Research On Cooperative Diversity Network Under New Communication Technologies

Meeting the heterogeneous requirements,such as low latency,high speed,large-scale connectivity and high throughput,etc.is an important sign that the fifth generation(5G)mobile communication realizes the "Internet of Everything" effect.Therefore,using novel communication techniques to establish a new heterogeneous network with multiple functions is a breakthrough to address the above requirements.Among the new techniques,non-orthogonal multiple access(NOMA),unmanned aerial vehicles(UAV),energy harvesting,and free space optical(FSO)communication have been proved that they can effectively hence the performance,like spectrum efficiency,data transfer rate and energy efficiency.In particular,UAV can supplement the existing communication network to form a communication system with emergency missions,while the FSO systems have the advantages of large capacity,high data rate and no interference with the RF system and so on.In addition,compared with the traditional orthogonal multiple access(OMA)cases,the breakthrough of the NOMA system is to be able to serve the receivers sharing the same resource block(such as time/frequency)by allocating the power resource,thereby significantly improving the spectral efficiency.At the same time,cooperative communication is one of the necessary means to build new heterogeneous networks because it can link existing wireless communication systems with these emerging technologies.However,until now,cooperative communication systems using these novel techniques still lack of sufficient consideratio ns,such as physical layer security for cooperative NOMA systems and UAV cooperative communication systems in urban environment.Therefore,this thesis will study the NOMA,UAV and FSO techniques based on cooperative communication.The main contents of this work are given by the follows:1.In this thesis,we first study a static NOMA system with multiple AF-based relays and compare it with the cooperative NOMA system under decode-and-forward(DF)mode.At the same time,a two-stage relay selection(TSRS)algorithm for the amplify-and-forward(AF)cooperative NOMA system is proposed.The performance of the TSRS is then compared to the traditional max-min relay selection(MMRS)strategy in the simulation section.On this basis,we provide the performance analysis and asymptotic analysis of this static cooperative NOMA system.Also,the diversity gain of the system is given according to the asymptotic result.Finally,we investigate the relay coverage area of the NOMA relaying system to demonstrate how the rela ys improve the coverage of the cooperative NOMA system.2.Based on the static cooperative NOMA system mentioned before,we propose a dynamic cooperative NOMA system with spatially roaming relays and compare its performance between adopting AF and DF protocols.Then,by modeling a Rayleigh fading,the wireless propagation environment that a large number of radio signals scattering are caused by urban obstacles can be effectively simulated.From practical consideration,we adopt a stochastic geometry-based model and assume that the spatial topology of relays is modeled by using homogeneous poison point process(HPPP).Based on such a setting,an effective coverage area of the relays modeled by HPPP in cooperative NOMA systems is developed and performance comparison between TSRS and the conventional MMRS strategies is also presented.According to the locations of the NOMA users,we develop the complete strategies for calculating the effective coverage area of the relays.Furthermore,asymptotic analysis is provided to show that the outage probability tends to a constant which is only related to the density of HPPP and the effective coverage area of the relays.For a given outage probability,we reveal the relationship between the shortest and longest radiuses of the effective district of the relays.Finally,Monte Carlo simulations are provided to verify the accuracy of the analytical results.3.We comprehensively study the physical layer security for cooperative NOMA systems,where both AF and DF protocols are considered.Secondly,we use Rayleigh fading to model the wireless channel in urban environment.More concretely,the closed-form lower bound expressions of secrecy outage probability(SOP)and strictly positive secrecy capacity(SPSC)are proposed as the theoretical results in this thesis.Results show that AF and DF almost achieve the same secrecy performance.Finally,asymptotic results and diversity orders for the cooperative NOMA systems are demonstrated in this work.4.We investigate a UAV-assisted relaying system with energy harvesting over different urban environments.More specifically,we provide the outage performance of this system and analyzes the parameters related to the altitude of UAV.Since the transmission environment between the UAV and the ground node in urban is complicated,two simple approximated statistical models are used to characterize the channel statistics of Air-to-Ground(AtG).In addition,given the parameters of the approximate statistical model,the mean and variance of the AtG channel can be solved by using the numerical root finding method inside MATLAB.Finally,the outage probability can be minimized by adjusting the height of the UAV.5.Finally,we design a new type of optical energy harvesting system based on FSO system.This optical energy harvesting device can collect the direct current(DC)component from the subcarrier intensity modulation in FSO system to charge the receiver.In previous,DC bias signals are usually filtered out,thus causing waste of energy resources.Then,the effects of such energy harvesting techniques on the performance of the dual-hop FSO-RF system were investigated.From practical considerations,the FSO channel is assumed to be impaired by both atmospheric turbulence and pointing errors.Based on the settings,the outage probability expression of the system is derived in terms of a bivariate Fox-H function.Finally,we demonstrate the asymptotic analysis process and disclose the diversity order of the system in this thesis.By comparing the theoretical results with simulations,we provide the effective theoretical support for the practical systems and propose the viable ways to enhance the performance of the novel cooperative communication techniques.