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Adaptive M-PAM for Multiuser MISO Indoor VLC Systems

In multiuser multiple input single output (MISO) visible light communication (VLC) systems using LEDs with limited rise-times, it is essential to find effective M-ary modulation schemes to increase the bit rate. In this paper, we propose an adaptive M-ary pulse amplitude modulation (M- PAM) algorithm to support multiple users. The proposed algorithm can adjust the modulation constellation size for each user to maximize the bit rate under different channel environments such as shadowing, light dimming, and the impact of multiple access interference. In our MISO approach, multiple LED lamps coordinate to provide users with maximum data rates. We compare optical code division multiplexing access (OCDMA) using our adaptive M-PAM with time division multiplexing access (TDMA). The OCDMA technique can offer a higher bit rate when the number of users is larger than the length of the OCDMA code.