**Antennas for Body Centric Wireless Communications at Millimeter Wave Frequencies**

Nacer Chahat, Maxim Zhadobov and Ronan Sauleau

Additional information is available at the end of the chapter

http://dx.doi.org/10.5772/58816

## **1. Introduction**

Over the past few years, wearable antennas for body centric wireless communication systems have been increasingly gaining attention. Body area networks (BANs) are wireless communi‐ cation systems that enable communications between wearable and/or implanted electronic devices.

Such systems are of great interest for various applications such as healthcare, entertainment, military, identification systems, sport, smart home, and space [1, 2]. Indeed, portable devices, incorporating antennas close to the human body, have been used for many decades by the military. Nowadays, in order to increase overall effectiveness of soldiers on the battlefield, works are undertaken to integrate wireless systems to all equipment, such as weapons, sighting systems, helmet, and so on. Besides, wearable or implanted sensors increase the ability of doctors to monitor their patients at long distance and in real time. This monitoring capability is also used in sport applications and in rescue worker interventions.

Recently, studies carried out at millimeter waves have grown rapidly. This is due to the fact that many advantages can be found in operating such systems at millimeter waves compared to microwaves. First, because of the large available spectrum (7 GHz worldwide), very high data rates can be reached (up to 5 Gb/s) [3]. Second, it provides a high level of security and low interference with adjacent networks [4]. Finally, compared to on-body devices operating at microwaves, the size of similar millimeter-wave systems is significantly reduced.

Hence, the use of millimeter wave systems for BANs will have a high impact, in particular in the defense sector (Fig. 1), where communications emanating from a dismounted soldier leads to detection, location and vulnerability to enemy attack. The high atmospheric attenuation in

the 60-GHz band will lead to much higher levels of security against detection, interception and jamming. Fig. 1 illustrates a scenario of soldier-to-soldier communications for covert battlefield operation where co-located soldiers are wirelessly networked to allow high-speed communi‐ cations within a cluttered urban warfare environment. Besides, every soldier is equipped with advanced technology significantly improving situational awareness, lethality and survivabil‐ ity such as GPS, helmet mounted display, RADAR bullet detector, etc.

model of the human body is introduced using a Debye model. Finally, a semi-solid phantom

The primary biological targets of 60-GHz radiations are the skin and eyes. Exposure of the eyes leads to the absorption of the EM energy by the cornea characterized by a free water content of 75% and a thickness of 0.5mm. Ocular lesions have been found after high-intensity exposure

demonstrated no detectable physiological modifications [6], indicating that millimeter waves

Hereafter we will essentially consider the interactions with the skin as it covers 95% of the human body surface. From the EM viewpoint, human skin can be considered as an anisotropic multilayer dispersive structure made of three different layers, namely, epidermis, dermis, and subcutaneous fat layer (Fig. 2). The skin also contains capillaries and nerve endings. It is mainly

Knowledge of the dielectric properties of the skin is essential for the determination of the reflection from, transmission through, and absorption in the body, as well as for EM modeling. In contrast to frequencies below 20 GHz, existing data on the permittivity of tissues in the millimeter-wave band are very limited [8]-[11] due to some technical difficulties. In the 10–100 GHz range, the dispersive dielectric properties of the skin and biological solutions are primarily related to the rotational dispersion of free water molecules. In particular, high losses

In contrast to frequencies below 20 GHz, the already-existing data on the relative permittivity of human tissues at millimeter waves are very limited. In addition, the results reported so far in the literature strongly depend on the measurement technique, the sample type (*in vivo* or *in vitro* study) and other experimental conditions such as skin temperature, location on the

are related to the free water relaxation with the peak at 23 GHz at 33°C.

, 6min) [5]. However, studies performed at 60 GHz (10mW/cm2

Antennas for Body Centric Wireless Communications at Millimeter Wave Frequencies

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http://dx.doi.org/10.5772/58816

is introduced for antenna measurement in close proximity to the body.

composed of 65.3% of free water, 24.6% of proteins, and 9.4% of lipids [7].

**2.1. Interaction of millimeter waves with the human body**

act on the cornea in a dose-dependent manner.

**Figure 2.** Schematic representation of the skin structure.

body and thickness of different skin layers.

of the eye (3W/cm2

In addition, millimeter wave BANs will also benefit civilian sectors such as healthcare, personal entertainment, sports training, and emergency services. In hospital, clinics, entertainment venues, and public transport, there is a need to relay personalized data to and from individuals, in confined areas, or in crowds, and the high frequency and highly directive beams from small millimeter-wave antennas will reduce interference between users and other communication equipment.

The aim of this book chapter is to provide a review of recent progresses and outstanding challenges in the field of antennas for body-centric communication at millimeter waves.

**Figure 1.** Soldier-to-soldier communications for covert battlefield operations. The black arrows represent some possi‐ ble wireless links allowing data transfer from one soldier to another.
