Energy-efficient communication algorithms for wireless sensor networks

Abstract

This thesis is motivated to tackle the problem of decreasing sensor nodes' energy consumptions in wireless communications and extending the lifetime of wireless sensor networks (WSN) in providing satisfactory services of data sensing and transmission. To this end, studies in this thesis are focused on theoretical aspects, aiming at developing communication algorithms in the network layer to effectively organize sensor nodes and in the physical payer to directly reduce a node's energy expenditure in wireless communications, respectively. Via simulations the theoretically developed algorithms and the performances of the sensor networks based on the developed algorithms are evaluated. As for the network layer algorithm, the Slotted Waiting period Energy-Efficient Time driven (SWEET) clustering algorithm is developed. The SWEET algorithm aims at organizing sensor nodes in the form of clusters where energy-rich Cluster Head nodes are selected and distributed evenly over the network area to coordinate the communications among cluster member nodes. The SWEET algorithm uses the distribution of nodes' remaining energies to achieve its design goal. To organize densely deployed sensor nodes, the SWEET algorithm is decentralized using the distribution of the residual energies of nodes in a node's neighbourhood area. The empirical probability density function of neighbourhood node energy distribution is obtained via Hello Message Exchange (HME). The procedure of HME is carried out based on the Birthday protocol and the Carrier Sensing Mini-Slot algorithm which is modified from the solution for the initialization problem. The time and the node energy required for the procedure of HME based on the considered methods are investigated with respect to the message exchange sufficiency. By simulations, the effectiveness of the SWEET and the decentralized SWEET algorithm is confirmed. Also by simulations, the performances of the networks based on these two algorithms are evaluated. The simulation results show that the developed algorithms outperform several competing clustering algorithms in significantly improving the network lifetime and data capacity at various cluster radii and network node densities. As for the physical layer algorithm, chip-interleaving signal processing is employed to save a node's energy in transmitting data in fading channel. The Bit Error Rate (BER) expressions of two Direct Sequence Code Division Multiple Access (DS-CDMA) systems with embedded chip interleaving are derived to determine how effective the chip interleaving is in decreasing the signal power loss due to the channel fading. With the derived BER expressions, the energy savings of networks based on sensor nodes that use the direct sequence spread spectrum (DSSS) transceivers with or without embedded chip interleaving are analyzed. The considered DSSS transceivers are compliant with the physical layer specifications in the IEEE 802.15.4 standard. The randomly deployed nodes are organized based on the studied clustering algorithms, in particular the SWEET algorithm. By simulations, the correctness and accuracy of the developed BER expressions are confirmed. Simulation results also show that the lifetime of a cluster-based WSN can be extended to a great extent when the chip interleaved transceivers are used by nodes to transmit data in flat Rayleigh fading channel. In summary, the energy efficiency of a sensor network can be significantly improved by utilizing the SWEET algorithm and the chip interleaving technique, individually or in combination.