Experimental Demonstration of Real-Time High-Level QAM-Encoded Direct-Detection Optical OFDM Systems

In this paper, high-level quadrature amplitude modulation (QAM)-encoded real-time orthogonal frequency division multiplexing (OFDM) transceivers are implemented with two field programmable gate arrays and high-resolution digital-to-analog converter (DAC) and analog-to-digital converter (ADC). Some key digital signal processing (DSP) algorithms for real-time direct-detection optical OFDM (DDO-OFDM) system are presented and described in detail. To improve the effective number of bits of ADC and reduce quantization noise, the DAC operates at 5 GS/s with an oversampling factor of 2. Meanwhile, the optimal digital clipping ratio at the transmitter is also investigated by numerical simulation to optimize the performance of the real-time transmitter. The results show that the real-time measured BERs after 10-km SSMF are below the hard-decision forward error correction threshold of $3.8 \times 10^{-3}$. For comparison, the off-line BER performance is also analyzed using off-line DSP approaches. It shows that there is a negligible power penalty between the offline and real-time processing results. To the best of our knowledge, we have achieved the highest modulation format (1024-QAM) for real-time optical OFDM systems.