On Optimization Techniques For Improving Performances Of MmWave-NOMA Systems

Mobile communication techniques are developing rapidly.Moreover,data rates and the number of devices are growing explosively.However,limited resources need to be utilized reasonably,and thus performances of communication systems are expected to be improved all the time.Recently,millimeter wave(mm Wave)and non-orthogonal multiple access(NOMA)technologies are widely considered.It is important to joint mm Wave and NOMA.In mm Wave bands,spectrum is abundant.Moreover,the paired users in mm Wave-NOMA systems may have strongly correlated channels due to the high direction of the mm Wave transmission.In addition,NOMA encourage users share the same resource,supporting massive connectivity and further improving spectral efficiency in mm Wave systems.However,the mm Wave channels are subject to high propagation loss and susceptible to blockage in general.Moreover,the gain of NOMA over orthogonal multiple access(OMA)is related to its key components,such as user paring and successive interference cancellation(SIC).Thus,to improve the performances of mm Wave-NOMA downlink systems,the challenges are involved in how to maximize the minimum signal-to-interference-plus-noise(SINR),maximize spectral efficiency,and balance energy efficiency and spectral efficiency.To address these three challenges,the optimization techniques are investigated in mm Wave-NOMA downlink systems.Correspondingly,three contributions are as follows:1.Max-min SINR among users in mm Wave-NOMA downlink systems is investigated.Consider the massive multiple-input single-output(MISO)mm Wave-NOMA downlink communication systems of the single cell,where the BS located in the center of the cell transmits directional beams to cover multiple single-antenna users uniformly distributed in the cell.Users in different positions may obtain different beamfroming gains and suffer different interference from other beams.To guarantee user fairness,we formulate an optimization problem to maximize the minimum SINR,which is non-convex.However,through variable substitution,it can be converted into an equivalent maximization problem.Moreover,based on the constraints in the converted problem,we propose the sum of power allocation coefficients based iterative algorithm(SPACIA)to obtain the max-min SINR.Furthermore,it is proved that NOMA systems outperform OMA systems in terms of max-min SINR.Moreover,we investigate how max-min SINR varies with systems parameters.Specifically,we prove that max-min SINR decreases as the number of paired users in a cluster increases.We also derive when the number of BS antennas tends to infinity and the transmit signal-to-noise ratio is high enough,max-min SINR is the function of the number of beams in the cell and the number of users in a cluster.In addition,we derive the upper bound of max-min SINR.Finally,simulation results verify effectiveness of SPACIA algorithm as well as correctness of theoretical analyses,and demonstrate that the proposed scheme outperforms existing ones in terms of max-min SINR.2.Spectral efficiency is maximized in simultaneously transmitting and reflecting reconfigurable intelligent surface(STAR-RIS)assisted mm Wave-NOMA downlink systems.Consider the BS with multiple antennas is communicating with two single-antenna users located in the reflection space and the transmission space,respectively.Because the direct communication links between the BS and users are blocked by obstacles,the two users only receive the signals from STAR-RIS.Moreover,to exploit the gain of NOMA over OMA,assume one user is closer to STAR-RIS than the other one.We propose the spectral efficiency maximization problem subject to users’ quality of service(Qo S)requirements,maximum available transmit power,the amplitude and phase shift constraints of STAR-RIS elements’ transmission and reflection coefficients,and so on,where active precoding,passive precoding and power allocation are optimized.The non-convex problem is hard to solve because of optimizing the coupled variables and designing the amplitudes and phase shifters of STAR-RIS elements’ transmission and reflection coefficients.Thus,we propose two algorithms to solve the problem,i.e.,successive convex approximation(SCA)based iterative alternating algorithm and semidefinite programming(SDP)based iterative alternating algorithm.The basic idea of the two algorithms is the same,i.e.,the original problem is decoupled and further converted into three convex sub-problems,which are alternatingly solved optimally in each iteration.Iterations terminate until the stationary solution of the original problem is obtained.Additionally,assuming only the line of sight(Lo S)path exists,based on the features of mm Wave channel,we derive the closed-form expression of spectral efficiency in OMA systems.Simulation results demonstrate the two proposed algorithms outperform existing algorithms,and NOMA is superior to OMA in term of spectral efficiency.3.The energy efficiency-spectral efficiency tradeoff is investigated in reconfigurable intelligent surface(RIS)assisted mm Wave-NOMA downlink systems.In communication systems,energy efficiency also needs to be improved.However,energy efficiency and spectral efficiency may not be improved simultaneously,and thus optimizing them jointly involves a tradeoff design.Moreover,by designing RIS elements’ amplitudes and phase shifts,energy efficiency and spectral efficiency can be improved.Therefore,consider the energy efficiency-spectral efficiency tradeoff in RIS assisted mm Wave-NOMA downlink systems.The BS is communicating with two users with higher and lower Qo S requirements,respectively.The two users can receive both direct signals from the BS and indirect signals from RIS.Firstly,we propose a Qo S-based requirement decoding strategy.The key idea is that the user with lower Qo S requirement is decoded first.Next,to pursue the joint optimization of energy efficiency and spectral efficiency,we formulate a multi-objective optimization problem,where spectral efficiency is maximized and the consumed power is minimized jointly.Based on the weighted-sum method,the original multi-objective optimization problem is combined as a single objective one,which is non-convex and hard to solve because the variables are coupled together and the unit modulus constraints in RIS elements are non-convex.Thus,we propose a SDP-based iterative alternating algorithm to solve the single objective problem,and a stationary solution is obtained.Finally,Simulation results demonstrate that compared with exsiting schemes,the proposed scheme improves both energy efficiency and spectral efficiency.In summary,to maximize the minimum SINR among users,maximize spectral efficiency,and balance energy efficiency and spectral efficiency in mm Wave-NOMA downlink systems,the corresponding schemes are proposed,respectively.Moreover,the proposed schemes are superior to existing schemes and thus contribute to the improvement of system performances,which are verified by simulation results.