Flexible Double-Layer Optical OFDM for IM/DD-Based Visible Light Communications

In this letter, a flexible double-layer optical orthogonal frequency division multiplexing (OFDM) scheme is proposed for visible light communications (VLC). At the transmitter, the proposed scheme provides a flexible means of arranging the data subcarriers between the two layers based on a subcarrier activation factor to meet different spectrum efficiency requirements. Additionally, the first layer modulated by quadrature amplitude modulation (QAM) is bipolar without negative clipping to reduce the clipping noise. Therefore, an adaptive DC bias is employed to make the time-domain superimposed bipolar signal positive. At the receiver, a noise suppression algorithm is first applied to the received signal, followed by a single fast Fourier transform to extract the data subcarriers of QAM and pulse amplitude modulation schemes (PAM). Simulation results show that, the proposed scheme exhibits a higher spectrum efficiency, a lower computational complexity, a lower peak-to-average power ratio, and a competitive bit error rate under the linear and nonlinear model compared to current optical OFDM schemes, demonstrating its potential for VLC applications.