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Human RF Exposure and Communication Systems

"Something is not just because it is law. But it must be law because it is just".

MONTESQUIEU

1.1. Introduction

Over the past 30 years, wireless communication systems have been increasingly used in our daily lives (see Figure 1.1). Worldwide, cellular phone users are more than 6 billion and mobile subscriptions will reach 9.3 billion in 2019, with more than 5.6 billion using smartphones (Figure 1.1). The versatile use of new smart mobile phones and tablets, the development of home wireless LANs as well as the emergence of pervasive wireless communication systems, such as machine-to-machine, are strengthening this tendency. At the end of 2013, the mobile broadband subscription was 2 billion, which is expected to reach 8 billion by 2019 (3G technology at 4.8 billion and 4G at 2.6 billion). By 2018, the global mobile data traffic will increase nearly 11-fold. Twenty-six billion communication devices will be on the Internet of Things by 2020, with a large proportion of these being wireless.

Figure 1.1. Mobile phone subscriber's progression (left) [ICT 14]; number of devices versus years (right) [CIS 15]. For a color version of the figure, see www.iste.co.uk/wiart/radiofrequency.zip

Despite the increasing use of wireless communications, public concerns about the possible health impacts of exposure to the radiofrequency (RF) electromagnetic field (EMF) have appeared, even if no risk has been proven to date.

In this context, the monitoring and management of EMF exposure has become a key question. Based on scientific knowledge, international organizations, such as the International Commission on Non-Ionizing Radio Protection (ICNIRP) and the Institute of Electrical and Electronics Engineers (IEEE), have established limits to protect the public against known health effects associated with EMF exposure [ICN 98, IEE 05].

In Europe, a council recommendation, based on ICNIRP guidelines and adopted in 1999, provides legal framework for the limitation of the exposure of the general public to EMFs. Equipment that intentionally emits or receives radiowaves for the purpose of radio communication has to comply with the Radio and Telecommunications Terminal Equipment (R&TTE) European Directive [DIR 14]. This directive will be replaced in 2016 by a new directive, 2014/53/EU [EU 99] (known as the Radio Equipment Directive), but the main objectives are similar. They aim to put equipment and devices onto the market and into service that satisfy the essential requirements imposed by the European Council [ECR 99].

1.2. Metric and limits relative to human exposure

1.2.1. Human RF exposure and specific absorption rate

The EMF induced by an RF source S is composed of an electric {E} and a magnetic field {H} that are governed by the Maxwell equations. In the RF domain, E and H are highly correlated. Close to the source, in the "near field", the relationship between E and H can be complex since the phase and polarization of the electric and the magnetic fields can vary with location. Far from the source, in the "far field", the EMF has, locally, a structure of a plane wave. In this case, E and H are orthogonal and the relationship between them is given by equation [1.1], where ? is the free space impedance equal to 377 O.

In the "far field", the incident power density, linked to the Poynting vector, is given by [1.2]:

The human exposure to an RF-EMF is quantified through the specific absorption rate (SAR) that is the ratio of the electromagnetic power absorbed (watts) by tissues to the mass (kg) of these tissues [1.3]:

The SAR is often averaged over the whole body or over a specific organ. The IEEE and ICNIRP standards, which have been established to limit human exposure to EMFs, use the whole body SAR (i.e. SAR averaged over the whole body). They also use the maximum SAR averaged over a mass of 10 or 1 g. In this case, the objective is to estimate the maximum SAR over a continuous volume of tissue having a mass of 1 or 10 g. The shape of the volume depends on the standard: IEEE recommends a cube shape, while ICNIRP prefers continuous tissues.

The electromagnetic energy deposited in tissues included in a volume V can be estimated through the electric field or the measurement of the rise in temperature. The first approach explains the conductivity, whereas the second approach needs information on the calorific mass.

The SAR assessment using temperature is less used than the method based on the electric field. In addition to sensitivity, another problem of the SAR measurement via the temperature is linked to the need of a steady state before each measurement. In case of a large number of measurement points, this constraint can induce long durations for the measurement, which is not always compatible with other constraints such as the life of wireless phone batteries. Because of this, the compliance of mobile phones is performed through the electric field assessment.

Eectric field measurement using small antennas, detection sensors or optical probes is nowadays the most common method used to experimentally assess SAR. Equation [1.4], that will be explained in section 2.2.5.2, provides the relationship between the SAR and the electric field.

where s, ? and E represent, respectively, the conductivity of the body tissue (S/m), the mass density of the tissue (kg/m3) and the peak electric field strength in the tissue (V/m). Depending on the use of r ms (root mean square) or maximum value of the electric field strength, the coefficient ½ exists or not. In this book, coefficient ½ will be used.

1.2.2. Protection limits

1.2.2.1. Basic restrictions

To protect humans from the adverse health effects of EMFs, ICNIRP, IEEE and the International Committee on Electromagnetic Safety (ICES) have agreed on a set of recommended limits [ICN 98, IEE 05].

ICNIRP limits are composed of fundamental ones - the basic restrictions - and derived ones - the reference levels.

Basic restrictions are, on the one hand, the local exposure, and, on the other hand, the exposure averaged over the whole body, using SAR. These limits are established to protect humans from the known health effects.

In the RF domain, these are the thermal effects. As stated in the ICNIRP guidelines, with an exposure higher than 4 W/kg for longer than 6 min, the rise in human body temperature can be higher than 1°C, which can induce possibly adverse health effects.

To protect from such thermal effects, an exposure limitation of 0.4 W/kg has been recommended for a healthy adult (and, by extension, also defined for workers). The maximum recommended exposure is therefore 10 times below the level which includes a thermal effect. With regard to the general public, taking into account a possible specificity of young children, elderly or sick people, an additional safety factor of 5 has been also defined.

Ultimately, the whole body averaged SAR (WBSAR) limit for general public is 0.08 W/kg and for workers is 0.4 W/kg. A similar approach has been used to define local limits. Health effects have been reported with local exposure above 100 W/kg ICNIRP. Therefore, for head and trunk, a limit of 2 W/kg (50 times below the health effect) has been recommended for the general public and 10 W/kg (10 times below the health effect) for workers. For the limbs, the general public and worker limits are, respectively, 10 and 20 W/kg. All these limits are summarized in Table 1.1.

Table 1.1. ICNIRP basic restrictions

1.2.2.2. From basic restrictions to reference levels

The measurement of SAR is complex and requires a laboratory. Reference levels have been defined to help reinforce the basic restrictions. They define a limit for the incident field strength that is the level inducing an exposure compliant with the basic restrictions.

Since SAR assessment could not be performed in situ in the 1960s, studies were conducted to characterize a transfer function of the incident EMF to the power absorbed by the human body. The initial studies were carried out with analytical approaches and mathematical structures such as spheroids. This relationship was then revisited in the 2000s using advanced numerical methods and phantoms [WU 11, CON 08].

As an antenna, the "equivalent surface" of the body evolves with the frequency; as a consequence, while the basic restrictions do not depend on the frequency, the reference levels are frequency dependent.

Human morphology is variable and body shape, as well as internal organ proportions, can vary; because of this, as shown in Figure 1.2, the power absorbed by a human body depends on the frequency and morphology. Figure...