Assessment by software simulation
Since assessment of exposure levels by measurement is not always possible due to one or more reasons, several other methods are used for the assessment, such as, by calculations or by software simulations. Many EM simulation software like CST Microwave Studio, High Frequency Structure Simulator (HFSS), Orthoslice, IE3D, WASP-NET, MicroWave Wizard, FEKO and EM Simulator are commercially available. These are being used for microwave analysis, design and optimisation. These simulation software make use of different numerical methods like finite-difference time-domain (FDTD), method of moments (MOM), multiple-region finite-difference time-domain (MR-FDTD), numeric EM code (NEC) and ray tracing model.
There is no one EM software application or approach that solves all problems equally well. They all have their individual strengths and weaknesses. Selection of the appropriate numerical method depends on the structure to be simulated (antenna, microstrip lines, transmission lines, waveguide structures), availability of computer resources (memory), simulation time constraints, quantity to be observed (SAR/E field/H field/power density), field zone (near/far-field) where exposure assessment is required, topology (open/closed) of the surrounding, accuracy of measurements and so on (given in the table).
FDTD method can be used to predict field values in complex scattering environments by specifying appropriate boundary conditions, or to predict SAR by specifying the dielectric properties and dimensions of the human body and appropriate boundary conditions for closed or open spaces.
For geometries having sparse regions, MR-FDTD is more suitable because the problem space is sub-divided into several independent sub-regions. Fields in the sub-regions are calculated with the use of localised FDTD lattices.
Ray tracing is useful for evaluation of fields in far-field region, especially in large open areas including complex scattering environments. Ray tracing does not enable calculation of SAR. So, a combination of FDTD and ray tracing is used for taking advantages of each of these, that is, calculation of SAR and other field quantities.
MOM method is also an efficient technique to evaluate field quantities in near- and far-field regions (open/closed) emanating from thin-wire conductive structures. But it is not suitable for determining field penetration through dielectric materials. So, MOM is not used for SAR assessment.
Another approach for exposure assessment is synthetic modelling of an antenna system. In such modelling, each antenna is considered to be made of several elementary sources (say, dipole antenna). And each of these is considered as a separate radiating source. But, accuracy is somewhat lower due to negligence of coupling between elementary radiating sources.
For calculations of field quantities, point source model of the antenna system can also be used, but with some limitations. In this model, the radiating source is represented by one-point source situated in the antenna electric centre, having a radiation pattern of the considered radiating source. Boundaries of field regions have to be calculated by taking the actual size of the radiating source. This is because this model does not take into account antenna size. Hence, applicability of this model, especially in near-field calculations, is limited. If the results of calculations are to be accurate, minimum distance between the point of investigation and transmitting antenna has to fulfil requirements for the far-field region (distance =max (3, 2D2/).
Assessment by calculations
Assessment of exposure by EIRP calculations
Calculation of equivalent isotropically radiated power (EIPR) in the direction of maximum antenna gain is an efficient method for the assessment of exposure levels of a base station antenna. EIRP is the product of power supplied to the antenna and maximum antenna gain relative to an isotropic antenna. In this method, site survey is done and several parameters like location parameters, operating parameters and environment parameters are recorded.
Assessment of EIRP and threshold of EIRP (EIRPth) are done at various publicly accessible points (on ground, rooftop, adjacent building, etc) in the environment surrounding the base station antenna.
Threshold value of EIRP should be calculated at the position within the exposure assessment area where power density is maximum. The most important parameter for determining exposure due to elevated antennae (broad coverage antenna—omni directional or sectional) is the vertical (elevation) antenna pattern. Since exposure assessment assumes exposure along the direction of maximum radiation in the horizontal plane, horizontal (azimuth) pattern is not relevant.
For a shared site, EIRP and EIRPth have be to calculated. Cumulative ratio should be less than unity at all points outside the exceedance zone for normally compliant site.
The beauty of exposure assessment by calculation is to pre-estimate the exposure from non-existent radiation sources. Calculations consider maximum possible radiation (ERP) that leads to maximum possible exposure levels. It is also possible to make calculations in areas with no access. It gives an opportunity to apply mitigation techniques to reduce radiation levels, if required.