Design And Implement Of OFDM System Based On USRP And Auxiliary External Synchronization

In a wireless communication system, with the development of digital technology, more and more system functions are transferred by the software, which evolved out of the Software Defined Radio concept. As a flexible, low-cost communication solution, Software Defined Radio currently have caught more and more attention. The basic idea of this technology is that the modular, systematic hardware modules in communication systems are integrated on a common hardware platform, in addition to the development of modular software programs, this paper also runs baseband and Communications algorithms upper, which embrace reconfigurable, programmable, scalable and easy system-upgrade advantages.Orthogonal Frequency-Division Multiplexing (OFDM) technology is a special physical layer technology, it can be seen as a modulation technique and be viewed as a multiplexing technique. B3G to 4G OFDM is one of the key technologies of the process of evolution, it presents that channel is divided into a number of Orthogonal sub-channel, high-speed signals convert into parallel low-rate sub-streams for the transmission modulation to each channel (subcarriers), due to the Orthogonality of the sub-carrier, they will not interfere with each other. Because of OFDM’s good performance characteristics have been widespreadly concerned in the anti-multipath effects and anti-frequency selective fading aspects. In addition, OFDM is a broadband, efficient modulation scheme to improve the bandwidth efficiency, there is a good prospect for OFDM.This paper aims to design OFDM modulation and demodulation system and combine software radio technology with OFDM multi-carrier modulation technology based on GUN Radio and USRP, this paper proposed an OFDM multi-carrier transmission system based on universal software radio platform, built an architecture based on the IF transceiver machine, detailedly studied the key technologies in OFDM systems-Synchronization. The article first introduced the software radio key figures involved in signal processing techniques, including key technologies such as multi-rate system used in some USRP sampling, upconversion, downconversion. After highlighting the basic principles and key technologies in the OFDM modulation technique, such as guard interval, cyclic prefix, FFT transform and QAM mapping. OFDM synchronization has always been a hot spot of research. Based on the demands, this paper highlighted several OFDM symbol synchronization algorithm, and analyzed its performance.Besides, depending on the specific USRP platform, this paper presents two methods that achieve synchronization based on self-synchronization and external synchronization strategies based on UBNT+Wireless network cards, through practice tests,it found that USRP platform will introduce uncertain processing delay, the traditional self-synchronization program will not get good results, while the adopted external synchronization scheme can reduce the error from the platform itself, which improved the synchronization accuracy of system.The actual test presents the synchronization accuracy’s maximum size is 0.208ms,less than lms,which can meet synchronization requirements.Finally,this paper tested the bit error rate with and without external synchronization,the bit error rate declined from 13.5% to 0.8%.Thus,the scheme that has adopted external synchronization has achieved better transmission performance.