The Research And Implementationt Of Spread Spectrum Modulation/Demodulation Based On TDMA

Spread spectrum communication system has many advantages compared with other systems, such as anti-jamming, low interception rate, confidentiality, and high precision measurement, easy to implement code division multiple access, anti-multipath interference. Therefore, the spread spectrum communication system through research and development has been attached great importance and widespread application of civil and military communications. Time Division Multiple Access system requirements within a very short time to complete the synchronization and can start to receive data, which put forward higher requirements for synchronization in spread spectrum communication system.In the beginning of this thesis first introduces the basic principles of spread spectrum communication system and the existing research. Then research the QPSK modulation and demodulation techniques and spread spectrum systems synchronization technologies. Synchronization of spread spectrum system consists of three levels, namely, carrier frequency synchronization, code synchronization, bit synchronization. Based on the above established the focus of research:synchronization in direct sequence spread spectrum communication system and its modulation and demodulation system FPGA implementation.To synchronization in direct sequence spread spectrum communication system, the first study adopted by the frequency-locked loop and phase-locked loop combination of carrier frequency synchronization method, which frequency-locked loop using the cross product automatic frequency control tracking loop, phase-locked loop using Costas loop. They determine the size of the phase difference to switch to each other to complete carrier synchronization and study the synchronization process and the mathematical derivation. This thesis also researches the synchronization capture and tracking of the spreading code sequence which mainly through the research and comparison of various spreading code synchronization acquisition method, select the capture method suitable for the background of this thesis issue. Since the spread spectrum system is based on time division multiple access communications, demanding high synchronization speed, so chose the digital matched filter as the spread spectrum code acquisition method. At the same time in order to reduce the system hardware resources using folding digital matched filter. Via increasing the working frequency folded digital matched filter complex used shift register to reduce resource consumption. At spreading code tracking stage, Using the delay locked loop to further reduce the phase difference between the received spreading code and locally generated spreading code sequence.Finally through the ISE environmental design to compile each module of a DSSS communication system, the sending end including mapping module, the spreading code generator and the spreading code modulation and digital up conversion. Receiving end of the digital down-conversion, carrier multiplying, digital matched filter, the spreading code tracking loop, the spreading code generator, carrier frequency synchronization loop, differential decoding. And using timing simulation gives the results of each module. Then application the designed system on XILINX’s VIRTEX II PRO XC2VP30chip. And eventually use online logic analyzer to prove the feasibility of the system.