Trinchero D. Exposure assessment procedures in presence of wideband digital wireless networks. Rad Prot Dos. Nov 13, 2009 Ahead of print.
The precise assessment of population exposure to radiofrequency radiations requires the identification of reliable methods for the measurement of electromagnetic fields, especially in the presence of complex modulations. Earlier measurement techniques developed for this purpose were based on the standard procedures applied for electromagnetic compatibility radiated measurements, where the object of the measurement is a sequence of incoherent, narrowband, non-modulated signals. However, last generation radio services make use of more and more complex modulations that limit or even preclude the use of those procedures that often lead to inadequate results. The purpose of the current article is to analyze and discuss the previous methods that have appeared in the literature for measurement of electromagnetic fields modulated with complex wideband (spread-spectrum) digital signals.
Methods and Results
Methods available for the measurement of non-ionizing radiation in the radiofrequency and
microwave bandwidths can be grouped in two main categories: the first involves the use of broadband field probes and the second implies selective investigations by means of spectrum analyzers. Both categories can be implemented for short-term (spot) measurements or long-term (continuous) ones, even if narrowband analysis is rarely carried out over long periods. In the broadband field probes, electromagnetic field measurements are typically performed by means of broadband probes, to characterize the cumulative value of electromagnetic noise over a wide portion of the frequency spectrum. The acquisition is realized by means of a detector that can be constructed either by the use of temperature detectors or diode rectifiers. Independently from the choice of the detector, to construct a direct relationship between the incident field and the detector output, a calibration process is used. This procedure allows a partial extension of the dynamic range over a wider interval especially for probes making use of diode detectors, but it is valid only if the dynamics of the modulation is limited. However, some preliminary results reported demonstrate that discontinuous signals can produce an underestimation that grows with the field amplitude.
Selective measurements by means of spectral analysis use vector signal analyzer to obtain an accurate measurement of digital signals. By using a spectrum analyzer, it is possible to measure with adequate accuracy the electromagnetic field, averaged during the period of measurement. For simplicity, this value will be referred as ‘instantaneous’. Several procedures are available to characterize the average signal power by means of a spectrum analyzer. However, despite their applicability to any kind of digital signal, vector signal analyzers are expensive, difficult to transport and they require strong expertise of the operator. For these reasons, they cannot be regularly used for exposure assessment and they are mainly dedicated to laboratory measurements.
In conclusion, measuring wideband digitally modulated signals requires the use of advanced exposure assessment techniques, if compared with the traditional ones. Despite their cost and usage complexity, the most accurate measurement is obtained by using a vector signal analyzer equipped with digital demodulators. Nevertheless, the literature reports several examples and cases of study that avoid the use of such instruments, owing to the application of techniques based on an exact knowledge of the transmission standard.