This paper presents a novel position based access scheme for indoor optical wireless communication systems. In this scheme, we apply the Intensity Modulation-Direct Detection (IM/DD) with Quadrature Phase Shift Keying (QPSK) modulation. The I (in-phase) and Q (Quadrature) signal are transmitted separately in Additive White Gaussian Noise (AWGN) channel. The probability of error is analyzed with consideration of different mode numbers of the radiation lobe of Lambertian source, and position of the transmitters. The analytical result of this paper demonstrates that high level security for optical wireless communication systems can be achieved by specifying the receivers' position where the signal can be demodulated successfully.