Research On Frequency Offset Of Uplink Non-Orthogonal Multiple Access

Massive Machine Type Communication(mMTC)is one of the typical application scenarios of the Fifth-Generation Mobile Communications System(5G).The main feature of this scenario is the connectivity of massive small packets.Due to the defects such as low spectral efficiency,limited connections,and high signaling overhead,the conventional Orthogonal Multiple Access(OMA)schemes cannot meet the requirements of mMTC.Therefore,5G needs to introduce more flexible and efficient multiple access technologies.Among them,Non-Orthogonal Multiple Access(NOMA)has gained wide attention because of its significant advantages,such as large number of supported connections,high spectral efficiency,and low latency.Considering the complexity of implementation,NOMA systems often use Serial Interference Cancellation(SIC)receivers for Multi-User Detection(MUD).It is noted that the local oscillators of the system transceivers are inevitably non-ideal,which will introduce sampling frequency offsets and carrier frequency offsets.Even if the synchronization technology is used,the residual frequency offset will still exist.These residual frequency offsets lead to imperfect signal reconstruction and imperfect interference cancellation in the SIC receiver.The residual caused by interference cancellation will accumulate during the iteration,which may seriously degrade the transmission performance of the system and limit the access capability of the system.At present,the research on the NOMA system mainly focuses on the resource scheduling strategy,and pays less attention to the non-ideality of the system.Our paper focuses on the problem of residual frequency offset of transceivers in the uplink NOMA system.We will analyze the influence of residual frequency offset on system performance,and design frequency offset estimation and compensation algorithms to improve the transmission performance.This paper can be divided into two parts:(1)Study the uplink two-user power domain NOMA system.The impact of the residual frequency offset on the transmission performance of the system is analyzed.(2)Investigate the influence of the residual frequency offset on the system performance of the uplink multi-domain(including power domain,code domain,space-time domain and frequency domain,etc.)NOMA system and explore ways to improve system performance.We firstly analyze the performance degradation caused by residual frequency offset theoretically,which is verified by simulation results.Then we propose frequency offset estimation and compensation algorithm considering the characteristics of different systems and their typical receiving algorithms.The performance of the proposed algorithm is evaluated by numerical simulation.The results have shown that the residual frequency offset may degrade the performance of the uplink NOMA system.And the more users there are,the more serious the impact of frequency offset is.The frequency offset estimation and compensation algorithm proposed in the paper can solve the multi-user interference caused by residual frequency offset and can effectively improve the system transmission performance.