Efficient Transmission Strategies Based On Non-Orthogonal Multiple Access

Non-orthogonal multiple access(NOMA)is a significant technique to realize high spectral efficiency and massive connectivity in 5G wireless communication networks;hence it has recently received significant research attention both in industry and academia.Different from the traditional orthogonal multiple access(OMA),NOMA via power domain multiplexing allows multiple users to occupy the same resource block,and applies superposition coding at the transmitters for multiplexing users within the power domain and invokes successive interference cancellation(SIC)at receiver for detection.This academic dissertation focuses on NOMA and its applications for intensive study.With several impacting factors comprehensively considered,such as beamforming technology,power allocation and multiuser fairness,the applications of NOMA in wirelss powered transfer,D2D communications,secure broadcast systems and mobile edge computation(MEC)systems are investigated in this dissertation.The main results and contributions are summarized as follows:1.To investigate the energy consumption of the MISO-NOMA downlink system with wireless power supply from the energy transmitter,a design based on the service(QoS)requirements of the users and the power constraint of the wireless device is proposed to minimize the power of the energy transmitter.The energy-constrained full-duplex wireless device harvests energy from a dedicated energy transmitter and its loop channel,and transmits the superposition coded signal to the downlink users.To depress the interference of the energy signal to information signal,a joint optimization of the energy beamforming and the information beamforming is adopted to minimize transmission power of the energy transmitter.The technique of semi-definition relaxation(SDR)is applied to solve the nonconvex optimization problem and the optimal solution is obtained Furthermore,the tightness of the SDR is proved by showing the existence of the rank-one optimal solution.Besides,a suboptimal scheme with lower computational complexity is proposed,and the suboptimal solution based on interference-nulling in closed form is derived by applying the Lagrangian dual method.The simulation results show that the proposed scheme has lower energy consumption at the energy transmitter than the orthogonal multiple access(OMA)scheme2.For a NOMA-based D2D system in a small cell,a precoding design scheme based on NOMA users' QoS and fairness is proposed.Under the given decoding order,each ordinary user can decode information by SIC.By employing fractional program and iterative algorithm,the optimal precoding vectors of the multi-antenna user with information transceiver are designed to maximize the rate of latter decoded information.Meanwhile,a suboptimal scheme based on singular value decomposition(SVD)is derived to reduce the computational complexity of the algorithm.The simulation results show that the proposed design of precoding vectors has higher achievable rate and communication efficiency than the conventional OMA scheme,such as time division multiple access(TDMA).3.In terms of secure and efficient transmission of unicasting information in multi-user downlink broadcast system,a secure beamforming design for the NOMA-assisted multicast-unicast transmission is proposed.The multi-antenna base station(BS)transmits the multicasting information to each user and simultaneously transmits the unicasting information to the specific user.Each user can decode the multicasting information first,and then decode the unicasting information.Thus the other users are all the potential eavesdroppers of unicasting information.Subject to the QoS requirements of the multicasting users and the power budget constraint of the base station,secure achievable rate of the unicasting information is maximized by jointly optimizing the beamforming vectors of multicasting and unicasting information.Note that the formulated problem is non-convex and hard to tackle.And a two-stage procedure is proposed to solve it optimally by dealing with two sub-problems iteratively.First,given the maximal signal to interference plus noise ratio(SINR)of unicasting information in other users,the corresponding SINR of unicasting information at the particular user is obtained by using SDR.Then,the maximal secret unicasting rate is derived iteratively by a one-dimension search over the intercepted SINR of unicasting information of other users.At last,according to the tightness of SDR,the corresponding secure beamforming design of multicasting and unicasting is presented.Furthermore,two suboptimal solutions with lower complexity,based on zero-forcing(ZF)beamforming and maximal ratio transmission(MRT)respectively,are also proposed.Numerical simulation shows that the proposed NOMA assisted schemes always have a better security performance than conventional OMA.4.To investigate secure offloading and energy efficiency of the mobile edge computation(MEC)system with eavesdropping,an uplink NOMA-based MEC system consisting of one access point(AP)integrated with an MEC server,multiple end users and an external eavesdropper is proposed.Under the NOMA and partial offloading setup,all the users can simultaneously offload partial computation tasks to the AP over the same resource(time/frequency)block.Since the passive eavesdropper's instantaneous CSI cannot be known by the AP in practice,we take the secrecy outage probability as the secrecy metric to measure the secrecy offloading performance of the NOMA-based MEC system.Based on the constraints of the secrecy offloading rates,the computation latency and the secrecy outage probability,the users' weighted sum-energy consumption is minimized by jointly optimizing the numbers of locally computed bits,the power allocation,the codeword transmission rates and the confidential data rates at the uplink users Leveraging the state-of-the-art optimization approaches to solve the nonconvex optimization problem,the optimal solution in a semi-closed form is derived.Numerical results are provided to evaluate the performance of our proposed design,compared with that of the secure NOMA full offloading scheme and the secure OMA-MEC scheme.It is shown that our proposed design can significantly reduce the energy consumption.