Optimal sizing of QAM constellation for indoor optical wireless OFDM transmissions without bandwidth limitation

This paper investigates the performance of indoor optical wireless transmissions using orthogonal frequency division multiplexing (OFDM) with intensity modulation and direct detection (IM/DD). To obtain unipolar OFDM signals compatible with IM/DD, DC biased optical OFDM (DCO-OFDM) and asymmetrically clipped optical OFDM (ACO-OFDM) are considered with DCO-OFDM as the baseline. Known to be more power efficient, ACO-OFDM is optimized over the size of standard QAM constellations subject to the constraints on a fixed transmission bit rate and on a fixed target bit error rate (BER). However, there is no constraint on the utilized bandwidth; this assumption holds for any room with a single transmitter, e.g., for broadcasting. For the transmission bit rates of 10 and 20 Mbps, using system parameters according to typical operating environments, it is observed that there is an optimal QAM constellation size that is neither the smallest nor the largest.