Adaptive Three-Dimensional Optimization for Optical Direct-Detection OFDM

We propose an adaptive three-dimensional (3-D) optimization including bit loading, power loading, and trellis-coded modulation for optical direct-detection orthogonal frequency division multiplexing (OFDM) in the fading channel. This approach could improve the bandwidth of the various devices such as electro-optical components, digital to analog convertors (DACs), and analog to digital convertors (ADCs) effectively. We conduct experiments to verify this 3-D optimization and compare the performance with that of 2-D optimization, i.e., bit loading and power loading only. Our experimental results show that the receiver sensitivity of 3-D optimization can be improved by 2 dB at the BER of 10 –3 compared with that of 2-D optimization in the fading channel. With the simple scheme we present of realizing the optimum 3-D optimization adaptively, the data rate is improved by 32% compared with using adaptive 2-D optimization in the fading channel. With the adaptive 3-D optimization, we also transmit a 16-GHz optical OFDM signal using a 10-GHz Mach–Zehnder (MZ) intensity modulator over a 20-km standard single-mode fiber link successfully. The modulation bandwidth of the MZ modulator is improved by 60% and the data rate is improved by 40%.