Investigation Of NOMA-based Coordinated Direct And D2D Transmission Scheme

Non-Orthogonal Multiple Access(NOMA)technology enables multiple users to multiplex the time/frequency/code domain using differentiated power level.As a result,NOMA has gained a wide attention from both academia and industry.Among the studies of NOMA,the NOMA-based coordinated direct and relay transmission(CDRT)system is deemed as a crucial cooperative relaying solution due to its higher spectral efficiency together with wider coverage.However,the traditional NOMA-based CDRT system requires the deployment of a dedicated relay node,which results in additional communication deployment costs.In addition,the existing researches on NOMA-based CDRT systems mainly focus on singlerelay assisted scenarios.Generally,the limited reliability of the single-relay cooperative scheme cannot meet the target requirements.Although the deployment of multi-relay combined with relay selection can improve the diversity gain and wireless transmission performance,it will inevitably incur greater communication overhead.Therefore,how to explore the potential gains of multi-relay system with low communication cost is another urgent problem to be solved.To tackle the aforementioned problems,Device-to-Device(D2D)technology is introduced in this thesis,which uses the D2 D devise to act as a relay to assist the transmission of the target user.Considering that the D2 D terminal has its own communication demand,a NOMA-based coordinated direct and D2 D transmission scheme is proposed to achieve a win-win situation for cellular users and D2 D users.Besides,a large number of D2 D resources are fully utilized by designing a D2 D selection mechanism to break the system performance bottleneck caused by single-relay cooperative,and further improve the reliability of wireless transmission.More specifically,the key contributions of this thesis are summarized as follows.First,a NOMA-based coordinated direct and single-pair D2 D transmission scheme is investigated,in which a pair of D2 D users is operationally arranged as a relay to assist the communication between the base station and the cellular edge user.As compensation,the D2 D device is allowed to transmit signal to the corresponding D2 D receiver by multiplexing the spectrum resources of the cellular network while relaying the signal.On the one hand,using the existing D2 D device in the cellular network to act as a relay for the cooperative system effectively reduces the communication cost.On the other hand,the spectrum resource utilization and system throughput are further improved by the collaboration between cellular system and D2 D communication.To evaluate the system performance,the exact expression of the system ergodic capacity and the asymptotic expression at high signal-to-noise ratio are analyzed.Numerical simulation results show that: 1)The proposed scheme achieves higher system ergodic capacity than the conventional NOMA-based CDRT scheme and the OMA-based coordinated direct and single-pair D2 D transmission scheme.2)The power allocation ratio at the base station and the D2 D transmitter as well as the location of the D2 D device dominate the system performance.Second,a NOMA-based coordinated direct and multi-pair D2 D transmission scheme is investigated,and a dynamic D2 D relay selection strategy is designed accordingly.This strategy aims to maximize the cellular center user’s successful decoding probability and the D2 D user’s signal reception quality with guaranteeing the reliable reception of the cellular edge user.Outage analysis shows that: 1)The cellular edge user can achieve full diversity gain while satisfying the quality of service requirements of the cellular center user and the D2 D user.The achievable diversity order is determined by the number of D2 D pairs.2)As the number of D2 D devices increases,the outage probability for all user signals decreases.In particular,the outage performance of the D2 D user improved significantly,which effectively overcomes the performance deterioration of the system due to cross interference and harsh decoding condition.3)Compared with the three traditional relay selection strategies,termed One-Stage Relay Selection,Two-Stage max-link and Two-Stage min-link,the proposed dynamic D2 D relay selection strategy achieves the optimal outage performance.