On The Channel Capacity Of Orbital Angular Momentum Based Wireless Communications

Popularity of smart phones has led to the rapid increase of information exchange through wireless channels.Furthermore,with smart devices becoming common and popular,people have growing needs for the capability of wireless access network.Rapid developments of new technologies such as Internet of things,cloud computing and mobile Internet promote further demands on the mobile data transmission.In spite of the innovation of novel signal processing method and resource management strategies,traffic congestion in the available bands is inevitable.Therefore,it is urgent to increase spectrum efficiency.One of the possible solutions to substantially improve capacity is using orbital angular momentum(OAM),a totally new resource dimension in radio communications.Since the infinite set of OAM modes forms an orthonormal basis,which can be used as information carriers for multiplexing,OAM is hopeful to increase the capacity of the communication system in proportion to the number of used OAM modes.The application of OAM to the wireless network faced challenges.For one thing,unlike traditional communication,measurement of OAM is highly dependent on beam axis in the far field.Once transceivers are not aligned,detection results are thus not trustworthy.It is noted that a single mode,subject to axis misalignment,is no longer described by a single value of OAM,but a series of distinct OAM states,which means that the misalignment of OAM broadens OAM power spectrum.Besides,the more seriously the axis is misaligned,the wider dispersion the power spectrum is subject to.Hence,the issue of axis alignment is worthy of studying in the open radio environment.For another thing,data transmission rate and quality of wireless communication system are restricted to the channel state.Only after thorough analysis and understanding of the characteristics of the wireless channel,can we adopt suitable techniques on the physical layer.The advantages of OAM communications are simultaneous transmission of superposition of orthogonal modes in a single beam.In free space environments,OAM remains conserved and modes’ orthogonality retains in transmission.With influences of reflection,scattering and refraction on the transmission route,the phase front will be distorted,and crosstalk will occur among OAM modes.This will lead to errors and mistakes in the communication and thus degrades the system performance.Therefore,analysis on the channel model and system capacity is of significance both on theory and in practice.Contributions of the thesis are summarized as following:Firstly,the thesis focuses on the influences of misalignment between transceivers and the automatic method of axis alignment in orbital angular momentum based communications.By means of signal analysis theory,we decompose OAM beam subject to axis misalignment into a linear combination of infinite set of OAM harmonics.The influence of axis misalignment on the OAM beam is quantitatively analyzed and validated by the simulation results.In order to overcome this problem,novel automatic method is proposed to accurately amend axis misalignment.The crux of axis alignment is an optimal problem which is aimed at optimizing the objective function.We propose that the variance of OAM power spectrum represents the degree of dispersion of OAM modes.Simulation results have validated its feasibility.Spatial misalignment with respect to the axis can be generally divided into two types:lateral displacement and tilt.Therefore,we propose that the adjustment is implemented on these two dimensions.Numerical simulations validate the feasibility and effectiveness of this method,which have shown that with our alignment method,the accuracy of detection reaches 99.8%.In practice,monotonous OAM beam that is previously agreed upon by transceivers can be exploited as training signal for the rectification of the axis misalignment.After aligning,multiple OAM modes can be used to transmit massive date.Secondly,it is concentrated on the OAM communication channel model and system capacity.In our analysis,at the receiver,the scattered electric field caused by blocks and objects on the path is modeled as the sum of contributions from all the independent sub-paths.Due to the different distances in the multiple sub-paths,time delays are created in each sub-wave.We have found that influences of reflection and scattering on OAM communication are to introduce a random phase which will distort the spiral phase structure of OAM carrying beam,resulting in the nonorthogonality of OAM and transmission of OAM modes from the original one to others with probability.We derived transmission probabilities of OAM subject to interference and noise.Based on characteristics of OAM communication,we employ the discrete memoryless channel in the Information Theory to depict OAM channel.Simulation indicates that increasing scattering leads to the degradation of the channel capacity.Furthermore,it is capable to mitigate the effects by increasing the spacing between detected OAM modes.This study has implications for OAM based wireless communication systems.