Abstract
Vacuum-propagation optical communication with high photon efficiency (many bits/photon)
and high spectral efficiency (many bits/s
$\cdot$
Hz) requires operation in the near-field power transfer regime with a large number of spatial
modes. For terrestrial propagation paths, however, the effects of atmospheric turbulence must be factored into the
photon and spectral efficiency assessments. In Part I of this study [N. Chandrasekaran and J. H. Shapiro,
“Photon Information Efficient Communication Through Atmospheric Turbulence—Part I: Channel Model and
Propagation Statistics,” J. Lightw. Technol., vol. 32, no. 6, pp.
1075–1087, Mar. 2014], modal-transmissivity statistics were derived for the turbulent channel that depend solely
on the mutual coherence function of the atmospheric Green’s function, and these bounds were evaluated for
$\sim$
200 spatial-mode systems whose transmitters used either
focused-beam (FB), Hermite-Gaussian (HG), or Laguerre–Gaussian (LG) modes and whose receivers either did or did
not employ adaptive optics. This Part II paper derives upper and lower bounds for the ergodic Holevo capacities of
classical and private information transmission over the multiple spatial-mode turbulent channel that can be evaluated
from Part I’s transmissivity statistics. Also included are bounds on the ergodic capacity for on–off
keying encoding and direct detection. It is shown that: 1) adaptive optics are not necessary to realize high photon
information efficiency and high spectral efficiency simultaneously; 2) an FB-mode system with perfect adaptive
optics outperforms its HG-mode and LG-mode counterparts; and 3) the converse is true when adaptive optics are not
employed.
© 2013 IEEE
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