to be covered and on global positions. In [15] the robots can disconnect as long as they regain connectivity periodically following a distributed but synchronous strategy. Authors address coordination implicitly through localisationdata exchanging. Robots are forced to wait for others before making a decision. The system works as an optimisation method where each variable is optimised at a time in a round-robin while the others remain unchangeable. In [27] the authors describe a heterogeneous multi-robot system for exploration tasks. They consider several explorer robots and conceive a particular robot playing the role of relay dispenser. This agent is in charge of place relays when and where it is necessary to support the video/audio streaming generated by explorers. In [28] the problem of exploration and mapping is addressed by using a _Decentralised POMDP_.