Performance Analysis of ACO-OFDM and DCO-OFDM Using Bit and Power Loading in Frequency Selective Optical Wireless Channels

In this paper, we present a detailed comparison of the performance of asymmetrically clipped optical OFDM (ACO-OFDM), ACO-OFDM with diversity combining, and DC-biased optical OFDM (DCO-OFDM) using a range of different bias levels. Comparisons are made for both an additive white Gaussian noise (AWGN) channel and for a typical frequency selective visible light communication channel. Adaptive bit loading is used to maximize the bit rate for the frequency selective channel using a target bit error rate of 10-3. Clipping noise in DCO-OFDM, unlike AWGN, is added at the transmitter, not the receiver. New analytical results are derived which include this effect in the bit-loading calculations. It is shown that diversity-combining alters the spectral distribution of the noise after equalization. The effect of this in bit loading is also analyzed. It is shown that for both the AWGN and the frequency selective channel ACO-OFDM with diversity combining is the most energy efficient at lower bit rates/normalized bandwidths but for higher data rates DCO-OFDM requires the smallest transmit power. For example, for the frequency selective channel for bit rates/normalized bandwidths up to 6, ACO-OFDM with diversity combining is the most power efficient. For higher data rates DCO-OFDM requires the smallest transmit power, but these data rates require very large constellation sizes, for example, the constellation size of 128 for DCO-OFDM for some subcarriers and 2048 for ACO-OFDM with diversity-combining.