  This paper proposes a practical method for the definition of communication
modes when antennas operate in the near-field region, by realizing ad-hoc beams
exploiting the focusing capability of large antennas. The beamspace modeling
proposed to define the communication modes is then exploited to derive
expressions for their number (i.e., the degrees of freedom) in a generic setup,
beyond the traditional paraxial scenario, together with closed-form definitions
for the basis set at the transmitting and receiving antennas for several cases
of interest, such as for the communication between a large antenna and a small
antenna. Numerical results show that quasi-optimal communication can be
obtained starting from focusing functions. This translates into the possibility
of a significant enhancement of the channel capacity even in line-of-sight
channel condition, without the need of implementing optimal but complex
phase/amplitude profiles on transmitting/receiving antennas as well as
resorting to intensive numerical solutions. Traditional results valid under
paraxial approximation are revised in light of the proposed modeling, showing
that similar conclusions can be obtained from different perspectives.
