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This paper proposes a practical method for the definition of communicationmodes when antennas operate in the near-field region, by realizing ad-hoc beamsexploiting the focusing capability of large antennas. The beamspace modelingproposed to define the communication modes is then exploited to deriveexpressions for their number (i.e., the degrees of freedom) in a generic setup,beyond the traditional paraxial scenario, together with closed-form definitionsfor the basis set at the transmitting and receiving antennas for several casesof interest, such as for the communication between a large antenna and a smallantenna. Numerical results show that quasi-optimal communication can beobtained starting from focusing functions. This translates into the possibilityof a significant enhancement of the channel capacity even in line-of-sightchannel condition, without the need of implementing optimal but complexphase/amplitude profiles on transmitting/receiving antennas as well asresorting to intensive numerical solutions. Traditional results valid underparaxial approximation are revised in light of the proposed modeling, showingthat similar conclusions can be obtained from different perspectives.

Communication Modes with Large Intelligent Surfaces in the Near Field