Abstract
In recent years, optical orthogonal frequency-division multiplexing (OFDM) system has become a popular topic for research. Because of the orthogonal characteristics in OFDM signal, the subcarriers will be arranged in an overlapping scenario. Therefore, the spectrum efficiency is much higher than traditional modulation techniques, such as OOK (on-off keying). However, most of research results for optical OFDM in laboratories are still far from realization in various fields. To apply OFDM system extensively, it’s important to implement real-time optical OFDM system. In this thesis, we design a direct detection receiving optical OFDM system with realizing through a FPGA of 16 parallelism. The baseband OFDM receiver including I/Q down-conversion, frame detection, CP removal, FFT, channel estimation and equalization. The throughput can achieve 0.632 Gb/s. In the experiment, we use two kinds of optical OFDM system respectively, the optical up-conversion optical OFDM system and electrical up-conversion optical OFDM system. For the experimental results, the electrical up-conversion optical OFDM system has better performance which OSNR sensitivity is 1-dB lower than the optical up-conversion scheme. To evaluate the benchmark of the employed FPGA, we use an electrical experiment to simulate optical system. We find that the OSNR sensitivity of the FPGA is 15 dB. We also discuss the effects of carrier frequency offset in optical up-conversion optical OFDM system. In the 16QAM experiment, we test the FFT size from 512 to 128 and symbol length from 100 to 32. As a result, we can reduce the FFT size and the symbol length to avoid the effect with carrier frequency offset. Finally, we successfully demonstrate a real-time receiver via FPGA for an optical OFDM system. In addition, it’s worthy to note that the optical up-conversion scheme needs reduce FFT size and symbol length to achieve a similar performance as the electrical up-conversion scheme.