Optimized Design Of Sparse Code Multiple Access Systems And Dense Networks

With the explosive growth of mobile services,the next generation wireless communication system is expected to face the challenge of a large number of connecting devices.On the one hand,the number of mobile users is increasing,and the services required by users are becoming more and more diversified;on the other hand,the space and frequency resources are limited.In this context,users also need to get higher transmission rate with lower delay.In the future,wireless communication transmission will be carried out more often in the high-density node scenario.Therefore,it is necessary to study non-orthogonal multiple access and dense communication networks.Compared with existing systems,non-orthogonal multiple access can not only provide higher spectrum efficiency,lower delay,but also support more access users.Firstly,in this thesis,a novel codebook design method for sparse code multiple access(SCMA)is proposed.In particular,to meet the codebook design criteria based on the pairwise error probability(PEP),a novel codebook with large minimum Euclidean distance employing the star quadrature amplitude modulation(Star-QAM)signal constellations is designed.The simulation results show that the presented SCMA codebook outperforms the existing codebooks and low density signature(LDS),and the proposed design is more efficient for the SCMA codebook with large size and/or high dimension.In addition,in this thesis,an optimized codebook design based on rotated angles(RAs)and extrinsic information transfer(EXIT)chart for a nonorthogonal multiple access scheme,called irregular sparse code multiple access(Ir SCMA),is presented.Unlike regular SCMA,in Ir SCMA,the defined degree of layer's resource nodes is different,which is beneficial to system performance.It is demonstrated that the new codebook can greatly improve the BER performance especially when the signal-to-noise ratio(SNR)is high,without sacrificing the low detection complexity,compared with the existing codebooks and LDS.Secondly,in this thesis,the average bit error rate(BER)performance of sparse code multiple access(SCMA)systems with codebooks based on star-QAM signaling constellations over additive white Gaussian noise(AWGN)channel and Rayleigh fading channels are analyzed and evaluated.Motivated by the fact that the phase rotation plays an important role in designing codebooks and thus can significantly affect the BER performance in SCMA system,we derive a theoretical expression for the BER performance based on the statistics of the phase angle in SCMA constellations.Numerical and simulation results corroborate the proposed analysis.In addition,with the aid of the phase distribution of the presented SCMA constellations,BER of SCMA system over downlink Rayleigh fading channel is obtained in closed-form expression.The derived theoretical BER results match well the simulation results,especially in the high signal-to-noise ratio(SNR)regimes.In addition,the simulation results show that SCMA has the same diversity order when the value of N is the same in the system.Thirdly,in this thesis,the average spectral efficiency(SE)of sparse code multiple access(SCMA)systems with adaptive codebooks based on star-QAM signaling constellations is analyzed and evaluated.Motivated by the fact that the different power allocations have different effects on SE in SCMA systems,we propose three different power allocation schemes to maximize SE based on the existing theoretical bit error rate(BER)expression.In the case of different codebook sizes,the numerical results of three different approximate models of BER match well with the analytical results.In addition,we compare SE between the SCMA and OFDMA systems under the same power allocation scheme.Numerical and analytical results corroborate the proposed power allocation schemes.Fourthly,in this thesis,the optimal design of downlink sparse code multiple access systems based on sum-rate maximization is studied.A design scheme of SCMA codebook allocation based on hypergraph model is proposed to minimize user-centric ultra-dense networks(UUDNs)interference and improve system performance.In UUDNs,a large number of access points(APs)are densely deployed in the network to provide service to a large number of user equipment(UEs).The network architecture of UUDNs is shifted from traditional cell-centric to user-centric,where many APs can serve for one single UE,with the density of APs higher than that of UEs.One main challenge faced by UUDN design is the large interference due to the dense AP/UE deployments.The objective of this paper is to identify the optimum SCMA codebooks allocation to minimize the interference that can maximize the system throughput,subject to the quality of service(Qo S)constraints for each UE in the network.The design is formulated as a mixed integer nonlinear program(MINLP)problem by employing different codebooks among APs serving the same UE,and it is known to be NP-hard.To tackle this problem,we use a weighted hypergraph model to transform it to a clustering problem,where machine learning(ML)algorithms are proposed to solve this MINLP problem efficiently.Simulation results show that the proposed hypergraph based ML algorithm outperforms the existing algorithm.Fifthly,as we know,employing massively distributed antennas brings radio access points(RAPs)closer to users,thus enables aggressive spectrum reuse that can bridge the gap between the scarce spectrum resource and extremely high connection densities in future wireless systems.User-centric ultra-dense networks(UUDNs)are usually designed in the form of fiberwireless communications(FWC),where the distributed antennas or RAPs are connected to a central unit(CU)through optical front-hauls.The large number of densely deployed antennas or RAPs requires an extensive infrastructure of optical front-hauls.Consequently,the cost,complexity,and power consumption of the network of optical front-hauls may dominate the performance of the entire system.This thesis provides a qualitative and quantitative analysis on the architecture,modeling,design and performance of FWC systems with massively distributed antennas.The complex interactions between optical front-hauls and wireless access links require optimum designs across the optical and wireless domains by jointly exploiting their unique characteristics.It is demonstrated that FWC systems with analog radio-frequencyover-fiber(RFo F)links outperforms its baseband-over-fiber(BBo F)or intermediate-frequencyover-fiber(IFo F)counterparts for systems with shorter fiber length,lower transmission power,and more RAPs,which are all desired properties for future wireless communication systems.Finally,in the thesis,we study the optimum designs of the downlink of user-centric ultradense networks(UUDNs)with fiber-wireless communications(FWC).A large number of low power radio access points(RAPs)are densely deployed in the network to provide service to spatially distributed UUDN user equipments(UEs).The user-centric network employs more RAPs than UEs such that each RAP can serve at most one UE.The RAPs are connected to a central unit(CU)through optical fiber front-hauls.Radio-frequency-over-fiber(RFo F)is employed in the optical front-hauls to reduce RAP complexity,cost,and energy consumption.With RFo F front-hauls,wireless signals received by UEs are subject to distortions accumulated through the optical and wireless links,including optical loss,optical chromatic distortion,optical and thermal noises,wireless pathloss,and small scale fading.The optimum designs are performed across the optical and wireless domains with the help of a newly developed model that quantifies the combined effects of the optical and wireless links.One of the main challenges faced by the design of a ultra-dense network is the high energy consumption due to dense RAP deployment.The objective of this paper is to minimize the total energy consumption of the entire network,including both optical and wireless links,by jointly optimize RAP power allocation and RAP-PD association,subject to quality-of-service(Qo S)constraints for each UE.The problem is formulated as a mixed integer programming(MIP)problem.We propose a low complexity binary forcing gradient search(BFGS)algorithm,which performs a gradient-based search based on the unique structure of the problem.Simulation results show that the optical front-hauls have significant impacts on the design of ultra-dense networks,especially for millimeter-wave communications that might experience significant power losses due to optical chromatic dispersion.