Research On CDMA Technology For Visible Light Communication And Its Application

With the rapid development of mobile Internet and Internet of Things technologies,traditional radio frequency communication can no longer meet the increasing demand for communication capacity,and the next generation of mobile communication is facing a great challenge of spectrum resource scarcity.Visible light communication(VLC)is an optical wireless communication technology using visible light as the information carrier,which has the advantages of wide spectrum,large capacity,wide coverage,high security and low energy consumption,and has the potential to become one of the key technologies in the next generation mobile communication architecture.However,like traditional wireless communication technology,multiple access interference and synchronization problems caused by multi-user access can directly affect the performance and practical procedure of the VLC system.Code division multiple access(CDMA)technology can effectively reduce multiple access interference by distinguishing users in the code domain,and can also enhance the tolerance of user signal synchronization by increasing zero correlation zone length of spreading code set,which is an excellent choice for multi-user access applications.In addition,while considering communication performance,it is also necessary to consider system illumination performance,transmission efficiency,complexity and other factors.Therefore,how to effectively reduce multiple access interference while meeting the other performance requirements of system is one of the most urgent problems in the VLC system.Based on the performance improvement of multi-user VLC systems with ideal synchronization,quasi-synchronization and multi-rate,this paper explores a variety of novel CDMA spreading code sets,and successfully performs system verification and application expansion.The main research contents and innovations are summarized as follows:(1)Aiming at the problem of communication and illumination multiplexing in multi-user system,a dimming control CDMA scheme for VLC is studied and verified.The scheme improves the traditional CDMA scheme by introducing the mapping module and the dimming module,which can ensure the transmission efficiency of the system and realize dimming control.Based on the CDMA scheme,a real-time multi-user VLC system is built by using field programmable gate array(FPGA).The results show that the scheme can realize multi-user transmission with low complexity and better illumination performance.(2)Aiming at the problem of synchronization damage caused by multipath effects in multi-user system,this paper constructs two new spreading code sets which are suitable for quasi-synchronous(QS)CDMA-VLC systems,and performs system verification.The two new code sets are the optical zero correlation zone(OZCZ)code set based on interleave iterative and the OZCZ code set based on iterative extension.They maintain good correlation properties in zero correlation zone.The transmitter and receiver of the system adopt code sets with different polarity to spread and despread.Combining the new constructed code sets,this paper further builds single-channel and two-channel QS-CDMA-VLC systems.The results show that the performance of the systems can be greatly improved in terms of dimming value,total bit rate,tolerance to transmission delay and bit error rate performance.(3)Aiming at the problem of diverse traffic requirements in multi-user system,this paper establishes a multi-rate QS-CDMA-VLC system model and proposes an orthogonal variable spreading factor OZCZ(OVSF-OZCZ)code set for this system model.The proposed code set has the characteristics of variable spreading factor and zero correlation zone,which can meet the multi-rate and quasi-synchronous requirements of multi-user transmission at the same time.By numerical simulation analysis,the new constructed code set can be used as a candidate code set for multi-rate QS-CDMA-VLC to support the multi-service requirements of large-scale heterogeneous devices in the future.(4)This paper further adopts the integrated development platform of ARM and FPGA to realize an integrated visible light communication and positioning system based on CDMA technology,which has the functions of illumination,communication and positioning at the same time.The results show that with the application of the OZCZ code set based on interleave iterative,the system performs well in terms of reducing multiple access interference,improving positioning accuracy,and ensuring illumination performance.