Study And Realization Of The Synchronization For OFDM System Based On Software Radio

Along with the contradiction between the requirement about much larger communication capability and the limited spectrum resource becoming much more austere,OFDM(Orthogonal Frequency Division Multiplexing) gradually becomes the kernel technique in the 4G mobile communication.It not noly can confront the frequency selectivity degradation,but also satisfy the require of multimedia communication.It could provide high quality and low cost transfer multimedia-service that contains the abundant information(for example:voice,data and image etc)through wide frequency channel.But the OFDM technique also has shortage,it is very sensitive to the system synchronization capability.All of the offsets with symbol timing,carrier wave frequency and sampling frequency can destroy the orthogonality of every subcarrier.It is not noly introduce the ICI(Internal Channel Interference) among subcarriers and ISI(Internal Symbol Interference) among symbols,and also reduce the whole system capability.So,it needs effectively estimation and compensates the offsets with symbol timing,carrier wave frequency and sampling frequency,in order to eliminate all kinds of asynchronization influence to the system capability.Firstly,this paper simply introduces the developing course of the wireless access and the principle of OFDM technique,then analyses and investigates the asynchronization offsets influence to the system capability.Then,this paper emphasizely introduces the simulation and design scheme for asynchronization by long training sequences,short training sequences and pilot freqency signals,according to characteristics of the IEEE802.11a frame structuer.Following issues were emphatically illuminated in this paper:·The algorithm and realization of frame detecting ,bring forward the combining of power detecting and delaying group,much more declining the probalility of false synchronization and missed synchronization.·The realization of coarse and precise symbol timing,present the long training sequence and the signal's FFT window positions,supplying the qualifications for channel estimation.·The estimation and correction of frequency offset,adopting coarse and precise two stages estimation,enlarging the rang of estimation and enhancing the precision of estimation.·The algorithm and realization of the phase tracking, using frequency domain pilot subcarriers to track and compensate phase error caused by residual carrier frequency offset after FFT operateFinally, the realization source code is given by VHDL. It has been synthesized in the chip of Altera and simulated. From the simulation and testing result, we know that its performance is good and satisfy the need of the system.