Research And Verification Of Adaptive Subcarrier Modulation Technology Based On OFDM System

In daily work and life,with the increasing demand for multimedia services,the requirements for wireless communication transmission efficiency are becoming higher and higher.Under the background of the growing shortage of spectrum resources,Orthogonal Frequency Division Multiplexing(OFDM)technology has gained a lot of attention due to its high spectrum utilization and strong anti multipath interference ability.What's more,when combined with adaptive modulation technology,OFDM system can adaptively allocate the bits needed to transmit to the subcarriers according to the fading of frequency selective channel,which significantly improves the system performance.Therefore,it is of great practical significance to study adaptive modulation technology based on OFDM system for enhancing the spectrum efficiency of the system.At present,the research of adaptive modulation technology is mainly focused on the algorithm level,and there are relatively few achievements based on FPGA.However,with the increase of signal bandwidth,the number of subcarriers in OFDM system also increases,which leads to the complexity of the adaptive modulation algorithm,so in addition to considering the reliability and effectiveness of the system,the problem of the high complexity of the system also needs to be solved.In this paper,firstly,aiming at the high complexity of the existing adaptive modulation algorithm,we study how to apply the adaptive modulation technology based on subband in an OFDM system,and design the baseband processing module and implement it in FPGA.In the specific design and implementation process,each subband uses the corresponding modulation method according to the channel quality estimation results.To allocate data bits correctly,this paper focuses on the design of the modulation and demodulation module,which makes clear the modulation mode used in each subband and the amount of data carried,to ensure the reliability of the data processing process.Besides,aiming at the accuracy and real-time of modulation feedback,this paper designs the estimation method of subband signal quality and the feedback mechanism of modulation.Finally,in the Xilinx Vivado integrated environment,this paper uses Verilog hardware description language to complete FPGA implementation and functional simulation verification.The results show that the proposed scheme can improve system capacity while maintaining system sensitivity.Then,to further improve the spectrum utilization of the system,this paper considers the complexity of the implementation,designs a scheme of packet modulation according to the difference of signal quality between subcarriers in the subband,completes the baseband processing unit of adaptive subband packet modulation,and realizes the mode that there are two modulation modes in the subband.Compared with the adaptive modulation system based on subband,this scheme has a more considerable throughput performance improvement.It does not need more multipliers in the actual FPGA design process,which will not affect the complexity of the system.Finally,the expected results are obtained through simulation and application test.Finally,considering that in the practical application process,it is difficult to transplant the algorithm under different bandwidth systems.To facilitate the application of adaptive modulation technology and avoid the complicated transplantation work,this paper uses the relationship between parameters of varying bandwidth systems,combined with the RF unit structure,to design an adaptive modulation parameterization implementation scheme,which can realize the application of adaptive modulation technology in the system with variable bandwidth by changing the limited initialization parameters.The simulation results show that the parameterized design can process the data correctly.