## 3 Networked Mobile Robot System Design Issues
### Communication  
Communication between robot units in the network is essential for real world applications because of situational awareness. In particular, cooperation and coordination in networked systems requires a robust communication ability to accomplish a given mission accurately. Communication methods in networked robot systems are classified as implicit communication, also called stigmergy, and explicit communication. The effect of communication on the system performance is shown in a variety of works; non-verbal communication efficiency in human-robot teamwork [96], target search task performance evaluation with no communication, reflexive and deliberative communication [97], communication range effects on robot search task on two distinct search algorithms which are spiral search and informed random search [98]. [PERSON] _et al._[98] developed a cooperative search method in complex environments and shows the effect of communication in the target search mission on real experimental setups. In these experiments, e-puck robots try to find a hidden object via explicitly communicating with their local neighbors. They concluded that the system performance improves with increasing communication range, in terms of the task accomplishment time. Robots can exchange information about the other robots' internal states and environmental conditions via explicit communication protocols, which also improves the system performance. Meanwhile, this yields a considerable computational burden to the robot team and these systems may not be robust to single point failures. Explicit communication is achieved using special standard communication protocols. Environmental conditions are important for explicit communication especially in indoor applications such as search and rescue operations; reliability and robustness may be corrupted due to noisy or failed signals in explicit communication techniques as seen in some special cases of references [99]. For example, in a semi-collapsed building, communication signal strength can be very strong only until one meter, whereas in outdoor environments, communication signals can generally easily cover 50 m with full bandwidth. Explicit communication approaches are also not suitable in the sense of scalability characteristics because of communication load and computational complexity [76]. In implicit communication techniques, information transmission is accomplished through the environment or via the observation of robot behaviors. Implicit communication based approaches were first introduced by [PERSON] _et al._[84], who showed that there is no need for explicit communication in some task executions. Their communication strategy is inspired from biological systems such as social behavior of animals, and they proved that explicit communication is unnecessary in graze tasks. Although implicit communication is simple, it can suffer from limitations in robots sensing abilities. Swarm robotic researchers have focused upon implicit communication to obtain emergent cooperation. This provides the opportunity for the colony's control algorithm to be scalable to large numbers. [PERSON] and [PERSON] [100] presented an implicit cooperation methodology for networked robots search in unknown areas with a reactive, layered architecture composed of three behaviors; namely, obstacle avoidance, stall recovery, and search. Each robot selects its next search area in the border of a locally sensed area so the algorithm bases upon local experience rather than the collective experience. It is proved that selection of local search goals increases the system performance, because it reduces interference between robots. Implicit communication between humans and robots in heterogeneous teams is extremely important in order to understand the internal state of the robot. [PERSON] _et al._[96] introduced such an implicit non-verbal communication technique between robot and human by using behaviors, social cues and gestures, without any deliberate communication.