Non-Orthogonal Multiple Access Technology For Ultra-Reliable Wireless Communications

In wireless communication systems,due to the fading and interference of wireless channels,the communication reliability plays an important role in the design of wireless communication systems.In fifth generation(5G)communication systems,high reliabili-ty communication design will become more complicated.Unlike previous generations of wireless communication systems,5G communication systems not only need to provide high throughput but also support massive connectivity in massive machine type com-munication(mMTC)scenarios and also need to provide ultra-reliable and low-latency communications(URLLC)services for some special scenarios,such as industrial inter-net,automatic driving,etc.These requirements bring great challenge in ensuring reliable communication of the system.For example,in the mMTC scenario,the system needs to provide reliable communication while supporting several hundreds of connectivities.The URLLC scenario requires a much higher communication reliability,which requires the system to guarantee a delay of 1 millisecond while ensuring no more than 10-5 packet er-ror rate.Traditional communication systems are based on the orthogonal multiple access(OMA)schemes,such as time division multiple access(TDMA),frequency division mul-tiple access(FDMA)or orthogonal frequency division multiplexing access(OFDMA).However,such OMA schemes would waste system resources,which is not conducive to the realization of 5G system requirements.NOMA allows multiple users to concurrently access to the same system resources,thereby improving the spectral efficiency of the sys-tem and also supporting more connections.Therefore,it is of great practical significance to consider the design of high reliability systems based on NOMA.This dissertation proposes three strategies to improve the communication reliabil-ity based on NOMA.Specifically,we first propose a cooperative NOMA transmission strategy to improve the reliability of cell edge user.Then,we introduce hybrid automatic repeat request(HARQ)technique into the design of the NOMA system to improve sys-tem reliability by jointly optimize the transmission of multiple time slots.Finally,we further apply the NOMA technique to the design of the URLLC system and use the finite blocklength codes to precisely characterize the system transmission delay and reliability.The contents of these three chapters are aimed at the non-features and requirements of 5G communication scenarios,and different communication technologies are used to ensure the communication reliability of systemThe main innovations and contributions of this dissertation are listed as follows1)For mMTC scenario,we propose to use the cooperative NOMA to improve the com munication reliability of the cell-edge user.Considering that the users in mMTC scenario is usually energy-limited sensors.Therefore,in order to increase the energy efficiency,we introduce wireless energy transfer technology into the cooperative NO MA system,where the cell-center user acts as an energy harvesting relay to assist the communication of the cell-edge user.First we investigate the multi-antenna scenario and propose a suboptimal but with low-complexity algorithm to solve the considered problem.Then,for practical system considerations,we consider the system design in the single antenna scenario.By analyzing the special structure of the problem,we propose an low complexity algorithm which can guarantee the global optimality of the solusions.This means that our proposed strategies and algorithms have high value of practical applications2)We introduce HARQ technique into the design of NOMA-based systems.First,one advantage of this strategy is that it does not need to know the exact information of the channel,but only the statistical information of the channel.This will reduce the sig naling overhead of the system.Secondly,by jointly designing a transmission strategy for multiple time slots,the system reliability and energy consumption performance are improved.However,the system analysis and design based on HARQ are challenging We first use the probability and statistics theory to obtain the probability expression of the system.Then based on the derived results,we propose an effective algorithm to guarantee the system performance.In addition,we extend our proposed strategy to multi-user scenarios,which makes our algorithms more practical3)We propose the NOMA based strategy for URLLC system.By introducing the finite blocklength codes into the design of the NOMA system,the relationship between the communication reliability and delay of user are precisely described.In addition,con sidering the diversification of user requirements in the system,we consider hetero geneous user systems where users have different delay and reliability requirements This poses a great challenge to the design of the NOMA-based system.Under dif,ferent system conditions,we propose several new SIC strategies.Then,by analyzing the potential structure of the problem,we propose an efficient algorithm with low complexity to solve the considered problem.