Research On Transmission Strategy And Performance Of Buffer Cooperative Non-orthogonal Multiple Access Systems

Non-orthogonal multiple access(NOMA)can achieve high spectral efficiency,which is considered to be one of the most promising multiple access technique in the future wireless communication systems.Cooperative NOMA(C-NOMA)can utilize dedicated relays or exploit the cooperation between intended users to enhance the transmission performance of wireless communication systems,and buffer cooperative communication technique provides additional freedom for C-NOMA system,which can overcome the bottleneck effect of traditional C-NOMA system.However,the design and performance analysis of buffer-aided transmission strategy with low delay and high reliability in C-NOMA system is still a challenging problem due to the influence of the spectrum resource shortage and fading characteristics of the wireless channel.For buffer C-NOMA system,the specific research contents of this thesis are summarized as follows:1.A buffer C-NOMA system model is established,which includes a base station and two users equipped with buffers.Firstly,six potential transmission modes are proposed,considering the direct transmission mode from the base station to the users and the buffer cooperative transmission mode between two users.Then,the channel state information(CSI)and buffer storge state requirements for each transmission mode are analyzed,and the probabilities of satisfying the CSI requirements for each transmission modes are derived.Furthermore,the buffer C-NOMA system is modeled by Markov,and the performance index such as the system outage probability and average delay of the system are defined.2.For the buffer C-NOMA system,the performance of traditional buffer-aided transmission strategy is investigated.Firstly,the traditional buffer-aided strategy is applied to the buffer-aided C-NOMA system,which adaptively selects between the direct transmission mode and the buffer cooperative transmission mode according to the instantaneous CSI and buffer storage state.Then,the state transition matrix of the traditional buffer-aided strategy is calculated when the base station adopts the fixed or dynamic power allocation scheme,and the closed-form expressions of the system outage probability and the average delay are derived.Furthermore,the diversity gain performance of the traditional buffer-aided transmission strategy is analyzed.It is proved that the full diversity gain(i.e.,three)can be achieved when the length of the buffer storage units is greater than or equal to three,while the diversity gain of the existing non-buffer-aided transmission strategy is only two.3.For the buffer C-NOMA system,an improved buffer-aided transmission strategy is designed and its performance is investigated.Firstly,the priority of direct transmission mode and buffer cooperative transmission mode is changed based on the traditional buffer-aided transmission strategy,and an improved buffer-aided transmission strategy is proposed: direct transmission mode is preferred when the buffer storage state is not empty and full;buffer cooperative transmission mode is preferred when the buffer storage state is empty or full.Then,the state transition matrix of the improved buffer-aided transmission strategy is calculated,and the closed-form expressions of system outage probability and average delay are derived.Furthermore,the diversity gain performance of the improved buffer-aided strategy is analyzed.The theoretical analysis results show that the minimum buffer storage units length required by the improved buffer-aided strategy is only two in order to achieve full diversity gain,while the buffer storage units length required by the traditional buffer-aided strategy is at least three.Finally,numerical results show that the improved buffer-aided strategy is outperforms to the traditional buffer-aided strategy in terms of system outage probability and average delay.