Research On Four-Dimensional Trellis-Coded Modulation Technology Based On Sphere Packing

At present,the telecommunications industry is in a critical period of digital transformation.As the backbone of modern communications network,optical transmission network has become the top priority of this transformation.Large capacity and long distance are the eternal theme of optical transmission system.Due to the high investment cost,long construction period and difficult laying of optical fiber and cable,upgrading the modulation format has become one of the effective measures to meet the current challenges of optical communication.On the one hand,in order to alleviate the contradiction between spectral efficiency and asymptotic power efficiency in traditional two-dimensional modulation formats,four-dimensional modulation has become a research hotspot.It modulates jointly X and Y polarization states(each polarization state contains in-phase and quadrature components)in the light field,and can optimize the constellation in four dimensions.Among them,four-dimensional modulation formats based on sphere packing ensure the arrangement of the constellation point set is denser while the minimum Euclidean distance between the constellation points is maximized.Because their constellation points are selected from the lattice points obtained by the sphere packing theory in the four-dimensional space.Therefore,compared with other four-dimensional modulation formats with the same spectral efficiency,sphere packing four-dimensional modulation formats tend to have lower average symbol energy and higher asymptotic power efficiency.On the other hand,trellis-coded modulation technology combines coding and modulation.Set-partitioning is the core of trellis-coded modulation,which increases the Euclidean distance between constellation points.Then,different subsets are selected by convolution code and mapped by set partitioning.In this way,the coding gain is obtained while the sensitivity of the receiver is improved effectively.The existing four-dimensional trellis-coded modulation is based on the quadrature amplitude modulation(QAM)constellations.However,there is no corresponding effective set-partitioning method for the four-dimensional modulation formats based on sphere packing,so the trellis-coded modulation cannot be realized.Therefore,this paper proposes a four-dimensional trellis-coded modulation technique based on sphere packing.This paper carried out the following research work:1.Based on the main problems of optical transmission technology,theadvantages of four-dimensional modulation based on sphere packing and trellis-coded modulation are discussed.The significance of four-dimensional trellis-coded modulation based on sphere packing is explored.2.The carrying technology and performance parameters of four-dimensional modulation are introduced.Several typical four-dimensional modulation formats are listed.Meanwhile,the key steps and performance advantages of four-dimensional modulation format based on sphere packing are described in detail.3.The basic idea of trellis-coded modulation technology is deeply analyzed.The set-partitioning process of two-dimensional constellation,convolutional code and mapping by set partitioning at the transmitter,subset decoding and viterbi decoding at the receiver are introduced in detail.Finally,the existing four-dimensional trellis-coded modulation formats are summarized.4.A four-dimensional trellis-coded modulation technique based on sphere packing is proposed.Firstly,the characteristics of four-dimensional constellations based on sphere packing are analyzed.Secondly,the principle of set-partitioning are determined according to the key factors,the minimum Euclidean distance between constellation points and the average number of nearest neighbors,affecting the decision of receiver in constellation.On this basis,the steps of set-partitioning are formulated and the process is described in detail.By comparing the parameters in the constellation before and after partitioning,it is proved that the set-partitioning method is effective.After that,the appropriate convolutional code and mapping by set partitioning schemes are selected,and the subset decoding and viterbi decoding processes of the receiver are completed.Finally,the four-dimensional trellis-coded modulation based on sphere packing is realized.5.A four-dimensional trellis-coded modulation system based on sphere packing is built and simulated.The results show that the four-dimensional trellis-coded modulation formats based on sphere packing can improve the optical signal-to-noise ratio and transmission distance compared with the conventional four-dimensional modulation formats based on sphere packing on the condition that the same constellation or spectral efficiency.