  The clipping noise of the photon-level detector for both direct
current-biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM
(ACO-OFDM) is investigated. Based on Bussgang theorem and central limit theorem
(CLT), we obtain the approximate closed-form SNR of each subcarrier, based on
which we further formulate the power allocation among the subcarriers.
Numerical results show that the SNR obtained from theoretical analysis can well
approximate that obtained from simulation results, and uniform power allocation
suffices to perform close to the optimized power allocation from Genetic
Algorithm (GA) with significantly reduced computational complexity.
