Abstract: This study proposes a multi-rate Optical Code Division Multiple Access (OCDMA) downlink system in Visible Light Communication (VLC) based on Orthogonal Variable Spreading Factor (OVSF) codes in indoor diffuse channel. In a realistic indoor environment, there are many reflected light waves due to walls or obstacles, and that is cause by Inter-Symbol Interference (ISI) owing to multipath dispersion. This channel model is referred to as diffuse channel. Impulse responses of the diffuse channel depend on Root Mean Square (RMS) delay spread values, so, the adaptive multi-rate transmission systems are required according to channel states. OCDMA system is an attracting attention as one of the promising multiple access technologies in VLC because entire asynchronous optical channel can be shared simultaneously between multiple users. Furthermore, transmission capacity can be improved by using the specific orthogonal spread codes assigned to each user. Based on these characteristics in order to support a variety of transmission data rates in accordance with the RMS delay spread values, the OVSF codes are adopted to an OCDMA system. A mainly feature of the OVSF codes are able to ensure orthogonality between different lengths of spreading code sets. Therefore, the orthogonal codes which have different Spreading Factor (SF), (i.e., length of spread codes) can be allocated to users according to channel states. Our proposed system can improve the total through put of the downlink VLC system by allocating the spreading codes with appropriate length according to the RMS delay spread values.
Kyuntak Kim, Kyujin Lee and Kyesan Lee, 2018. Multi-Rate Optical CDMA System Using OVSF Codes for Visible Light Communication in Diffuse Channel. Journal of Engineering and Applied Sciences, 13: 5258-5265.