Power efficient ultra-wideband based wireless communication for wireless body area network applications

Abstract

Ultra-wideband (UWB) is a lucrative wireless technology for Wireless Body Area Network (WBAN) applications requiring a power restricted operation, high data rate and high level of miniaturisation. An Impulse Radio-UWB (IR-UWB) system transmits data by means of short duration pulses. Because of the pulse-based nature of this data transmission, less-complex modulation schemes, such as Pulse Position Modulation (PPM) and On-Off Keying (OOK), can be used in IR-UWB based systems resulting in significant power savings. Because of the simplicity of pulse generation, IR-UWB transmitters require a reduced hardware complexity compared to other candidates such as Orthogonal Frequency Division Multiplexing (OFDM) based UWB, enabling a low form factor and improved power efficiency. This dissertation contributes to the state of the art in the IR-UWB technology for WBAN applications in three major areas, namely: • Power efficient Medium Access Control (MAC) protocol design for IR-UWB based WBAN applications. • Development of a low-power sensor platform and evaluation for on-body communication. • Feasibility analysis of the IR-UWB technology for human implant applications through study of the electromagnetic and thermal effects caused by UWB signals. Although IR-UWB transmitters provide excellent benefits, such as high data rate transmission, low power consumption and simple hardware implementation, IR-UWB receivers remain a bottleneck for the use of IR-UWB technology in WBAN applications because of the following reasons. Firstly, IR-UWB receivers are complex in design and consume excessive amount of power. Secondly, the synchronisation of IR-UWB pulses at the receive stage using low power front-end circuitry is a major difficulty that restricts the use of IR-UWB receivers in WBAN applications. An IR-UWB based communication system has to be designed in a manner such that it enhances the advantages provided by IR-UWB transmitters while avoiding the complexities introduced by IR-UWB receivers. In order to achieve this objective, this dissertation presents a dual band communication system that uses IR-UWB for data transmission from sensor nodes while using narrow band technology for data reception. The MAC protocol plays a vital role in determining the data transmission efficiency of IR-UWB-based WBAN applications. Unlike in wireless narrow band applications, MAC protocols for IR-UWB applications should be designed in a manner so as to incorporate the unique advantages provided by the physical layer properties of IR-UWB signals. This dissertation presents the design and evaluation of a new dual band MAC protocol for WBAN sensor nodes. This MAC protocol improves the advantages provided by IR-UWB signals, such as high data rate transmission and low power operation, while avoiding the complexities introduced by IR-UWB reception at the sensor node end. In addition, data priority is taken into consideration during the design of the MAC protocol and a guaranteed delivery mechanism is utilised to transfer high priority data. The performance of the suggested MAC protocol is analysed extensively using simulations in terms of critical parameters, such as packet error rate, throughput, packet delay and power consumption. These simulation studies have provided optimised design parameters for hardware implementation of sensor nodes that use the proposed MAC protocol for WBAN applications. Although IR-UWB has been identified as a viable wireless technology for WBAN applications, there are no full implementations of low-power and small size IR-UWB sensor platforms for healthcare monitoring applications in both commercial domain and research domain. This dissertation presents the implementation and evaluation of a complete communication platform for WBAN applications that includes sensor nodes, coordinator nodes and interfacing computer software. In order to overcome the limitations created by the use of IR-UWB receivers in IR-UWB based sensor platforms, this dissertation presents an implementation method for WBAN sensor nodes that uses IR-UWB for data transmission from sensor nodes to coordinator nodes (up-link) and a narrow band link to receive control messages from coordinator nodes to sensor nodes (down-link). This unique technique provides the means of achieving low-power consuming sensor nodes with high data rate capability. Wireless communication for implantable devices is another potential area for the use of the IR-UWB technology in WBAN applications. Implantable systems, such as neural recording systems and wireless endoscopy devices, require high data rates, low power consumption and small form factor. These requirements can be fulfilled by the use of IR-UWB technology. This dissertation demonstrates the feasibility of the IR-UWB technology for implant applications through the study of electromagnetic and thermal power absorption of human tissue that is exposed to IR-UWB signals. These studies are conducted through finite element simulations using human body models. In addition, the path loss of UWB signals for various implant applications, such as neural recording units and capsule endoscopy systems, are studied using numerical modelling techniques and provides a predictive model that can be used as a guide in designing IR-UWB systems for implant applications. The findings of this research contribute to the improvement of wireless healthcare monitoring systems by introducing a high data rate, low power and efficient communication mechanism. The dual band sensor nodes that are developed as the final product of this research are capable of operating at scalable data rates up to 5 Mbps with a power consumption as low as 11 nJ/bit. The unique features of the MAC protocol suggested in this dissertation enable the dual band communication system to operate in varying channel conditions with a controllable BER around 10-4. Simulation based analysis carried out in order to investigate the feasibility of IR-UWB for implant communication shows that it is possible to set the IR-UWB transmit power in the in-body channel at a higher level than the FCC spectral mask such that tissue power absorption levels fall within the regulatory levels while outdoor power levels meet the FCC spectral requirements. These peak transmit power levels depend on the position of the implant within the human body. The path loss analysis carried out for UWB based Wireless Capsule Endoscopy (WCE) devices shows an average path loss of 80 dB for an in-body propagation distance of 80mm.