Optimizing Adaptive Modulation for Symmetrically Clipped Band-Limited DCO-OFDM

A multicarrier transmission system with clipping distortion on the transmitting side, additive white Gaussian noise on the receiving side and a frequency-selective channel is considered with respect to adaptive modulation. The investigations are made based on an optical intensity modulated band-limited DC-offset orthogonal frequency-division multiplexing (DCO-OFDM) system. Usually, the frequency-selective channel transfer function determines the bit allocation. In the present scenario, this approach must be adapted, because the clipping noise is distorted by the same channel as the information signal. Therefore, adaptive modulation can no longer achieve a gain when clipping becomes the dominating source of distortion. In this paper, the total signal-to-interference-and-noise power ratio (SINR), considering clipping noise at the transmitter and additive noise at the receiver, is calculated. Based on this result, the optimal bit allocation table (BAT) for each scenario, ranging from no clipping to very strong clipping, can be determined. It is shown that the error probability of a band-limited DCO-OFDM transmission can be approximately halved based on this concept. Additionally, the optimal transmit power to achieve the lowest error probability in a given system can be theoretically calculated. The results are verified by simulations.