A non-uniform QAM OFDM modulation system based on probabilistic shaping (PS) and geometric shaping (GS) is proposed in this paper, which can effectively resist the interference among signal waveforms and improve spectrum utilization and channel capacity. First, Huffman coding is used in the scheme for probability shaping to reduce the average energy of the signal. Uniform 16QAM is adjusted to non-uniform 12QAM and uniform 32QAM is adjusted to non-uniform 12QAM. Then geometric shaping is applied. Taking generalized mutual information (GMI) as the objective function, pairwise optimization (PO) algorithm is applied to continuously adjust the constellation. Finally, the coordinate point with the largest mutual information is obtained. The simulation results show the performance of non-uniform 12QAM and non-uniform 24QAM under different conditions, we obtain the signal-to-noise ratio (SNR) with generalized mutual information curve, signal-to-noise ratio with bit error rate (BER) curve under different QAM modulation formats and the bit error rate with the different length of the fiber under different transmission modes. The results show that the proposed non-uniform 12QAM-GS-OFDM has a gain of 0.02 bit/symbol over 12QAM-OFDM, the proposed non-uniform 24QAM-GS-OFDM has a gain of 0.02 bit/symbol over 24QAM-OFDM.
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