Abstract
A study is presented of wireless on-body communicationlinks, with the body in motion. This paper compares simulationsand measurements of the path gain (PG) on moving male andfemale bodies. Simulations using avatars derived from an animationsoftware have been performed at 2.45 GHz. Male and femaleavatars walking and a male rising from a chair have been simulatedand the results are compared with measurements carried outin an anechoic chamber. The results show that good agreement betweensimulation and measurement of slow fading features can beachieved for a reasonable computational effort.
Index Terms—Body area networks, computer simulations,on-body propagation channel, wireless communication systems.
I. INTRODUCTION
THE growing miniaturization of electronic devices combinedwith the recent developments in wearable computertechnology have been leading to the creation of a wide rangeof devices which can be carried in a pocket or attached to auser’s body [1]. These applications have allowed the removalof the need for wired interconnections and have led to the riseof the concept of the wireless body area network (WBAN) [2],[3]. Propagation channels may extend from the body to a localbase station or between two antennas on the body. In this workthe on-body channels are considered, although the conclusionsdrawn may also be applicable to off-body communication linkswith a local base station. In either case, a full understanding ofthe channel is vital for the design of very low power transceiversand associated baseband processing, which maximize the batterylife of body worn equipment. WBAN channel characterizationhas been performed using measurements on human subjects performing a number of common activities [4], [5]. Movementof the body in a scattering environment causes channel fading.This has been characterized into long and short term fading.Long term fading is due to relative movement of the variousbody parts which leads to shadowing of the antennas by the bodyparts as well as changes of the path length and of the relative orientationof the antennas (thus changing the antenna gain). Shortterm fading is due to multipath interference effects, where multipathmay be present due to the different signal paths aroundthe body and from scattering (or multiple reflections) of signalpower off the local environment.Simulation has the potential to allow channel characterizationand hence reduce the time and cost of system design.However, its application to WBANs is complicated by the needto model body movement, which plays an important part in thechannel behavior. There are two issues. Firstly, there is a needto get a body phantom which can assume the relevant postureof the human body during its motions. Secondly, the simulationof microwave antennas on the body is a multi-scale problemrequiring long durations of computational time. Much workon the latter problem has been performed. Finite differencetime domain (FDTD) and finite element method (FEM) havebeen used for simulation of whole bodies with wearable antennas[6], for a single or a few stationary postures. Advancedcomputational methods have been used to improve performance,including conformal grid methods [7], and equivalentsource models [8]. Various numerical phantoms are available,including the Taro and Hanako Japanese human voxel models[9], or Norman human model [10]. Several software packagesused to manipulate posture are also available [11], [12].However, extraction of channel fading characteristics requiresmodeling of small changes in body posture as it moves. Thesampling rate of the motion is determined based on whether itis desired to extract long term or short term fading information.To the authors' knowledge, the modeling of small changes inbody posture to characterize on-body communication channelshas not been demonstrated. It is the objective of this paper todemonstrate the feasibility of an approach that combines theuse of animation software with conventional electromagneticsolvers to allow long term channel fading to be characterized[13]. The method is, in principle, extendable to short term fadingcharacterization. The simulation methodology has been verifiedby comparison with measurements. The achieved resultsare found to depend on the quality of the phantom that hasbeen utilized. A comparison of the transmission path gains (PG)for static postures of different types of phantom are presented.


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