Cooperative Beamforming and Modern Spatial Diversity Techniques for Power Efficient Wireless Sensor Networks

Wireless Sensor Networks (WSN) have been attracting great attention recently. They are relatively low cost to be deployed and to be used in many promising applications, such as biomedical sensor monitoring (e.g., cardiac patient monitoring), habitat monitoring (e.g., animal tracking), weather monitoring (temperature, humidity, etc.), low-performance seismic sensing, environment preservation and natural disaster detection and monitoring (e.g., flooding and fire) Lewis (2004); Tubaishat & Madria (2003); Stankovic et al. (2003); Akyildiz (2002); Rashid-Farrokhi et al. (1998). TheWSN applications analyzed in this chapter have a topology where a large number of wireless sensor nodes are spread out over a large or small geographic area (e.g., disaster regions, indoor factory, large sports event areas, etc.). In this topology, an inefficient use of bandwidth and transmitter power resources is resulted if each wireless sensor is transmitting its measurement data to the base station (processing central). In this case, each sensor node would have to be assigned its own frequency channel and, if the base station is located a long distance from the sensor nodes, it would also demand a higher than average sensor node transmitter power. By using a coordinating cluster head, for each cluster of wireless sensor nodes, we can instead use the combined transmitter power of the node cluster through the use of beamforming to increase the transmitter-receiver separation and/or to improve the signal-to-noise ratio (SNR) of the communication link. Another advantage of using this cooperative transmission is that we can exert power control to minimize the power consumption of each individual sensor node, and thus maximizing network lifetime. In addition, in a cooperative network the measurement data could be sent by using Time Division Multiplexing (TDM) instead of Frequency Division Multiplexing (FDM) which improves the overall bandwidth efficiency of the system. The spatial properties of wireless communication channels are extremely important in determining the performance of the systems. Thus, there has been great interest in the application of beamforming andmodern spatial diversity techniques (ormultiantenna systems) since they 1