Helena Fernández-López

Performance evaluation of a ZigBee-based medical sensor network

Low power consumption and small footprint make 802.15.4/ZigBee based devices well suited for personal healthcare applications, representing a promising alternative to patient monitoring under important scenarios such as emergency, postop, continuous care, and chronic diseases. However, their use in a healthcare facility to monitor several mobile patients poses several difficulties, mainly because this protocol was primarily designed to operate in low data rate scenarios. This paper presents simulation results used to evaluate important quality of service (QoS) markers and, ultimately, estimate the maximum number of sensors that could integrate a wireless vital signs monitoring system. Results show that the system is able to carry the signals from 30 ECG sensors with delivery ratio higher than 99% in the considered scenario, provided that an adequate number of retransmissions are allowed.

Evaluation of the Impact of the Topology and Hidden Nodes in the Performance of a ZigBee Network

Low power and small footprint IEEE 802.15.4/ZigBee based devices are a promising alternative to 802.11a/b/g and proprietary protocols for non-critical patient monitoring under important scenarios such as post-op and emergency rooms. However, their use in a healthcare facility to monitor several mobile patients poses several difficulties, mainly because these protocols were primarily designed to operate in low traffic load scenarios. This work presents simulation results used to evaluate the performance of an IEEE 802.15.4/ ZigBee based wireless sensors network (WSN) in a vital signs monitoring scenario, for both star and tree based network topologies. The scalability problem in non-beacon enabled networks is addressed to quantify the degradation in quality of service (QoS) markers when the number of sensor nodes increase. Additionally, the impact of hidden nodes is assessed for the star topology. Results indicate that, to achieve a delivery ratio (DR) higher than 99%, the number of electrocardiogram (ECG) nodes in a star network must not exceed 35. However, considering a tree topology, the maximum number of nodes must be reduced to 18 to maintain the same DR. The network performance is severely impacted by hidden nodes. For instance, in the absence of hidden nodes, a star network consisting of 32 ECG nodes presents a DR higher than 99%; however, if the percentage of hidden nodes is increased to 5%, it drops to 94%. If the same percentage of hidden nodes is maintained, it is necessary to reduce the number of nodes to 13 to reestablish a 99% DR.

ZigBee-based remote patient monitoring.

This paper describes a developed continuous patient monitoring system based on the ZigBee protocol. The system was tested in the hospital environment using six sensor devices in two different modes. For electrocardiogram transmission and in the absence of hidden-nodes, the system achieved a mean delivery ratio of 100% and 98.56%, respectively for star and 2-hop tree network topologies. When sensor devices were arranged in a way that three of them were unable to hear the transmissions made by the other three, the mean delivery ratio dropped to 83.96%. However, when sensor devices were reprogrammed to transmit only heart rate values, the mean delivery ratio increased to 99.90%, despite the presence of hidden-nodes.

The Need for Standardized Tests to Evaluate the Reliability of Data Transport in Wireless Medical Systems

Wireless medical systems are comprised of four stages, namely the medical device, the data transport, the data collection and the data evaluation stages. Whereas the performance of the first stage is highly regulated, the others are not. This paper concentrates on the data transport stage and argues that it is necessary to establish standardized tests to be used by medical device manufacturers to provide comparable results concerning the communication performance of the wireless networks used to transport medical data. Besides, it suggests test parameters and procedures to be used to produce comparable communication performance results.

Experimental Evaluation of IEEE 802.15.4/ZigBee for Multi-patient ECG Monitoring

IEEE 802.15.4/ZigBee wireless sensor networks (WSNs) are a promising alternative to cabled systems for patient monitoring in hospitals. Some areas where monitoring systems based on WSNs can be successfully used are ambulatory, waiting and triage rooms, post-op, and emergency rooms. The low power and small size ZigBee devices have the ability to form self-configuring networks that can extend themselves through a hospital wing or floor. Using spatially distributed networks, it is possible to cover an extended area and serve several patients. However, the low data rate protocols provided by IEEE 802.15.4 poses several challenges, mainly because its protocols were primarily designed to operate in low traffic load scenarios, whereas some vital signs sensors generate a large volume of data. This work presents an experimental evaluation of the performance of multi-hop ZigBee networks comprised of several nodes that carry the traffic of wearable electrocardiogram (ECG) sensors. The results indicate that star networks can relay 100% of the traffic generated by at least 12 ECG nodes. In tree topologies, the increase of the network traffic load reduces the performance but even these networks can reliably relay the traffic of a considerable number of ECG nodes.