This paper proposes a channel ranking based spreading factor optimization for orthogonal frequency division multiple interleave division multiple access (OFDM-IDMA) on visible light communication (VLC) systems. VLC is under the global spotlight, since light emitting diode (LED) has recently become a part of building infrastructure and its functionality is quite easy by implementing communication components to LED lighting. To enhance the VLC capability, OFDM-IDMA was previously proposed as a multiple access method. It also employs a spectrum spreading, in which the symbol is orthogonally spread to a specified number of subcarriers. Here, an interleave pattern specific to the transmission signal of each user can improve the separation capacity of the source signal and therefore multiple access becomes possible in VLC-OFDM-IDMA. However, it requires a lower rate error correction code and a large spread factor to limit the throughput performance. To resolve this problem, we introduce a new channel ranking and allocate an appropriate spreading factor to each user according to their channel state. Overall, BER performance and throughput performance can be enhanced by optimising the spread code.

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