Investigation Of Non-Orthogonal Multiple Access Technique In 5G Wireless Communications Systems

Emerging as a new multiple access technique for 5G wireless communications systems,power-domain non-orthogonal multiple access(NOMA)enables simultaneous transmission of multiple signals over the same time/frequency/code domain,and applies successive in-terference cancellation to separate the individual signals.Compared with the conventional orthogonal multiple access(OMA),the use of NOMA can significantly improve spectrum efficiency,user access ability,and user fairness.Recently,NOMA has been attracted enor-mous academic and industrial attention.The current research about NOMA mainly focuses on the aspects of transmission efficiency,reliability,and security.On the one hand,the combination of cognitive radio and NOMA can further improve spectrum efficiency,nevertheless,designing reliable cognitive NOMA transmission is a challenging task.First,due to the coexistence of inter-network interference between primary and secondary networks and intra-network interference caused by power domain multiplexing of NOMA signals,the reception quality of NOMA users will degrade significantly.Hence,it is necessary to investigate a cooperative cognitive NOMA transmis-sion scheme to improve spectrum efficiency and reception reliability.Second,existing litera-ture is limited to unicast cognitive NOMA transmission,when considering unicast-multicast cognitive NOMA transmission,it is rather challenging to promise reliability since the multi-cast message needs to be successfully delivered to multiple users.Therefore,it is important to design a cooperative unicast-multicast cognitive NOMA transmission scheme.On the other hand,due to the broadcast nature of wireless channels,the confidential information intended for multiple NOMA users is vulnerable to passive eavesdropping,thus resulting in several physical layer security issues.First,if directly employing the artificial noise(AN)-aided secrecy beamforming scheme designed for OMA to secure NOMA transmission,AN will leak into the range space of legitimate channels,which leads to a poor reception quality.This motivates us to study new secrecy beamforming scheme that is applicable for secure NOMA transmission.Second,most of current works consider cooperative NOMA with trusted relays,however,in practical networks the relay used for cooperation is untrusted.In this situation,how to design efficient cooperative jamming and secure transmission strategy is of great significance in NOMA assisted untrusted relay networks.Based on the aforementioned observations,this thesis provides a systematic study of NOMA technique to improve transmission efficiency,reliability and security of wireless communi-cations.In particular,the main contributions of this thesis are summarized as follows.1.NOMA Enabled Unicast Transmission in Cognitive Radio NetworksNOMA enabled unicast transmission in cognitive radio networks is investigated.First,we consider a single-user cooperation scenario.A novel NOMA-based cooperative spectrum sharing scheme is proposed,whereby a secondary user serves as a relay and helps for-ward primary and secondary signals simultaneously by using NOMA principle.To evaluate the outage performance of the proposed scheme,outage probability and outage throughput for both primary and secondary networks over Nakagami-m fading channels are derived in closed form.Theoretical and numerical results manifest that:(1)a win-win situation is achieved for both primary and secondary networks,e.g.,the outage performance is enhanced for primary network and opportunity of spectrum access is granted to secondary network;(2)compared with the OMA based spectrum sharing scheme,the proposed scheme uses less time slots for cooperative spectrum sharing,thus significantly improving spectrum efficien-cy.Next,we focus on a multi-user cooperation scenario.Two secondary user scheduling schemes named R-SUS and F-SUS are proposed,where a secondary user is opportunis-tically scheduled to enable the NOMA-based cooperative spectrum sharing,with targets at minimal primary/secondary outage probabilities and superior user fairness,respectively.Under independent but non-identically distributed Rayleigh fading channels,we derive exact closed-form primary and secondary outage probabilities along with its asymptotic expres-sions.The results reveal that:(1)the R-SUS and F-SUS schemes can achieve a full diversity order which equals to the number of secondary users;(2)although the F-SUS scheme en-hances user fairness,it suffers a secondary outage probability loss compared with the R-SUS scheme.2.NOMA Enabled Unicast-Multicast Transmission in Cognitive Radio NetworksThe application of NOMA enabled unicast-multicast transmission in cognitive radio net-works is investigated,where a base station simultaneously serves a unicast primary user and multiple multicast secondary users by NOMA principle.On this basis,a dynamic coopera-tive MCR-NOMA scheme is proposed.With the available CSI,the best multicast secondary user is scheduled as a relay to minimize the outage performance of primary and secondary networks.Analytical expressions of primary and secondary outage probabilities are derived over the independent but non-identically distributed Rayleigh fading channels.Furthermore,we introduce a new metric,referred to as mutual outage probability to characterize the co-operation benefit of the overall network.The results demonstrate that:(1)significant perfor-mance gains are obtained for both networks,thanks to the use of our proposed cooperative MCR-NOMA scheme;(2)higher spatial diversity order can be achieved by opportunistical-ly utilizing the CSI available for secondary user scheduling;(3)with full instantaneous CSI,the diversity orders of primary and secondary networks are equal to the number of multicast secondary users.Thus,their outage probabilities are exponentially reduced with number of multicast secondary users,indicating the proposed scheme overcomes the drawback of multicast transmission that the reliability degrades with number of multicast users.3.Physical Layer Security of NOMA in Multiple Antenna NetworksWe study the physical layer security issue of a two-user NOMA transmission in multiple antenna networks.A novel secrecy beamforming scheme using AN is proposed to protect the confidential information of two NOMA assisted legitimate users,where AN is generated in the null space of the legitimate users,such that only the wiretap channel is degraded.Con-sidering a practical assumption of the imperfect worst-case successive interference cancella-tion which is a unique character in employing NOMA transmission,we derive closed-form expressions of secrecy outage probabilities for the two users.After that,we carry out an analysis of secrecy diversity order in high main-to-eavesdropper ratio regimes.Numerical results are provided to demonstrate the accuracy of the developed analytical results and the effectiveness of the proposed scheme.4.Physical Layer Security of NOMA in Untrusted Relay NetworksPhysical layer security issue of NOMA in untrusted relay networks is investigated.First,we consider a conventional three-node cooperation scenario.A novel NOMA inspired re-laying protocol is proposed,targeted at maximizing the multiplexing gain by allowing the source node and the untrusted relay node to transmit simultaneously for security enhance-ment.Based on this protocol,we present two transmit antenna selection schemes to further improve the secrecy performance.The secrecy rate performance of the proposed protocol is examined under Rayleigh fading,where an approximation of ergodic secrecy sum rate(ESSR)are obtained with the help of the Gauss-Chebyshev quadrature.For further insights,the ESSR scaling law is also derived.The results manifest that the ESSR of the proposed protocol scales as log((?)log Nt),where p denotes the system average signal-to-noise ra-tio(SNR),and Nt denotes the number of transmit antennas at the source node.Next,we study a two-user cooperative NOMA scenario in which the untrusted relay has the same signal detection capability as the legitime users.An adaptive friendly jamming scheme is proposed where the far user adaptively transmit a jamming signal to intentionally misguide the untrusted relay while not affecting reception of the near user.The lower bound of ESSR and ESSR scaling law are obtained in Rayleigh fading channels.Theoretical results show that,the ESSR scales with the system average SNR according to 3/4 log ?,thus significantly improving the secrecy performance of cooperative NOMA systems.