**1.1. Wireless multi-robot sensor networks**

In wireless MRSN, from a viewpoint of sensor network communication, each robot senses data and transmits data to the adjacent lower node. To collect all data at the sink, data are sent by relay nodes in a multi-hop manner. However, due to the mobility of the robot,

sometimes robots are out of the network area so that break the network connectivity. Hence, how to keep the connectivity all the time is a crucial issue so that it already became a hot research topic (Mi et, al., 2010). Besides, the reliability and robustness as well as secure et al. are some other concerned research topics. With respect to communication techniques in MRSN, a network with the goal of search and rescue is described in (Reich & Sklar, 2006). In this paper, they proposed an entirely distributed gradient propagation (GP) algorithm. Each sensor in the paper independently executes the GP algorithm and broadcasts after some independent, randomly chosen time interval. The robots sensors estimate their target by "hot" values and "cold" values, where the "hot" values become the searching target. In (Sheng, et al. 2006), for reliability and robustness, a distributed biding algorithm was proposed for multiple robots in exploration tasks to address the problems caused by the limited communication range. In this algorithm, all the robots work asynchronously. There are three states for each robot that (1) sensing and mapping, (2) bidding and (3) traveling. A distributed algorithm that makes mobile robots in a multi-robot system aware of network connectivity was discussed in (Leyzx, et al., 2009). The basic idea is to take a "fixed" robot as the reference robot that keep in touch with at least one neighboring robot from which a communication path to the reference robot can be established.
