Research On Key Transmission Technologies In SCMA Systems

In recent years,the popularity of mobile intelligent terminals,mobile Internet and new services has been urging communication systems to provide services which meet the need of users.However,the existing communication systems can not meet the requirements of low latency and high rate for massive users.In this background,the sparse code multiple access(SCMA)was proposed.It makes the massive users reuse the spectrum resources in an overloaded manner,while these users can adopt the grant-free mechanism to reduce latency.Therefore,it will become a way of multi-user access with huge potential in the 5th generation(5G)systems.The current researches on SCMA mainly focus on the codebook design and the design of low complexity receiver,while there are few researches on the transmission techneques in SCMA.As we all know,the transmission techneques in communication systems play important roles in determining performances of throughput,energy efficiency and reliability of the systems,hence related researches on transmission techneques are of great significance.Considering the great importance of SCMA on future wireless networks and the lacking of transmission techneques in SCMA,this paper mainly focuses research on energy efficiency optimization design and hybrid automatic repeat request(HARQ)technique in SCMA systems.Firstly,the energy efficiency optimization of SCMA system is studied.Two optimization problems concerning energy efficiency of SCMA systems are proposed.Where,the objective function of the first optimization problem is to maximize the energy efficiency of all the users in SCMA systems,while the objective function of the second optimization problem is to maximize energy efficiency of the user with worst link.Both two energy efficiency optimization problems guarantee the minimum transmission rate requirements for all SCMA users.Based on these,two algorithms are designed,whose basic idea is iteratively solving parameter optimization problem.That is,by introducing the parameter,the fractional objective function is transformed into a concave function containing the parameters.The optimal power allocation is obtained according to the karush-kuhn-tucker(KKT)condition,then the parameter are updated,continually solve the optimization problem until the converges of parameter.The simulation results show that the proposed two algorithms of energy efficiency optimization solve the energy efficiency optimization problem under given model,and they can quickly get convergent.In addition,comparing the performances of two energy efficiency optimization algorithms,we can see that energy efficiency and transmission rate are compromised.Later,we research on improving the reliability of SCMA systems.Based on the traditional message passing algorithms(MPA)processes,this paper presents a kind of partial codeword information MPA known as PI-MPA,and qualitatively analyzes its principles of improving decoding accuracy,compared with the traditional MPA decoder.Then a highly reliable SCMA-HARQ system is constructed based on the proposed PI-MPA.By analyzing the effect of the number of retransmitting correctly decoded codewords on the system throughput and retransmission delay,this paper designs an algorithm considering adptively retransmissions of partial correctly decoded codewords.Specifically,All users are divided into two groups according to their quality of service(QoS).Only the group with lower QoS can conduct redundant retransmission in the next transmission round,and the number of users performing redundant retransmissions is adaptively adjusted based on the the number of incorrectly decoded users in the current transmission round.Simulation results show that the proposed reliable SCMA-HARQ system has only a small latency performance loss,while significantly enhances its reliability.