Interpolation and Extrapolation is to calculate/estimate the data points with in range and out of range for the observations made respectively.
I dont know how this is related to Cell charaterization here , but by extrapolation only scientists calculate the distance of stars and others where normally we cannot reach.
these 2 come under statical characterization only, data assumptions you calculate might not be exact in extrapolation as 100% but to approximation it will be true.
though interpolation is with in range , the more interpolate you do the more signal noise and makes system work to heavy load, this may / may not be 100% true but to a considerable error becomes true. refer for video processing techniques(interpolation) and signal analysis with GPS(extrapolation) to understand easily why they use.
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if I guess if your referring to some mobile network, may be this be related how much distance the signal can travel and at what distance repeaters should be placed, analysis of strength of the signal in NON - Line of Sight and so on.