Multi-carrier Ofdm System Frequency Synchronization Study

With the development of mobile multimedia service, modern society calls for the mobile communication technology with strong anti-multipath, good spectrum efficiency, quick transmit speed and high quality reliability. As a multiple-carriers modulation technology. OFDM has gained increased interest because of its various advantages such as high bandwidth efficiency, robust performance in Multipath dispersive environment and simple equilibrium.In many advantages of OFDM, the two are very important: one is high spectrum efficiency; the other is excellent anti-multipath ability. The first one is more important in modern society, when frequency resource is precious. The second one makes OFDM be used in mobile communications. Now OFDM is being used in terrestrial DTV standard (DVB-T, DVB, DMB-T), WLAN (802.11a, HIPERLAN/2) in a large scale and as the core technology in 4G and next generation WLAN.As a kind of multi-carries technology OFDM has some problem need to be solved against with single-carry system: Synchronization, Channel Estimation, Peak-to-Average Power Control, Inter-Carrier Interference. In order to gain high bandwidth efficiency, the sub-carrier must keep orthogonal in multi-carriers technology OFDM. So, the system requires high precious synchronization. The frequency offset between receiver and transmitter and Doppler frequency shifting can destroy the orthogonality among the sub-carriers and lead to Inter-Carriers Interference (ICI), which would degrade the performance even to unable to work. So synchronization is very important and urgency to be solved for the use of OFDM.From basic theory of OFDM, the paper analyses the effect of synchronization that acts on performance of OFDM system. Then we present new carrier frequency acquisition and channel estimationalgorithms for OFDM systems. Both new algorithms utilize only one frame of pilot data to perform carrier frequency acquisition and channel estimation. The simulation results show that both algorithms are immune from timing error with a high precision and low computation complexity.The basic idea of the combined algorithms is choosing a kind of time domain data-aided data block:d = [j(O),j(ix ■■,s(L-Y)]=cx[i,p,p2,..^pLA],o< pz),c is a constant parameter and large than one. Defining a special structure frame s=[d,d,d,...,d,...]rNxi, so we can use just one frame to perform carrier frequency acquisition and channel estimation algorithms for OFDM systems. At the end we use statistic value as the end estimation value, p is attenuation factor. Itdoesn't affect carrier frequency acquisition but it enhances the realization. At the same time it will affect the channel estimation algorithm.Carrier frequency acquisition has the advantage of high precision, wide range of acquisition and dealing the conflict between precision and range easily. Channel estimation algorithm has the advantage of low complexity and high precision. It is very fit for length-known channel. For length-unknown channel, we can use the length of CP as channel length to perform algorithm.