  Detector arrays are commonly used for free-space optical communications in
deep space. Such detector arrays---by virtue of their size---help in the
collection of the optical signal even when there is some misalignment between
the transmitter and receiver systems. In this paper, we argue that for the
common Gaussian beam profile, a detector array receiver is more useful for
minimizing the probability of error than a single detector receiver of the same
dimensions. Furthermore, the improvement in the error probability is more
pronounced for low signal-to-noise ratio conditions, and the probability of
error decreases monotonically as a function of the number of detectors in the
array. However, communication with detector arrays results in a larger
computational complexity at the receiver. Additionally, such detector arrays
are also more advantageous for beam position tracking on the detector array in
order to minimize the pointing loss.
