Performance analysis of spectrally shaped DDO-OFDM based on nonlinear differential coding and real-valued precoding

This paper presents a cost-effective spectrally shaped quadrature amplitude modulation (SS-QAM) signal in bandwidth limited direct detection optical orthogonal frequency division multiplexing (DDO-OFDM) systems. The concatenation of the nonlinear differential coding (NLDC) and real-valued precoding sequentially implements spectral-shaping and multi-carrier modulation. Real-value discrete cosine transform (DCT) and discrete Hartley transform (DHT) are used to realize the uniform distribution of the fluctuation signal-to-noise ratio (SNR) and reduce the peak-to-average power ratio (PAPR). To broaden the flexibility of constellation mapping and improve the fine granularity of signals, six modulation formats are analyzed and evaluated in DDO-OFDM systems. Results show that SS-QAM is superior to uniformly distributed QAM in terms of reducing bit error rate (BER) and increasing normalized generalized mutual information (NGMI). Compared to the uniform 25/36/49/64/81/100 QAM OFDM signal, when the transmission distance is 20 km, SS-25/36/49 QAM signal can achieve error free transmission and SS-64/81/100 QAM can achieve 0.9971/1.0037/1.0255 Q-factor improvement and 0.0009/0.0014/0.0015 NGMI gain. When the transmission distance is 50 km, SS -25/36/49/64/81/100 QAM can achieve 0.4182/0.5169/0.9311/0.9988/1.0216/1.1487 Q-factor improvement and 0.0004/0.0008/0.0031/0.0043/0.0066/0.0083 NGMI gain. When the transmission distance is 80 km, SS -25/36/49/64/81/100 QAM can achieve 0.4754/0.7486/1.181/1.2012/1.2302/1.3904 Q-factor improvement and 0.0015/0.0046/0.0108/0.0152/0.0172/0.0205 NGMI gain. In addition, the Q-factor and NGMI gain increases as the fiber length increases, which verifies that the presented scheme is robust to the nonlinear effect.