On Non-orthogonal Multiple Access In Coordinated Multi-Point Communications

Coordinated Multi-Point(CoMP)technique,which utilizes the cooperation among spatially distributed sites,can improve the quality of service(QoS)of users(especially cell-edge users)in wireless communications.However,traditional CoMP techniques are based on orthogonal multiple access(OMA),which results in low spectral efficiency.For example,when multiple sites are cooperatively serving a cell-edge user via CoMP,each site has to allocate a channel resource block for this user,and these resource blocks cannot be accessed by other users according to OMA principle.Thus,the spectral efficiency becomes worse as the number of cell-edge users increases.Applying non-orthogonal multiple access(NOMA)to CoMP transmissions,which is known as CoMP-NOMA,can effectively ease the aforementioned dilemma.The key idea of CoMP-NOMA is that when multiple sites cooperatively serve cell-edge users,each site is allowed to serve additional cell-center users via NOMA by using the same resource blocks allocated to the cell-edge users.Thus,CoMP-NOMA can support larger connectivity and improve the spectral efficiency,while the QoS of the cell-edge users can be guaranteed.In addition,from another point of view,CoMP-NOMA can also effectively solve the drawback caused by single cell NOMA,where the cell-edge user's performance is limitted.Due to the above characteristics,CoMP-NOMA has been recognized as a valuable research topic.In this dissertation,under different application scenarios,several key issues on the application of CoMP-NOMA are studied,including precoding design,transmission strategy design and performance analysis.The main work of this dissertation is as follows:1)The precoding design of a CoMP-NOMA system with two sites is considered,which aims to minimize the total transmission power under the constraints on users' data rates and highest transmission power of a single site.Firstly,consider a scenario with two sites and three users,the problem can be classified into two cases according to whether the two sites are synchronized.For each case,the precoding design problem is firstly formulated as an optimization problem.Through problem transformation and theoretical analysis,efficient precoding algorithm is proposed.Then,the conditions of channels(known as quasi-degradation conditions),under which CoMP-NOMA can achieve the same minimum transmission power as dirty paper coding(DPC),is provided.Finally,the study is extended to the scenario with two sites and multiple users.By applying the obtained results in the three-user scenario,low complexity user pairing algorithm is proposed,based on which users are grouped into multiple clusters.Further more,a hybrid multiple access CoMP-NOMA transmission strategy is proposed,where each cluster is allocated with a orthogonal sub-channel and in each cluster CoMP-NOMA or zero-forcing beamforming(ZFBF)is applied.2)Feasible uplink and downlink CoMP-NOMA transmission schemes are proposed for dense networks,which aim to increase the system connectivity and improve spectral efficiency.Firstly,according to the distribution characteristics of the nodes in dense networks,a Poisson cluster process(PCP)based stochastic geometric model is utilized to model the topology of the network.Then,novel uplink and downlink CoMP-NOMA schemes are proposed,respectively.Moreover,based on the expressions of the signal to interference plus noise ratios(SINRs)achieved by the users and corresponding stochastic geometric theories,comprehensive analysis are developed for performance evaluation of the proposed schemes.Finally,computer simulation results are provided to verify the accuracy of the developed analytical results and also demonstrate the superior performance of the proposed CoMP-NOMA schemes compared to traditional OMA based schemes.3)The performance evaluation is studied when CoMP-NOMA is applied to a vehicular communication network,where the property that sites and vehicle users are distributed along roads is seriously considered in the system topology modeling and analysis.Conventional modeling methods which are based on Poisson point process(PPP)are suitable for the modeling and analysis of traditional dense networks.However,they fail to capture the topology characteristic of vehicular communication networks where sites and vehicle users are distributed along roads.To fulfill this drawback,this work models the topology of the considered vehicular network by applying a Poisson line Cox process(PLC)model.In this model,the road system is modeled as a Poisson line process(PLP),and the sites and vehicles are modeled as one dimensional PPPs on each road.Then,the signal modeling for an Alamouti coding based downlink CoMPNOMA scheme is provided.Moreover,the outage performance achieved by cell-edge and cell-center users are analyzed.Further,the impact of power allocation coefficient on the outage performance is obtained,based on which an optimal power allocation algorithm is developed,where the outage probability of the cell-center user is minimized by ensuring the performance of the cell-edge user.