Research On Key Techniques Of Non-orthogonal Multiple Access

Non-orthogonal multiple access(NOMA)is a compectitive technique to deal with the booming mobile traffic,massive amounts of connected devices,and diverse service requirements in for future wireless communication systems.Although NOMA has been widely investigated,there are still many problems to be further studied.In view of this,this thesis focus on the key techniques such as combination of multiple-input multiple-output(MIMO)and NOMA(MIMO-NOMA)and orthogonal frequency division multiplexing(OFDM)based NOMA(OFDM-NOMA).The details are stated as follows.Firstly,a scheme combining quasi-orthogonal space-time block code(Q-OSTBC)and power multiplexing according to NOMA principle is designed.The proposed MIMO-NOMA scheme utilizes Q-OSTBC with unequal diversity order to mitigate the error propagation in successive interference cancellation(SIC)based detector,and power multiplexing for users with different channel gain to improve the spectrum efficiency.Simulation results show that the designed pattern can effectively mitigate error propagation in SIC based detector,and the proposed scheme can significantly improve the system capacity compared with the scheme combining Q-OSTBC and OMA.OFDM-NOMA is a competitive solution to achieve a capacity gain over orthogonal frequency division multiplexing based orthogonal multiple access(OFDM-OMA).However,one major drawback of OFDM based systems is the high peak-to-average power ratio(PAPR).Clipping is widely used for PAPR reduction,but it will degrades the capacity performance.Motivated by this fact,the capacity of each paired user in OFDM-NOMA with consideration of PAPR reduction using clipping is derived.Based on the derived capacities of paired users,a fair user signal power allocation region to OFDM-NOMA with clipping is derived,from where the power allocation factor is selected to fulfill the demand that OFDM-NOMA capacity can always outperforms OFDM-OMA capacity for each paired user,regardless of the user pairing criteria,making it applicable for implementation using any scheduling paradigms.Both the theoretical and numerical results indicate that,although the capacity performance of OFDM-NOMA as well as OFDM-OMA is decreased and restrained at high signal-to-noise ratio(SNR)by clipping,applying the fair power allocation scheme on OFDM-NOMA can still meet the aforementioned demand.In addition,it is shown that both the lower and upper bounds of the fair region are increased with deceasing of clipping ratio.Another drawback of OFDM based systems is that it is extremely sensitive to the inter-carrier interference(ICI)caused by RFO.Yet the impact of RFO on the sum rate performance of OFDM-NOMA systems hasn't been discussed in the existing works.Due to this fact,the ergodic sum rates of uplink and downlink OFDM-NOMA systems with RFO are derived and presented as integral forms.Moreover,the loss ratio is adopted to evaluate the impact of RFO on the ergodic sum rate performance,which can also be regarded as a reference for RFO mitigation to maintain the ergodic sum rate loss at a certain level.The numerical results indicate that,the ergodic sum rate loss ratio induced by RFO is considerable with a high RFO value,e.g.,for both uplink and downlink with the SNR of 10 dB,the ergodic sum rate suffers a 30% loss when the normalized RFO value is 0.45.In particular,it is recommended to control the normalized RFO value within 0.05 through RFO mitigation such that the ergodic sum rate loss may be neglectable.Besides,the power allocation proportion between the paired users is shown to have little impact on the sum rate loss ratio of OFDM-NOMA systems.In summary,this thesis designs a MIMO-NOMA transceiver scheme using Q-OSTBC,which improves the system capacity and effectively suppresses the error propagation of SIC receiver.Morover,the impacts of clipping on the capacity of OFDM-NOMA systems are analyzed,and a fair user power allocation region is given to achieve a good tradeoff between transmission rate and fairness of each user.Finally,the impacts of RFO on the capacity of OFDM-NOMA systems are studied,aiming to provide a reference for frequency offset suppression.