Chapter 1 - Introduction
1-1 Background
Wireless Body Area Network (WBAN) is wireless communication systems which transmit information and data near the human bodies. In WBAN, some proper devices and sensors are applied to human body and everyday clothing for short range communication and monitoring the human health. In other words, the WBAN sensors can supervise the vital signs of patients, police, or fire personnel, and then send the data to a center device for remote monitoring [1]
as shown in figure 1.1. Moreover, the police, fire personnel and military can also employ the WBAN system for short range communication within the group. Since the Body Area Network communication system can be applicable in various environments and purpose, the topic becomes an active and worth research.
With increasing attention toward WBAN, the ultra-wideband (UWB) technology is an attractive solution for WBAN applications because of its low transmiting power and high data
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Chapter 1 Introduction
rates. The IEEE 802.15.4a task group [2], who has developed an ultra-wideband (UWB) standard regarding Physical and Media Access Control layer, involves the body area network as a relevant scenario. In 2002, the Federal Communication Commission (FCC) had also allocated the spectrum from 3.1 GHz to 10.6 GHz for UWB unlicensed applications.
For Body Area Network, the antenna is placed on the human body for transmission so the application confines to the antenna design. The antenna should be unobtrusive and conformal in order to avoid interfering in the human‟s daily activities. Furthermore, the antenna performance such as bandwidth and radiation patterns could be interfered after placing on the human body. The Specific Absorption Rate can be also an issue which should
Fig. 1.1 The schematic of Body Area Network
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Chapter 1 Introduction
be considered in antenna design. Owing to these restrictions on antenna characteristics, designing the UWB antennas used in BAN is a challenge and a worth topic.
1-2 Related Works and Motivations
In past few years, there have been various works [3, 4] which discuss the channel model of UWB system near the human body and compare the measured results of different antennas.
In [3], the UWB channels were measured from 3 GHz to 6 GHz for Body Area Network in anechoic chamber and an office room. The measured delay spread and mean excess delay are shown that the multi-path components are obvious in office environments compare to in anechoic chamber. Moreover, the path loss was calculated from transfer functions, the results reveal that the path loss is highly relative to the position of antennas on body. In [4], it
compares the on body measurements of two different antennas. One is the 3-D planar inverted cone monopole antenna (PICA) where the antenna is perpendicular to the human body, and the other is a printed horn shaped self-complementary antenna (HSCA). The measured results shows that the 3-D PICA represents lower path loss compare to the ones of HSCA. However, the PICA radiator is perpendicular to the human body so that the antenna is impractical when applied to the human body.
Several studies provide various antenna designs used in WBAN [4-10]. Planar monopole antennas with compact form and low height are widely developed in UWB community.
However, the operating bandwidth and radiation characteristics are easily affected by the body when the planar antennas are placed on the human body directly [5]. The Specific Absorption Rate is also a problem because of its omni-directional patterns. In [7], the simulation results show that the omni-directional antenna exhibits low radiation efficiency and high SAR values compared to the directional antenna when placed on the muscle model.
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Chapter 1 Introduction
Compact profile, Low backward radiation, and low mutual effect between the antenna and the human body are three major requirements [7-10] for WBAN antennas. These features increase the difficulty of antenna design. To solve this issue, some studies propose using a reflector in antenna design to reduce the backward radiation and enhance directionality. In [7], a reflector was added to a slot antenna whose bandwidth is from 3GHz to 6GHz. By using this additional reflector, it can enhance the antenna directionality and reduce the backward
radiation. But the additional reflector affects the antenna bandwidth which becomes 4GHz to 6.5GHz.
The related UWB antennas designed for Body Area Network are summarized in Table 1.1.
These three types of UWB antennas all have some drawbacks (marked in grey) when used in BAN. Therefore, a UWB antenna fits the all requirements is strongly desired and the antenna height should be restricted to 10 mm for unobtrusive to the human daily activities.
TABLE 1.1 Summarize the related UWB antennas designed for BAN
Antenna prototype bandwidth Antenna profile
Proximity
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Chapter 1 Introduction
1-3 Thesis Organization
This thesis comprises five chapters including the introduction. Chapter 2 introduces the basic theory of the broadband antennas and some UWB antenna prototype. The proposed UWB antenna and its design concept are discussed in chapter 3, and some parametric studies are shown in this chapter to validate the radiation mechanism. The simulated SAR values of the proposed antenna are also compared with omni-directional antenna in chapter 3. Chapter 4 represents the proposed UWB antenna with band-rejected property and we also discuss the design guideline of the band-rejected mechanism. Chapter 5 draws the conclusion of this thesis.
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Chapter 2 The basic theory of broadband antenna