Multiuser Detection Schemes And Performance Analysis For Non-Orthogonal Multiple Access Systems

Non-orthogonal multiple access(NOMA)techniques can improve the channel capacity gained from the constrained frequency resource.As the time and frequency resources are shared,signals from NOMA users superpose on each other,which brings challenges to the multiuser detection at the receiver.The synchronous power-domain NOMA systems usually employ the successive interference cancellation(SIC)detection,research on the theoretical performance of which has attracted extensive attention.As to the superior jointly optimal maximum likelihood(JOML)detection,the related theoretical error rate analysis is limited.As to the asynchronous NOMA,the detection schemes bear high complexity.How to analyze the error rate performance of asynchronous detection in an efficient and accurate manner becomes another tough issue.Moreover,asynchronous NOMA can achieve larger throughput compared to synchronous NOMA.Hence,how to compose artificial asynchronous NOMA to maximally improve the detection performance is also a worthy issue to explore.In view of the above problems,we focus on the multiuser detection schemes and their detection performance of both synchronous and asynchronous NOMA systems.The main contributions of this paper are listed as follows:(1)Due to the analysis on the exact error rate of JOML multiuser detection is limited in the existing literature,error rate performance of the JOML detection of two superposed quadrature phase-shift keying(QPSK)signals with an arbitrary relative phase offset is investigated in this paper.Exact,analytical symbol error rate(SER)and bit error rate(BER)formulas are derived for two users with no less than 3 dB power difference between them.Closed-form union bounds on the user SER and BER with an arbitrary phase offset and an arbitrary user power ratio are also given.Furthermore,insights on the potentials and properties of JOML regarding its error rate behaviors are revealed through comparisons with the SIC receiver.Compared to SIC detection,the results show that the superiority of JOML detection becomes more obvious as the user powers become comparable.(2)On account of the limitations of the existing analysis on the triangular SIC(TSIC)detection,the error rate performance of TSIC detection is investigated,where more accurate SER expressions are derived,and the iterative detection algorithm is improved.Considering the error propagation of TSIC detection,the average SER formulas of a two-user system with arbitrary M-ary quadrature amplitude modulation(M-QAM)scheme over Rayleigh fading channels are derived.These solutions are shown to have higher accuracies compared to the existing solutions.Furthermore,a novel iterative detection algorithm is proposed,i.e.,the parallel TSIC(PT-SIC)method,which improves the detection efficiency and reliability.The simulation results also show that the most significant improvement of the iterative detection reflects on the SER for strongest user,and the reduction is always within 50%of the SER compared to the primary detection.The gains obtained by the iterative detection is negligible for higher order M-QAM cases.The above results show the real benefits of iterative TSIC detection under realistic assumptions and correct the conclusion that a rate of 80%of the errors can be corrected by the iterative detection resulted under the ideal assumptions in the existing literature.(3)To obtain the optimal performance achieved by asynchronous NOMA,the optimal relative one-half symbol period time-offset for twouser NOMA is proposed,and the BER performance of two QPSK users in the TSIC detection using the proposed asynchronous NOMA is also investigated.Considering the detection error propagation,exact BER solutions for two asynchronous NOMA users operating in the additive white Gaussian noise(AWGN)channel are derived.A simpler closed-form BER approximation for the weaker user is also derived.Compared to the symbol-synchronous scheme,results show the proposed asynchronous scheme reduces the BER for the stronger user by a factor of one-half when the signal-to-noise ratio(SNR)is in good condition,while that for the weaker user can also be lowered since the error propagation is suppressed.(4)To improve the performance of asynchronous multiuser detection,we propose a simple synthetic detector for two-user asynchronous NOMA,i.e.,the JOML detection aided TSIC(JOML-TSIC)detector.By exploiting the proposed optimal artificial one-half symbol period asynchrony to compose uplink asynchronous NOMA,analytical BER solutions are obtained for the two-user case using JOML-TSIC detection.Closed-form BER solutions for the case using the TSIC detection method in the same scenario are also derived for comparisons.Results show that the JOMLTSIC detector for the asynchronous system outperforms the TSIC and the asynchronous symbol-level JOML detection in a general sense,it also outperforms the SIC and JOML detection in the synchronous case.When the signals experience flat Rayleigh fading,we show that the improvement of the JOML-TSIC receiver over other receivers is superior to that obtained in the AWGN channels.In summary,we resolve multiple theoretical research difficulties in power-domain NOMA and extend the theories of the existing work.The current research provides several theoretical benchmarks for the future research,and it also provides strong theoretical supports and insights for the development of NOMA in the next generation communication networks.