In this paper, the concept of Chase Combining (CC) is extended to photon communications systems. It is
assumed that intensity modulation (IM) along with on-off-keying (OOK) schemes are utilized to generate signals
at the transmitter. At the receiver, direct-detection (DD) mechanisms are utilized to recover the transmitted
signal. In particular, it is assumed that the optical detection is achieved via an avalanche photo-detector (APD).
It is assumed that the photon channel is impacted by clear-air (log-normal) turbulence and that the major
impairments of the receiver include receiver thermal noise, shot noise due to optical detection, background
radiation, and random gain distortion caused by APD. Performance is established in terms of bit error rate
(BER) using analytical and simulation means. The numerical results demonstrate that CC technique offers a
significant improvement in performance at a cost of increased hardware complexity.