Low Complexity Decoding Scheme Without SI For Reducing PAPR In SCMA-OFDM Systems

At present,the rapid development of mobile Internet business and Internet of Things(IoT)business has promoted scientific and social progress and brought convenience to people.However,more and more terminal equipment access and users'pursuit of the ultimate communication experience have brought new requirements to mobile communication technology and brought great challenges.Therefore,in order to meet the new communication needs,the fifth generation of mobile communication technology(Fifth Generation,5G)came into being.Among them,the multiple accesses in air interface technology is a key link to solve the demand for massive connections.It is a important research topic of the research center and universities at home and abroad.In 5Q the non-orthogonal multiple access(NOMA)technology has been proposed,which enables more users to access limited spectrum resources and solves the problem of massive connections to a certain extent.Among them,multiple users in Sparse Code Multiple Access(SCMA)technology can share frequency resources,which greatly improve the utilization of spectrum resources,and can easily separate the sending information of each user at the receiver,so it can be considered as a candidate for 5G air interface technology.In this article,we modulate the SCMA-encoded information onto subcarriers of an Orthogonal Frequency Division Multiplexing(OFDM)system for transmission,that is,to build a SCMA-OFDM system.This system also has the advantages of OFDM system such as resistance to multipath fading,high spectral efficiency,easy implementation and large number of users accessing like SCMA system,but this system also inherits the disadvantage like high Peak-to-Average Power Ratio(PAPR)of the OFDM system.The higher PAPR of the SCMA-OFDM signal will cause the high power amplifier(HPA)at the transmitter to enter the saturation region,resulting in in-band distortion and out-of-band radiation.The in-band non-linear distortion will degrade the system's Bit Error Rate(BER)performance,and the out-of-band radiation will interfere with the spectrum of adjacent channels.Therefore,the PAPR reducing of the SCMA-OFDM signal at the sending end and how to find the original signal after receiving the processed SCMA-OFDM signal at the receiving end become the research focus of the SCMA-OFDM system.The existing PAPR suppression technologies can be divided into three categories,among them SLM and PTS technologies will not cause non-linear distortion of the signal.Therefore,in this paper we choose SLM and PTS methods to reduce the PAPR of the signal at the receiving end.The main tasks of my paper is as follows:(1)First we use SLM to reduce the PAPR of SCMA-OFDM signals at the receiver and give the theoretical derivation of the PAPR suppressing performance with SLM.It can be seen from the derivation and simulation results that the PAPR of the signal is indeed effectively suppressed,but if the receiver wants to recover the original transmitted signal correctly,the candidate sequence selected by the sender to process the signal must be used as the sideband information to sent to the receiving end,and the receiving end performs MPA decoding according to the candidate sequence and finally obtains the sending information.The transmission of sideband information undoubtedly occupies a part of the spectrum resources.This is contrary to our goal of improving the utilization of spectrum resources.Therefore,a blind detection algorithm sing sparse SCMA codebooks that does not need to transmit sideband information is proposed in this paper.The receiver estimates the candidate sequence at the receiving end,and uses the estimated candidate sequence to recover the original transmission information.This algorithm avoids the transmission of sideband information,improves the spectrum utilization rate,and has no performance loss when compared with a decoding algorithm that transmits sideband information at high SNR.(2)PTS brings in less extra cost at the sendding end than SLM,which can effectively reduce the system load.But the three grouping methods of PTS applicable to OFDM systems have poor PARP suppression performance in the system proposed in this paper and the above blind detection algorithm cannot be used too.Therefore,a new PTS grouping algorithm based on SCMA codebook and a blind detection algorithm corresponding to the receiver are proposed in this paper.This grouping method makes the signal PAPR decrease and the original signal recovery function at the receiving end significantly improved.(3)Considering that the blind detection at the receiving end will make the decoding complexity of the receiving end increase sharply,we use the sparsity of the SCMA codebook to propose MPA algorithms for SLM and PTS.This algorithm estimates the candidate sequence while performing MPA iterative decoding at the receiving end and amends the candidate sequence ineach iteration.This operation uses the transmission probability of each codeword calculated in MPA.The algorithm does not introduce additional calculation overhead,and achieves the purpose of reducing calculation complexity,and the MPA iteration also makes the estimation of the candidate sequence more accurate.At the same time,a blind detection algorithm using reliable nodes for sequence estimation is proposed for Rayleigh channels,this algorithm can accurately recover the original signal through blind detection after the SCMA-OFDM signal passes through the Rayleigh channel.