Study On Guard Interval For OFDM System

Orthogonal Frequency Division Multiplexing is a kind of special modulation technique for multi-carrier. Although Orthogonal Frequency Division Multiplexing in theory has been in existence for more than thirty years, because of the limit of the hardware technique and the signal processing theory at that time, it was difficult for really implement. Not until DFT technique was introduced into Orthogonal Frequency Division Multiplexing, did recent developments in DSP and VLSI techniques make it a feasible option. All these techniques promote the rapid development for Orthogonal Frequency Division Multiplexing. Nowadays, Orthogonal Frequency Division Multiplexing has been one of the hotspots of investigating communications, and the key technology for the fourth generation mobile phone system is likely to be Orthogonal Frequency Division Multiplexing.The guard interval of Orthogonal Frequency Division Multiplexing is very important. We can completely eliminate the inter-block interference by using the guard interval of Orthogonal Frequency Division Multiplexing. Moreover, if we insert cyclic prefix into the guard interval, we can reduce the inter-carrier interference that result from the multipath signals.In this thesis, we look back the phylogeny of Orthogonal Frequency Division Multiplexing, and introduce the principle, the merit and shortcoming and some key technology of Orthogonal Frequency Division Multiplexing system. Following then, we present the function of the guard interval of Orthogonal Frequency Division Multiplexing system, and we use graphs to explain the reasons that Orthogonal Frequency Division Multiplexing system can eliminate the inter-block interference by inserting the guard interval between the Orthogonal Frequency DivisionMultiplexing symbol blocks.We present two main methods about how the guard interval of Orthogonal Frequency Division Multiplexing is inserted, that is cyclic prefix method and zero padding method. Then we use digital signal processing to explain the reason why the cyclic prefix can remove the inter-carrier interference. Furthermore, we analyze and compare the merit and shortcoming in Gaussian Noise tolerance, multipath delay spread immunity and peak power sum for the two methods in theory. The results of theory analyze show that the method of cyclic prefix has better performance in the multipath delay spread immunity, the method of zero padding possess better performance in the Gaussian noise tolerance and the peak power sum, and they take on the same performance in the peak-to-average ratio.We propose a novel method, in which we pad zero in the starting part of the guard interval and insert cyclic prefix in the rest part of the guard interval. The results of analyzing in theory indicate that the proposed method has good performance between those of cyclic prefix method and zero padding method in the multipath delay spread immunity or Gaussian Noise tolerance, or peak power sum, and possess the same performance with those of cyclic prefix method and zero padding method in peak-to-average ratio, moreover the novel method can access the best performance in some conditions. Also, we present the results of the computer simulation using Matlab. It is found that the results of computer simulation are identical entirely with the results of analyzing in theory.