Proposal for superposed ACO-OFDM using several even subcarriers

In this paper, we develop the conventional Asymmetrically Clipped Optical OFDM (ACO-OFDM) to propose a new OFDM for optical wireless communication with Intensity Modulation / Direct Detection (IM/DD). Although the conventional ACO-OFDM multiplexes only odd subcarriers, we reveal that some even subcarriers do not interfere with odd ones even if we clip their negative signal part. Then, we can superpose those even subcarriers on odd ones. In this paper, we show that the proposed system is able to use 1.5 times as many subcarriers as the conventional ACO-OFDM does in the same bandwidth. However, the superposed even subcarriers are affected by the odd ones in the demodulation process. Hence, we introduce an interference canceller to reduce the interference from the odd ones. We investigate the bit error rate (BER) of the proposed system, and compare it with that of the conventional ACO-OFDM. When we apply 8QAM and 4QAM for the conventional ACO-OFDM and the proposed system, respectively to achieve the same data transmission rate, the proposed system has a better BER about 1 dB than that of the conventional ACO-OFDM.