In this paper, to enhance the data transmission rate of the optical wireless hierarchical transmission system, the new hierarchical modulation scheme is proposed. The proposed system is designed based on multi-pulse pulse-position modulation (MPPM) using modified pseudo orthogonal M-sequence sets as a pseudo-noise code. The symbol error rate (SER) performance is derived taking into account the effect of the scintillation, background-noise, avalanche photo-diode (APD) noise, thermal noise, and signal dependence noise. Our results show that the data transmission rate performance improves significantly compared with the conventional MPPM system. Moreover, it is found that the proposed system can realize the hierarchical modulation scheme, and the SER performance of the proposed MPPM system can achieve that of the conventional MPPM system by increasing the number of slots for MPPM.
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