of Freedom Set of all the possible current density distribution on the source Set of all the possible electric field distribution on the observation domain Length of observation time Space-varying signal effective bandwidth Space-varying signal effective angular bandwidth Speed of the light in vacuum Frequency Central frequency Curvilinear abscissa along the observation curve Tangent unit vector on the observation curve kth left singular function of the radiation operator kth right singular function of the radiation operator Functional point in the X set Vector collecting the Xk coefficients kth coefficient of the expansion of the current density distribution J Functional point in the Y set Vector collecting the yk coefficients Angular spatial bandwidth Minimum angular spatial bandwidth assuring that the band limitation representation error of the field tends to zero (continued) M. D. Migliore Table 2.2 (continued) kth coefficient of the expansion of the electromagnetic field E Nyquist spatial step Length of the parameterized observation domain Observation domain Free space wavenumber Free space wavelength Observation curve parameterization function in the reduced field representation Observation curve parameterization function evaluated in kr/w' kth singular value of the radiation operator Phase function along the observation line in the reduced field representation 2.2 Communication at the DPL Using the Space Resource In this section the communication process will be analyzed using Kolmogorov information theory [35, 46-48]. In short, the waveform that encodes the transmitted information is represented by a point X in a proper functional space. On the receiver side, the received waveform is represented by a point y in a proper functional space. The sets of all possible transmitted and received waveforms are called X and Y, respectively. The communication process maps X E X to y E Y. The presence of noise is modeled as an open sphere of uncertainty having radius E around the "noiseless" point y. Received waveform