Biological Effects - RF/Microwave Interaction with Biological Tissues - page 134

The FDTD method is usually preferred for cellular telephone simulations
because it allows the inclusion of arbitrarily heterogeneous objects in the
region to be simulated. Highly realistic heterogeneous human head models
have been developed and successfully used. These models are characterized
by resolutions as low as 1 mm, and as many as 32 tissue types have been
identified.
The major drawbacks of these simulations are the execution time and com-
puter memory requirements, and methods have been proposed to reduce both.
Subgridding has been introduced for representing with high resolution the
regions of maximum absorption and lower resolution the regions where the
EM coupling is weak. In this approach, the major drawback is the need to
implement the subgridding scheme in the FDTD code. Methods have been
developed for accurate modeling of the region of interest (e.g., ear) and
increased coarser modeling of the weakly exposed regions of the human head.
The use of the absorbing perfectly matched layer (PML) for the purpose of
truncating the head model has been investigated, with the possibility of trun-
cating the head model in all the directions [120]. This method allows trunca-
tion along all three axes. It has been shown that by using only a half-truncated
head model it is possible to achieve accurate radiation patterns at both 835
and 1900 MHz. Therefore, when complete characterization of the handset per-
formance is needed, it is possible to use the proposed approach with a con-
siderable saving for SAR evaluation and a more limited saving for radiation
pattern calculation in both execution time and memory requirements. Reduc-
tions of the head volume down to 4% of the original volume have been
achieved, and memory savings of up to 82% have been obtained. Execution
times were no more than 7% of the original values.
A hybrid method between the MoM and the FDTD method has been
developed [121]. It is capable of analyzing a system of multiple discrete regions
by employing the principle of equivalent sources to excite their coupling sur-
faces. It has been shown that this theory has advantage for accurately model-
ing complex and arbitrarily oriented mobile telephone handset antennas.

3.11.4 Exposure of Body to Cell Phone and Base Station

Guidelines for limiting EM exposure provide protection against known
adverse health effects. Biological effects, on the other hand, may or may not
result in an adverse health effect. There is a serious concern among the pop-
ulation, in particular in Europe, about possible adverse biological effects due
to cellular telephone base stations. The question of the “microwave syndrome”
rises again, the possible effects of low-intensity exposure are discussed,
recommendations are analyzed in detail, and health issues are reviewed.
Surprisingly, however, there is no concern about TV and FM exposure. Main
conclusions are at the present: There are still a number of uncertainties; a sub-
stantial database is available on thermal biological effects; most countries have
protection standards; and there is a variety of these standards.