Abstract
Light-emitting diodes constitute a low-cost choice for optical transmitters in
medium-bit-rate optical links. An example for the latter is local-area networks. However, one
of the disadvantageous properties of light-emitting diodes is their nonlinear characteristic,
which may limit the data transmission performance of the system, especially in the case of
multiple subcarrier modulation, which is starting to attract attention in various
applications, such as visible-light communications and data transmission over polymer optical
fibers. In this paper, the influence of the nonlinear transfer function of the light-emitting
diodes on discrete multitone modulation is studied. The transfer function describes the
dependence of the emitted optical power on the driving current. Analytical expressions for an
idealized link were derived, and these equations allow the estimation of the power of the
noise-like, nonlinear crosstalk between the orthogonal subcarriers. The crosstalk components
of the quadrature and in-phase subcarrier components were found to be independent and
approximately normally distributed. Using these results, the influence of light-emitting-diode
nonlinearity on the performance of the system was investigated. The main finding was that
systems using a small number of subcarriers and/or high QAM level exhibit a large
signal-to-noise-ratio penalty due to the nonlinear crosstalk. The model was applied to systems
with white and resonant-cavity light-emitting diodes. It is shown that the nonlinearity may
severely limit the performance of the system, particularly in the case of resonant-cavity
light-emitting diodes, which exhibit a strong nonlinear behavior.
© 2009 IEEE
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