Development of a transmit power control scheme for Zigbee Healthnets

Abstract

With the advent of low rate wireless personal area networks (LR-PAN) and wireless sensor networks (WSNs), the standard ZigBee / IEEE 802.15.4 has been developed. Having been designed with power consumption as one of its main challenges to address, the standard does not, however, make full use of readily available features of transmission power adaptability. This seriously costs the network in terms of power, lifetime, as well as some issues with traffic such as the hidden terminal problem. Moreover, and due to the variety and nature of WSN applications and especially for indoors environments, simulation does not provide the most reliable means for testing and evaluating WSN behavior, a test-bed is required for reliable evaluation. The goal of this research is to develop a scheme to control transmit power in order to prolong the WSN lifetime and to test it in actual sensor network. The basic idea behind the transmission power control is for the sender to receive a feedback from the destination whose received signal strength is used to approximate to determine the transmit power level to be used for future packet transmissions. In order to achieve this, an experiment is carried out to study path loss in the RF Design Laboratory in Block E-4, Kulliyyah of Engineering as well as highlight issues with indoor propagation in general. Then the empirically calculated parameters (path loss index and shadowing) are used to create the relation between received signal strength indicator (RSSI) and transmitter-receiver distance in a path loss model using the log-distance method. The path loss model is then used in the mobile node to determine the transmit power level required for successful delivery at lower power consumption based on RSSI of a periodically broadcast signal from the base station. The scheme is tested within the RF Design Lab to evaluate power saving quantitatively. It is found that mobility would greatly affect the packet delivery rate. However, successful packet delivery was lower at 92.5% with the brisk movement compared to 98.9% in the default full power transmission mode. This drop in delivery success is accompanied by power saving of 70% in the brisk movement scenario and 87% in slow movement scenario. The RSSI proves to be a very reliable indicator and development in transceiver power control may provide further saving opportunities.