305
Mobile Communication Fields in Biological Systems
that distance to a base station was not an accurate measure to assess human RF exposure
(Neitzke et al. 2007; Neubauer et al. 2007; Schuz and Mann 2000). As a result, future
procedures for the assessment of base-station safety, with respect to human exposure,
were mainly based on a comparison of the average exposure field value with reference
levels suggested by exposure guidelines (EC 1999; FCC 2001; ICNIRP 1998; IEEE 2005).
Since reference levels have been derived from basic restrictions under the assumption of
uniform plane-wave exposure, their validity is demonstrated only under this particular
condition.
However, in the vicinity of the base station, in the radiating near-field region, the
antenna field is far from being uniform due to the high directivity on the vertical plane
of typical base-station antennas. Similarly, in an urban environment, due to environmental reflections, such as those produced by buildings and the ground, field nonuniformities arise again. Therefore, it is not obvious that compliance of the average field
level with reference levels ensures that basic limits on SAR are respected. For example,
in the study by Martinez-Burdalo et al. (2005), the procedures for safety assessment in
the close proximity of cellular communication base-station antennas at three different
frequencies (900, 1800, and 2170 MHz) were analyzed. For antenna–body distances in
the near zone of the antenna, the fact that averaged field values were below the reference
levels could, at certain frequencies, not guarantee guidelines compliance based on basic
restrictions. Similarly, in the research of Lacroux et al. (2008), a numerical dosimetry
study for a typical base-station antenna used in 3G systems was presented. The compliance of the averaged power density to the reference levels was not enough to ensure
compliance to localized SAR.
As a result, a problem arises regarding the field value that should be compared with the
reference one. This has resulted in different regulatory bodies issuing different standards
for the calculation and measurement of EM fields related to human exposure to basestation antennas. These standards, under some aspects, do not agree with each other.
According to the IEEE exposure guidelines (IEEE 2005), in the case of nonuniform
exposure conditions, the field should be averaged on a vertical surface equivalent to the
projection of the human body. These guidelines are largely at the base of the Federal
Communications Commission (FCC) regulations (FCC 2001). The ICNIRP guidelines
(1998) state that, in order to establish whole-body averaged SAR compliance (only), the
field should be averaged on a vertical surface equivalent to the projection of the human
body. On the other hand, local SAR compliance should be directly assessed through a
dosimetric study. The CENELEC has issued a compliance procedure in order to avoid
a complete dosimetric assessment (CENELEC 2002). This procedure allows local SAR
compliance evaluation through a comparison of the peak value of the exposure field with
the corresponding reference level (conservative approach). Finally, there are other procedures which suggest that, if compliance is evaluated numerically rather than experimentally, the numerical analysis can be performed neglecting the presence of the environment
(free-space condition), thus eliminating the problem of field nonuniformities (IEC 2002).
Due to the many contrasting approaches to base-station compliance assessment presented above, typical realistic urban environment exposure situations together with
exposure in close proximity of the antenna must be studied in order to characterize the
exposure to the field emitted by a base-station antenna.
Mobile Communication Fields in Biological Systems
that distance to a base station was not an accurate measure to assess human RF exposure
(Neitzke et al. 2007; Neubauer et al. 2007; Schuz and Mann 2000). As a result, future
procedures for the assessment of base-station safety, with respect to human exposure,
were mainly based on a comparison of the average exposure field value with reference
levels suggested by exposure guidelines (EC 1999; FCC 2001; ICNIRP 1998; IEEE 2005).
Since reference levels have been derived from basic restrictions under the assumption of
uniform plane-wave exposure, their validity is demonstrated only under this particular
condition.
However, in the vicinity of the base station, in the radiating near-field region, the
antenna field is far from being uniform due to the high directivity on the vertical plane
of typical base-station antennas. Similarly, in an urban environment, due to environmental reflections, such as those produced by buildings and the ground, field nonuniformities arise again. Therefore, it is not obvious that compliance of the average field
level with reference levels ensures that basic limits on SAR are respected. For example,
in the study by Martinez-Burdalo et al. (2005), the procedures for safety assessment in
the close proximity of cellular communication base-station antennas at three different
frequencies (900, 1800, and 2170 MHz) were analyzed. For antenna–body distances in
the near zone of the antenna, the fact that averaged field values were below the reference
levels could, at certain frequencies, not guarantee guidelines compliance based on basic
restrictions. Similarly, in the research of Lacroux et al. (2008), a numerical dosimetry
study for a typical base-station antenna used in 3G systems was presented. The compliance of the averaged power density to the reference levels was not enough to ensure
compliance to localized SAR.
As a result, a problem arises regarding the field value that should be compared with the
reference one. This has resulted in different regulatory bodies issuing different standards
for the calculation and measurement of EM fields related to human exposure to basestation antennas. These standards, under some aspects, do not agree with each other.
According to the IEEE exposure guidelines (IEEE 2005), in the case of nonuniform
exposure conditions, the field should be averaged on a vertical surface equivalent to the
projection of the human body. These guidelines are largely at the base of the Federal
Communications Commission (FCC) regulations (FCC 2001). The ICNIRP guidelines
(1998) state that, in order to establish whole-body averaged SAR compliance (only), the
field should be averaged on a vertical surface equivalent to the projection of the human
body. On the other hand, local SAR compliance should be directly assessed through a
dosimetric study. The CENELEC has issued a compliance procedure in order to avoid
a complete dosimetric assessment (CENELEC 2002). This procedure allows local SAR
compliance evaluation through a comparison of the peak value of the exposure field with
the corresponding reference level (conservative approach). Finally, there are other procedures which suggest that, if compliance is evaluated numerically rather than experimentally, the numerical analysis can be performed neglecting the presence of the environment
(free-space condition), thus eliminating the problem of field nonuniformities (IEC 2002).
Due to the many contrasting approaches to base-station compliance assessment presented above, typical realistic urban environment exposure situations together with
exposure in close proximity of the antenna must be studied in order to characterize the
exposure to the field emitted by a base-station antenna.