Interference Alignment For Downlink Non-orthogonal Multiple Access Systems

With the rapid development of communication and mobile Internet,the fifth generation mobile communication(5G)has higher and higher requirements for spectrum efficiency and number of connections.Non-orthogonal multiple access(NOMA)is considered to be one of the most promising technology to address challenges of 5G communication,as it can achieve high throughput,support massive connectivity,improve user fairness and meet other heterogeneous requirements.However,the interference caused by superposition affect s the accuracy and complexity of signal processing.Downlink multiuser signal detection and interference alignment(IA)for power domain NOMA is studied in this thesis.A semi-blind scheme based on IA and independent component analysis(ICA)is proposed for a two-user scenario.At the base station,one user's signal is aligned with the other users' through phase rotation,and to recover data,the rotated phase is tracked by linear precoding.At both users' receivers,only 1-2 pilot symbols are required to eliminate the ambiguity caused by ICA.Compared with conventional methods based on channel estimation and successive interference cancellation(SIC),the reliability and spectrum efficiency is improved.Closed-from expressions are derived for the users' symbol error rate(SER)performance.According to the expressions,an iterative power allocation scheme based on statistic channel state information(CSI)only is proposed to minimize the average SER.Simulation results show that the proposed power allocation scheme converges fast.Aiming at the inter-cell interference cancellation in multi-cell scenario,a twostep blind interference alignment(BIA)scheme is proposed.Compared with conventional IA schemes,global CSI and data sharing among different cells are not needed.Reconfigurable antenna is using at receivers to construct supersymbol by switching among different modes to realize interference alignment,and has a lower requirement of channel state.Aiming at the problem that the supersymbol length is too long,multi cells are divided into several groups,the intra-group interference is aligned first and then the inter-group interference is aligned.The analytical expressions of system degree of freedom(Do F)and supersymbol length with different number of transmit antennas and cells are derived based on the proposed two-step scheme.Results show that the proposed scheme reduces the supersymbol length exponentially and can achieve a high Do F with a limited supersymbol length.