Research On Adaptive Modulation Technology Based On SCMA System

With the increasing scarcity of spectrum resources for wireless communications,non-orthogonal multiple access(NOMA)technology can significantly improve spectrum efficiency.Sparse Code Multiple Access(SCMA)is one of the highly competitive non-orthogonal multiple access techniques.SCMA can non-orthogonally superimpose multiple users on resource blocks less than the number of users to achieve overload and greatly improve the spectral efficiency of the system.Adaptive modulation technology is also an effective means to further improve spectral efficiency.Under the condition of a given maximum bit error rate,the modulation method is adjusted in real time according to the current channel state information.It can maximize the spectral efficiency while ensuring reliability.Therefore,the combination of SCMA technology and adaptive modulation technology is very effective for further improving spectral efficiency and system throughput.SCMA codebook design is particularly important for SCMA system.However,there is no systematic study on SCMA codebook optimization for adaptive modulation.The modulation modes of all users in SCMA system under adaptive modulation mode are different,which makes the design method of phase rotation factor in traditional codebook design unable to be used.In this thesis,we propose an adaptive modulation SCMA codebook design based on constellation rotation(AMSCR).For each adaptive mode including different modulation schemes,the adaptive modulation SCMA codebook is optimized based on constellation rotation.The codebook optimization criterion is to maximize the minimum Euclidean distance between joint codewords.The adaptive modulation SCMA system contains a variety of adaptive modes,which will make the space set of the joint codebook larger.Since the space of the joint codeword set is too large,the calculation of the minimum Euclidean distance becomes an NP-hard problem.We use a genetic algorithm to solve this problem.Compared with the traditional SCMA codebook,the adaptive modulation SCMA codebook proposed in this thesis has better BER performance both in Gaussian and Rayleigh channels.In multiple adaptive modes,when the BER drops to 0.01,the corresponding SNR of AMSCR is about 0.5～7dB less than that of the traditional codebook.Aiming at the adaptive SCMA codebook designed based on constellation rotation in each adaptive modulation mode,this thesis further proposes an adaptive modulation SCMA transmission mechanism under different fading channels.Compared with the traditional SCMA system and the adaptive modulation SCMA system using the traditional codebook,the adaptive modulation strategy can maximize the throughput and have the best throughput performance.As compared with SCMA with QPSK and SCMA with 16QAM,the throughput of adaptive modulation SCMA can be increased by 13.6Mbps and 7.5Mpbs respectively in AWGN and Rayleigh channels.