Cooperative multi-hop wireless sensor-actuator networks: exploiting actuator-cooperation and cross-layer optimizations

Delay and energy constraints have a significant impact on the design and operation of wireless sensor-actuator networks (SANETs). Furthermore, preventing sensor nodes from being inactive is very critical. The problem of sensor inactivity arises from the path loss and fading that degrades the quality of the signals transmitted from actuators to sensors, especially in anisotropic deployment areas, e.g., rough and hilly terrains. Sensor data transmission in SANETs heavily relies on the scheduling information that each sensor node receives from its associated actuator. Therefore if the signal containing scheduling information is received at a very low power due to the impairments introduced by the wireless channel, the sensor node might be unable to decode it and consequently it will remain inactive. In this paper, it is proposed that each sensor node transmits its data to only one of the actuators. However all actuators cooperate and jointly transmit scheduling information to sensors with the use of beamforming. This results to an important reduction in the number of inactive sensors comparing to single actuator transmission for a given level of transmit power. The reduction is due to the resulting array gain and the exploitation of macro-diversity that is provided by the actuator cooperation. In order to maximize network lifetime and attain minimum end-to-end delays, it is essential to optimally match each sensor node to a particular actuator and find an optimal routing solution. A distributed solution for optimal actuator selection subject to energy-delay constraints is also provided.