Key Technology Research Of High-dimensional Modulation

With the constant development of cloudcomputing and bulk data handling techniques,and the new types of user experiences such as 4K resolution,Virtual Reality and high definition live,all the demand for high-bandwidth services like these,together with the expanding the scale of data center and enterprise network,multiple factors accelerate the development of optical communication transmission network to ultra high speed and large capacity.How to realize the upgrade of the present transmission network without additional trunk line rebuilding cost is one of the major problems of present optical communication system confront.Study shows that,employing high order modulation formats can improve the system transmission capacity,but spectral efficiency and power efficiency is a pair of contradictions,high order modulation formats make the distance between received constellation points very near,so that it makes receiver apt to error decision.Therefore,though the spectral efficiency of high order modulation formats is high,the power efficiency becomes low.The consequence of power efficiency becoming low is that at the same transmission power and bit error rate,the transmission distance will be decreased.To solve this problem,high-dimensional modulation formats emerge.High-dimensional modulation formats combined apply multiple degrees of freedom including orthogonal state of optical field,time slots,frequency of light waves and polarization state to modulate data.The increased degrees of freedom can maximize the minimum Euclidean distance without loss of spectral efficiency,and then increase the power efficiency,thus,the contradictions of spectral efficiency and power efficiency existed in present two dimensional modulation formats can be solved successfully.However,as an emerging technology,many problems should be solved for high-dimensional modulation in practical application,such as the problem of bit to symbol mapping,thecarrying of high-dimensional modulation symbol,and the signal processing.Only when these problems are solved well,high-dimensional modulation can give full play to function in optical communication system,and then solve the contradictions of spectral efficiency and power efficiency,finally the smoothly updating of optical communication system can be realized.In this dissertation,the key technologies of high-dimensional modulation are investigated.The generation mechanism and implement method of high-dimensional modulation signal are analyzed,based on the existing methods;we make a thorough study of the problems of bit to symbol mapping,the carrying of high-dimensional modulation symbols and the signal processing.The main summarized as contributions and the innovative points of this dissertation are follows.1.Aims at the problem of bit to symbol mapping,the method based on tree structure is proposed.By constructing a tree structure of constellation points,any neighboring constellation points become nearest-neighbor constellation points with minimum Euclidean distance,which in turn,changes the bit-to-symbol mapping problem in multidimensional signal modulation from random to orderly.Then,through the orderly distribution of labels,the minimum Hamming distance between the nearest neighboring constellation points is ensured,eventually achieving bit-to-symbol mapping optimization for multidimensional signals.Compared with the random searching mapping,the proposed method can effectively improve the bit error rate performance of high-dimensional modulation system without multiple searching,reducing the computational cost.Meanwhile,it can be realized at front end,and it can be used in many kinds of modulation formats.In this paper,we describe the principle of tree mapping,and derive the limited condition and analyze the lowest Gray penalty boundary.Simulation analysis indicates that,tree mapping can effectively achieve bit-to-symbol mapping optimization under various high-dimensional modulation formats.Additionally,the system performance can be improved by using the proposed method at low computational cost.2.Aims at the problem of the carry of high-dimensional modulation symbol,a new carry scheme of high-dimensional modulation symbol with orthogonal subcarriers frequency in Optical Orthogonal Frequency Division Multiplexing(OFDM)is proposed.As a special multi-carrier communication technology,OFDM contains multiple subcarriers which can provide multiple orthogonal spaces to carry the high-dimension signal.And high-dimensional modulation signal need multiple degrees of freedom to carry.The proposed scheme utilizes the orthogonal space of OFDM to carry the high-dimension modulation signal.The existing high-dimensional modulation optical communications has been limited to 4D case(two quadratures in two polarizations).Modulation dimensions expanded beyond 4 is implemented by using time-slots,propagating modes or wavelength to map symbols more than 4D onto a 4D carrier.Implementing is more complex.While,OFDM subcarriers can provide multiple orthogonal spaces to carry the high-dimension signal,compared to these existing methods,the proposed method is more flexible and simple.Simulation analysis indicates that the proposed scheme can fully exploit the advantages of OFDM as an efficient way to deal with chromatic dispersion,at the same time it can exploit the advantages of high-dimension modulation format.Meanwhile,it can realize the carry of different dimensional modulation formats successfully and it improves the performance of present optical OFDM system.3.Aims at the problem of high-dimensional modulation signal processing,a new“constellation basis” decomposition based blind equalization processing scheme for high-dimensional modulation format is proposed.In order to reduce the computation burden,the high-dimensional modulation signal are converted into several two dimensional signal by dimension reducing processing module,by introducing the conception of “constellation basis”,assigning the equivalent 2D signal from different“constellation basis” onto different orthogonal states.At receiver,these “constellation basis” are processed by 2D blind equalization algorithms separately.Compared to the present methods,the proposed method can save the cost of training signal,and it is available for variety of modulation formats.In this dissertation,a high-dimensionalmodulation based optical OFDM system is simulated to verify the performance of“constellation basis” decomposition scheme.Simulations show that compared to data-aided least mean square based compensation.The proposed scheme shows a better bit error rate performance in the case of different high-dimensional modulation formats for both in the back to back system and after long distance transmission.And it can realize flexible spectral efficiency.In this dissertation,based on the analysis of the generation mechanism and implement method of high-dimensional modulation formats,the key technologies of high-dimensional modulation including bit to symbol mapping,the carry of high-dimensional modulation symbol and signal processing are further investigated.We propose new methods aiming at the three problems.Simulations show that the proposed algorithms have the advantages in improvement of system performance,as well as reducing the computational complexity and different modulation formats compatibility,which will provide some reference for the further research on the high-dimensional modulation.