  Besides atmospheric turbulence and pointing errors which cause the signal
intensity fluctuation, background radiation also impairs the free-space optical
intensity-modulation / direct-detection link performance by introducing a noisy
photocurrent component in the receiver. Methods such as adopting some specific
optics systems, using pilot symbols or line codes to estimate the background
information and cancel it accordingly, and pulse-position modulation (PPM), can
be adopted to mitigate the impact of background radiation. However, purely
depending on the optics system, the background radiation can only be reduced,
but not completely cancelled; and the use of any of pilot symbols, line codes
and PPM reduces the system spectral efficiency drastically. In this paper,
based on the generalized likelihood ratio test (GLRT) principle, we develop a
Viterbi-type trellis-search sequence receiver (the GLRT sequence receiver) that
can estimate the unknown channel gain and the background radiation
simultaneously, and detect the data sequence accordingly. This receiver
requires very few pilot symbols, and therefore, does not significantly reduce
the bandwidth efficiency. Its error performance can approach that of detection
with perfect information of the channel state and the background radiation, as
the observation window length used for forming the decision metric increases.
Since a Viterbi-type trellis-search algorithm is adopted, the search complexity
is very low and is independent of the observation window size. However, the
search complexity of the GLRT sequence receiver grows exponentially with the
modulation order. To further simplify the implementation, we derive a more
efficient decision-feedback symbol-by-symbol receiver which retains the same
error performance as that of the GLRT sequence receiver.
