Optimal Combining for Optical Wireless Systems With Amplification: The $\chi ^{2}$ Noise Regime

We present novel analytical results on the optimal combiner (OC) for pre-amplified optical wireless receivers under chi-squared noise. The results show that the OC architecture that minimizes the average bit-error rate (BER) is determined by the level of the signal-spontaneous beating noise. Moreover, the OC provides increased gain to the diversity branches with the higher energies, up to the poin...