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Nonlinear Array detection strategies for optical beams in long range fee-space optics channels

In this study, we examine the problem of array detectors for free-space optical (FSO) communication systems when the optical field is detected using an array of detectors in the presence of large background noise and non-negligible pointing error. It is assumed that the spatial pointing errors are due to beam tilt and/or platform vibration. We propose and study a nonlinear detection mechanism for processing the outputs of the photodetectors (PD). The proposed strategy performs photodetector array combining that favors the PDs with non-negligible signal strengths, which are proportional to the spatial characteristics of the beam. Provided that the spatial profiles of the optical beams in single and multiple-spatial-mode scenarios are non-uniform, uniform aperture detector arrays will provide only a suboptimal performance. That is, the spatial feature of the detector is assumed to match the spatial feature of the received beam. This principle mirrors the concept of matched filtering in time domain for the detection of signals buried in additive white Gaussian noise. Performance in this paper is established in terms of the overall bit error rate for direct-detection receivers. It is shown that the proposed detection strategy results in a sizeable gain in performance in terms of reducing the bit error rate by an order of magnitude for all conditions, including when the pointing error is substantial.