## 1 Introduction  
Recent advances in pervasive computing, communication and sensing technologies are leading to the emergence of wireless sensor/actuator networks (WSANs) [1,2]. A WSAN is a distributed system of sensor nodes and actuator nodes that are interconnected over wireless links. Sensors gather information about the physical world, e.g., the environment or physical systems, and transmit the collected data to controllers/actuators through single-hop or multi-hop communications. From the received information, the controllers/actuators perform actions to change the behaviour of the environment or physical systems. In this way, remote, distributed interactions with the physical world are facilitated. Depending on the type of the target application, nodes in a WSAN can be either stationary or mobile. In many situations, however, sensor nodes are stationary whereas actuator nodes, e.g., mobile robots and unmanned aerial vehicles, are mobile. Sensor nodes are usually low-cost, low-power, small devices equipped with limited sensing, data processing and wireless communication capabilities, while actuator nodes typically have stronger computation and communication powers and more energy budget that allows longer battery life [3]. Regardless, resource constraints apply to both sensors and actuators. WSANs are not just an enhancement or complement to the intensively-investigated wireless sensor networks (WSNs) [4-8], but go beyond. They are a new generation of sensor networks [2,3]. While WSANs and WSNs share many common considerations concerning network design, such as reliability, connectivity, scalability and energy efficiency, the coexistence of sensors and actuators in WSANs causes substantial difference between these two types of networks. Applications in which some actions are introduced for the purpose of enhancing the monitoring capability of the sensor networks do not embody the essential characteristics of WSANs. On the contrary, actuators in a WSAN should be an integral part of the network and perform actions interacting with the physical world. As a consequence, WSANs have the ability to change the physical world, but WSNs do not. In WSNs, power consumption is generally the primary concern; however, this may not be the case in some WSANs where meeting the real-time, reliable communication requirements may be more important [9]. Although there are many situations in which only WSNs are required, for example, environment monitoring, product quality monitoring, and the like, there are an increasing number of applications that necessitate the use of actuators along with sensors [10-12]. That is, the network system needs to interact with the physical system or environment. Examples of application areas of WSANs include disaster relief operations, intelligent building, home automation, smart spaces, pervasive computing systems, and cyber-physical systems. Because of the use of both sensors and actuators, WSANs, by definition, exploit the methodology of feedback, which has been recognized as the central element of control systems [13]. The advent of WSANs has the potential to revolutionarily promote existing control applications. It can be envisioned that WSANs will become the backbone of many control applications enabling an unprecedented degree of distributed control. The use of WSANs in control applications has many advantages compared to wired solutions, which are dominant at the moment [14,15]. For instance, WSANs allow more flexible installation and maintenance, fully mobile operation, and monitoring and control of equipments in hazardous and previously difficult-to-access environments. Another important factor that instigates the deployment of WSANs is their relatively low costs [11]. Despite many advantages, WSANs also raise challenges for control applications. Wireless channels have adverse properties, such as path loss, multi-path fading, adjacent channel interference, Doppler shifts, and half-duplex operations [16]. WSANs are known to be notoriously unpredictable and inherently unreliable.