The vast acceptance of OFDM technology is not limited to the radio domain, but is also among researchers for wireline and wireless optical communications. For indoor wireless optical communications, the use of OFDM faces a major challenge in reducing the average transmit optical power to cope with the eye safety problem. We propose the use of the selected mapping (SLM) technique to reduce the average transmit optical power in indoor WOC systems. In particular, SLM is investigated by employing five different families of phase sequences, namely chaotic, Shapiro-Rudin, pseudo-random interferometry code (PRIC), Walsh-Hadamard, and random sequences. Simulation results indicate the attractiveness of chaotic and Shapiro-Rudin sequences in terms of the reduction in the average transmit power with the computational complexity that grows linearly with the number of OFDM subcarriers.

This research addresses the issue of average transmit optical power reduction in multi-carrier code division multiple access (MC-CDMA)-based indoor optical wireless communications employing intensity modulation with direct detection. The problem is treated in a novel way by investigating pre-and post-equalization-based subcarrier selection for transmit power reduction in downlink transmissions. Analytical expressions are derived for upper bounds of the required fixed DC bias for both cases. The fixed DC bias is used to reduce the system complexity on one hand and to devise optimal subcarrier selection criteria on the other. Simulation results based on the proposed subcarrier selection reveal significant power reduction subject to the 10 −4 bit error rate (BER) requirement for 10-Mbps 64-subcarrier MC-CDMA-based indoor optical wireless communication systems. In addition, the BER performance obtained from pre-equalization is shown to be no higher than that obtained from post-equalization for the same transmit power.light. The key advantages of such optical wireless systems are usage of flexible unlicensed spectrum, high data rate support, energy efficiency, no electromagnetic interference, low-cost front ends, and inherent security. Major issues are eye and skin safety problems which can be dealt with by limiting the average transmit optical power.The contributions of this research, which are not reported previously to the best of the authors' knowledge, are highlighted as follows:1. Average transmit optical power reduction is accomplished by subcarrier selection for the first time in MC-CDMA-based indoor optical wireless communication systems employing intensity modulation with direct detection (IM/DD). Expressions for upper bounds of a fixed DC biasusing pre-and post-equalization-based subcarrier selection are analytically derived for MC-CDMA-based IM/DD system. 3. Optimal subcarrier selection algorithms that minimizes the bit error rate (BER) for a fixed transmit optical power are proposed for both pre-and post-equalization.

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