The Research On Orthogonal Frequency Division Multiplexing Technique In WideBand Wireless Network

Orthogonal Frequency Division Multiplexing(OFDM)is one of the techniques which have been developed rapidly in recent years. The apparent advantages of it is the efficient using of spectrum and it can get over the inter-symbol interference (ISI) caused by multi-channel transmitting in wideband wireless communication . This technique has stronger ability of getting over interfere than single carrier wave system and has been used in several wideband wireless applications. It becomes one of the most important techniques in the fourth mobil communication.The basic principle of OFDM is converting high speed codes flowing in serial transmitting into low speed codes flowing in parallel transmitting, so the period of code is lengthened. It get over multi-channel delay using protective interval. If we deal with the received codes appropriately in receiver, the multi-channel can be matched with single channel, and the effect of channel to signal become multiplication from convolution, so OFDM can eliminate ISI effectively.A group of orthogonal frequency carriers are used in OFDM system to improve the using rate of spectrum. If the orthogonal character can be held and ignore the inter-channel interference (ICI), the symbol error character of OFDM depend on the modulate of source codes. This dissertation propose the theoretic formula of OFDM-QAM and OFDM-PSK in this precondition.Because there are many carriers in OFDM system, the orthogonal character can be destroyed by a little frequency offset. So synchronization is one of the important techniques in OFDM. Based on analysis of synchronization techniques in OFDM, this dissertation present a ML arithmetic to estimate time and frequency offset using symmetrical & conjugate prefix. The correlation peak of likelihood function curve is very sharp according this arithmetic , so the symbol synchronization by this arithmetic is exacter than that by general arithmetic using cycle prefix. In the synchronous techniques based pilot symbols, here propose a frequency offset estimating arithmetic using discrete pilot symbols. We pick up integer times sub-carrier frequency offset from the same OFDM symbol and estimate exact...