A modification to the Asymmetrically Clipped Optical OFDM (ACO-OFDM) technique, a well reported non-coherent optical implementation is proposed. A Modified ACO-OFDM (MACO-OFDM) system is developed to improve system performance at the expense of spectral efficiency. A MACO-OFDM system model is defined underpinned by a detailed mathematical framework verified through Monte Carlo simulations. System performance is compared to that of conventional ACO-OFDM. A 1.5 dB saving is achieved in the Bit Error Rate (BER) performance of 4-QAM ACO-OFDM after applying the proposed modification; the theoretical and simulation results are in good agreement. As the constellation size increases, the improvement in BER performance decreases. The research then treats the impact of atmospheric turbulence on the performance of both conventional ACO-OFDM and the proposed MACO-OFDM system. A Single Input Multiple Output (SIMO) approach using multiple receivers is employed to mitigate the impact of atmospheric turbulence. The performance of MACO-OFDM outperforms that of ACO-OFDM by nearly 3 dB, 4 dB, and 5 dB as the number of receiving apertures (nRx) increases as 1, 2, and 4 respectively in weak atmospheric turbulence; in moderate turbulence the performance is improved by 2 dB, and 4.5 dB for nRx=2, and nRx=4; and in strong turbulence, the BER performance is enhanced by nearly 2 dB and 4 dB for nRx=2 and nRx=4. The channel capacity of MACO-OFDM has been shown to be half that of conventional ACO-OFDM. The capacity of SIMO MACO-OFDM architecture in atmospheric turbulent channel is evaluated.