The effects of exposure to the electromagnetic field from base stations have received considerable attention. Currently, researches have shown that the exposure level from a base station varies with time due to the traffic. The traffic for mobile communications has a temporal and spatial correlation. In this paper, we develop an approach to study the variation law of exposure to the base stations and analyze the correlation of exposure in time and in space. We use a spectrum analyzer to measure the transmission power of the base stations at the different periods of a day. We obtain the analytical expressions for representing the variation of exposure with time using Genetic Algorithm. The self correlation of exposure to a single base station in time series and the cross correlation of exposure to base stations in the same area are both discussed. We find that the self correlation coefficients of exposure to a single base station are more than 0.9 in two hours and more than 0.5 in eleven hours. Particularly, the spatial correlation of exposure is slightly stronger than the time correlation and the cross correlation coefficients are up to 0.99.
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