Research On The Cutoff Rate Of Downlink SCMA System

Sparse code multiple access(SCMA)is one of the non-orthogonal multiple access techniques specifically proposed for the fifth generation mobile communication systems,which is capable of achieving non-orthogonal both in code domain and frequency domain.Based on the low density signature techniques,SCMA further contribute on extra system coding and fading gain by invoking the concept of complex multiple-dimensional codebooks/constellations.As a kind of promising NOMA techniques,SCMA achieves a remarkable high spectral efficiency,which can not only meet the hungry requirements of connecting to more user in future 5G communication system,but also achieve a high reliability,a short time latency,a low power cost etc.This thesis introduces the basic model and some related techniques of SCMA.Compared with LDS-CDMA system,the principle and performance improvement of SCMA transceiver are introduced,and then the basic ideas and operating principles are specifically explained.In this thesis,several design methods of SCMA codebook are discussed,especially for the mother constellation design of SCMA codebooks.And the common and different points between uplink and downlink in SCAM are revealed by comparing the physical layer model.The cutoff rate may be regard as a lower bound of channel capacity,which can be used to characterize the system reliable data rate,or use to characterize the performance of different SCMA codebooks under the same configuration.In this thesis,the cutoff rate of typical SCMA downlink and its associated important parameters are derived and analyzed,which will be further applied to evaluate the SCMA systems.In order to facilitate these derivations,a cloud partition concept of the entire synthetic SCMA codebook is proposed,which is suitable for deriving the appropriate approximation of cutoff rate of the SCMA downlink system.With aid of this,the theoretical estimates of cutoff rate under both AWGN channel and Rayleigh channel in SCMA downlink system have been provided.Through the simulation results,the feasibility and accuracy of our theoretical approximation of cutoff rate are evidenced.