Probabilistic Shaping Algorithm of Group Inversion Based on Symbol-Level Label
Objective With the continuous development of new services such as the Internet of Things,cloud computing,5th generation mobile communication,artificial intelligence,and virtual reality,a large number of terminal devices are connected to the network,and the required transmission capacity is increasing exponentially,which puts forward higher requirements for optical fiber communication systems.The core idea of probabilistic shaping(PS)is to both increase the sending probability of the inner constellation points and reduce the sending probability of the outer constellation points.This technology can improve the bit error rate performance and spectral efficiency of the optical fiber communication system,and make the channel capacity approach the Shannon limit.As PS is flexible,efficient,and easy to combine with other technologies,it is considered to be one of the key technologies in the future ultra-long distance,ultra-large capacity,and ultra-high speed optical fiber communication system.Since PS algorithms with low computational complexity are undoubtedly easier to implement in high-speed optical fiber communication systems,they have high research significance,especially for passive optical networks which require more low-cost solutions.Methods A PS algorithm of group inversion based on symbol-level label is proposed to realize PS of quadrature amplitude modulation(QAM)signals by inverting the data bits after grouping.According to different inversion rules,two methods of information bit inversion(IBI)and outer constellation point inversion(OCPI)can be divided.IBI is inverse according to the code weight after grouping,and an approximate gray-coded mapping rule is designed to let more constellation points with a small code weight in the inner circle and thus improve the shaping effect.OCPI takes the inverse operation according to the rule that the code group mapped by the outer and inner constellation points is exactly the inverse code.In the optical orthogonal frequency division multiplexing(OOFDM)system,the effects of different code group lengths(m)and inverse thresholds(n)are simulated.The results show that the algorithm not only features low complexity and high coding efficiency but also obtains a better shaping effect and improves the bit error rate performance of the system.Finally,the PS algorithm of group inversion combining hierarchical modulation(HM)technology further improves the performance of the system and gives it service classification advantages.Results and Discussions Since the PS algorithm of group inversion based on symbol-level label only needs the one-bit length of labels and does not need to establish a lookup table to achieve PS,it has low computational complexity and is easy to implement.The algorithm is successfully applied to the PS of 16 QAM and 32 QAM.Controlling the length of m can help flexibly strike a balance between coding efficiency and PS gain.The algorithm is combined with the HM technology,PS is only achieved in the low priority(LP)layer data,and the label bit is placed in the high priority(HP)layer with a low bit error rate,which further improves the coding efficiency and bit error rate performance of the system.Under the coding efficiency of 94.4%(Table 2),the hierarchical modulated 64 QAM can obtain a PS signal-to-noise ratio(SNR)gain of 1.3 dB(Fig.12).Conclusions The IBI and OCPI are proposed according to different inverse rules.Conducting the simulation analysis of the different values of n and m in the proposed algorithm helps indicate that the best shaping effect can be obtained when the optimal inverse threshold is half of the m.Additionally,the smaller m results in lower coding efficiency,better bit error rate performance,and better PS effect.The proposed PS algorithm of group inversion is characterized by low computational complexity,high coding efficiency,wide application range,high flexibility,hardware resource conservation,and easy implementation.Meanwhile,it has broad application prospects and provides an effective solution for the performance improvement of communication systems.
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