Study On Synchronization Of OFDM System

OFDM is the key technique for the fourth generation(4G) mobile communication. The great advantage of OFDM is that it is robust against frequency selective fading and that it improves frequency utilizing rate simultaneously. Not only does it increase system's capability, but also it can meet the demand of multimedia communication. It can transmit multimedia operation high quality including voice, data, video through wide-band channel.Many sub-carrier signals adding to each other produces OFDM symbol, which differs one from other by orthogonity. Therefore it is important thing to assure orthogonity for OFDM system, which needs accurate carrier synchronization. There are three demands in synchronization: carrier synchronization that guarantees the equality of oscillation frequency of the receiver to that of the tansmitter; signal synchronization that keeps the same moment for the beginning and ending of IFFT and FFT; sampling synchronization that sustain same sampling frequency for both receiver and transmitter. Since sampling synchronization cannot influence OFDM system seriously, only carrier synchronization and signal synchronization are mainly discussed in this paper.There are two kinds of synchronization algorithm regarding to timing and frequency offset in OFDM system: one is non-blind estimation, based on pilot symbol, which can lead to the waste of pilot width and power resource, thus decreases efficiency of the system; the other is blind estimation utilizing structure of signal such as cyclic prefix and virtual sub-carrier, which overcomes disadvantages of resources waste. In this paper, the joint maximum likelihood(ML) symbol-time and carrier-frequency offset estimator based on cyclic prefix(CP) and a synchronization algorithm utilizing training symbols based on standard IEEE 802.11a are presented.Firstly, the development and the basic principle of OFDM system are introduced in paper, and the research actuality of synchronization for OFDM is summarized. Then, the error parameters which affect the performance of system extremely are analyzed, we deduce the principle that error parameters imports inter-symbol interference(ISI) and inter-carrier interference(ICI) and the effects of error parameters for OFDM system. Following, the ML estimation arithmetic is analyzed that utilizes the redundancy information to synchronize symbol timing with the carrier frequency. The classical ML arithmetic is easy to carry out, but the timing estimation is too roughness. For the sake of increasing the available information of estimator to get the steady symbol time and the frequency of oscillator on that foundation, the ameliorated ML estimation arithmetic using sequential multi-symbols is analyzed and the performance of system can be increasing extremely by emulating in low-noise-ratio environment. Meanwhile, in order to reduce the computational complexity of ML arithmetic, ML arithmetic using iteration method is analyzed, which can predigest the execution steps for ML arithmetic and enhance the execution rate. Nowadays, IEEE 802.11a in WLAN is given more attention which uses OFDM in physical layer .Because OFDM can provide the data rate as high as 56Mbps. The system scheme which use the long and short training symbol to get the timing synchronization and frequency synchronization is analyzed on the basic of the frame structure of physical layer in protocol.