Design and Performance of Diversity based Wireless Interfaces for Sensor Network Nodes

Abstract: The main focus of the work presented in this licentiate thesis concerns antenna design, adaptive antenna control and investigation on how the performance of small wireless nodes can be increased by inclusion of multiple antennas. In order to provide an end-user suitable solution for wireless nodes the devices require both small form factor and good performance in order to be competitive on the marked and thus the main part of this thesis focuses on techniques developed to achieve these goals. Two prototype systems have been developed where one has been used by National Defence Research Agency (FOI) to successfully monitor a test-subject moving in an outdoor terrain. The other prototype system shows the overall performance gain achievable in a wireless sensor node when multiple antennas and antenna beam steering is used. As an example of how to include multiple antennas in a wireless node the concept of using dual conformal patch antennas for wireless nodes is presented. The proposed antenna showed an excess of 10 dB gain when using a single driven antenna element as would be the case in a system utilizing antenna selection combining. When used as a 2-element phased array, up to 19 dB gain was obtained in a multiscattering environment. Using the second order resonance the proposed antenna structure achieves low mutual coupling and a reflection coefficient lower than -15 dB. The presented antenna design shows how a dual antenna wireless node can be designed using discrete phase control with passive matching which provides a good adaptive antenna solution usable for wireless sensor networks. The inclusion of discrete phase sweep diversity in a wireless node has been evaluated and shown to provide a significant diversity gain. The diversity gain of a discrete phase sweep diversity based system was measured in both a reverberation chamber and a real life office environment. The former environment showed between 5.5 to 10.3 dB diversity gain depending on the detector architecture and the latter showed a diversity gain ranging from 1 to 5.4 dB. Also the performance of nodes designed to be placed in a high temperature and multiscattering environment (the fan stage of a jet engine) has been evaluated. The work was carried out in order to verify that a wireless sensor network is able to operate in such a multiscattering environment. It was shown that the wireless nodes are able to operate in an emulated turbine environment based on real-life measured turbine fading data. The tested sensor network was able to transmit 32 byte packages using cyclic redundancy check at 2 Mbps at an engine speed of 13.000 rpm.