Research On Key Technologies For Improving Air To Ground Link Communication Capacity Of Cluster UAV

Unmanned aerial vehicle(UAV)has the advantages of small volume,low casualties,high efficiency cost ratio and convenient deployment.As an effective means to realize unmanned,networked,information and intelligent,it is favored by more and more countries.Cluster UAV system is a system composed of several UAVs integrating intelligent perception,autonomous decision-making and cooperative control.Through division of labor and cooperation among multiple task execution units,cluster UAVs can quickly and accurately perform complex tasks such as path planning,cooperative reconnaissance,cooperative sensing and relay network services,so as to improve system performance.The air relay with cluster UAV as the carrier can be flexibly deployed at any position in 3D space to improve the coverage and the number of access users.The multiple in multiple out(MIMO)link formed by cluster UAV and ground station can use spatial multiplexing technology to improve the link communication capacity.The air to ground link between cluster UAV and ground station is usually dominated by line of sight(LoS)propagation,and the size of ground antenna array is usually limited.These factors will lead to highly correlated low rank channels,thus limiting the improvement of system capacity.When a large number of ground devices access the cluster UAV network at the same time,the contradiction between the excessive number of access and the limited wireless network resources makes the ground users need to compete for access,resulting in access collision and network congestion.Sparse code multiple access(SCMA)allows multiple users to share the same resources,so as to improve the number of users accessing the system.However,the traditional SCMA codebook is designed based on the mother codebook(MC),which leads to the same communication rate for all users.Although this simplifies the design process of codebook,it is difficult to meet the needs of diversified applications,waste time-frequency resources and reduce the communication capacity of the system.In addition,terminals sharing resources interfere with each other,reducing the bit error rate(BER)performance and communication capacity of the system.Aiming at the above problems,this paper studies the improvement of air to ground link communication capacity of cluster UAV from the location optimization of cluster UAV and SCMA codebook design.(1)For the low rank and low capacity channel formed by cluster UAV and ground station,the communication capacity is improved by optimizing the deployment of cluster UAV.In the scenario of virtual MIMO composed of cluster UAVs and ground stations,this paper uses the condition number(CN)and inner product(InP)of the channel matrix to measure the communication capacity of the system.At the beginning,the position constraint equation of cluster UAV is derived,and the particle swarm optimization(PSO)algorithm is used to optimize the deployment of cluster UAV to minimize the condition number of channel matrix;Then,the inner product expression of the channel matrix is derived,after that,the gradient of the inner product of the channel matrix with respect to the position of each UAV is derived.The deployment position of each UAV is iteratively optimized by using the gradient descent(GD)algorithm;Finally,PSO-GD algorithm is proposed to quickly optimize the deployment of cluster UAV.Simulation results show that the three algorithms can reduce the condition number and inner product of the channel matrix,and PSO-GD algorithm can converge to a smaller condition number and inner product in fewer iterations.(2)Aiming at the problem of equal user communication rate in traditional SCMA system,a method of dividing extended mother codebook(DEMC)is proposed to design SCMA codebook.Firstly,an extended mother codebook is designed according to golden angle modulation(GAM)and system parameters,which includes multi-dimensional codewords used by all users,and the power and phase correlation between symbols in different dimensions in the codebook is constructed;Then,a fixed rate codebook is designed by dividing the extended mother codebook into several mother codebooks with the same modulation order;Next,the extended mother codebook is divided into several mother codebooks with different modulation orders to design the variable bit rate codebook.The simulation results show that the BER performance of the fixed rate codebook designed in this paper is close to that of the existing codebook;Compared with the fixed rate codebook,the BER performance of different modulation order codebooks in the variable bit rate codebook has little change.Users can realize high-speed information transmission by applying for high-order codebook without increasing the transmission signal power and bandwidth,so as to flexibly meet the needs of applications with different data transmission rates.(3)Aiming at the serious interference between users in traditional SCMA system,which leads to the unsatisfactory BER performance of the system,a method of decomposing and superimposing constellation(DCSC)is proposed to reduce the interference between users,so as to improve the BER performance of SCMA codebook.Firstly,it is proved that the sum power of user constellation(UC)on each resource is determined by the power of superimposed constellation(SC).Therefore,the power of user constellation can be controlled by controlling the power of superimposed constellation,so as to improve user power efficiency;Then,a simple method is proposed to decompose a uniformly distributed large-scale quadrature amplitude modulation(QAM)constellation into several small-scale uniformly distributed QAM constellation,so as to ensure that the superimposed constellation has a large minimum Euclidean distance(MED)between superimposed symbols,so as to suppress the interference between users;Next,by repairing the symbols in different dimensions in the codebook to maximize the MED between codewords to increase the ability of anti-interference and channel fading suppression;Finally,linear correlation is constructed between symbols in different dimensions of codewords,SCMA communication system is transformed into MIMO communication system,and MMSE-SIC algorithm is proposed to demodulate user signals.Since there is obvious power diversity between user constellations on the same resource,serial interference cancel(SIC)can be used to demodulate user signals.Simulation results show that due to the reduction of interference between users,the BER performance of the proposed codebook is better than that of the traditional codebook.