Timing And Frequency Offset Algorithm In Ofdm Systems Research

Orthogonal frequency division multiplexing (OFDM) has recently become popular for wireless communication systems. In the last few years, OFDM has been employed in various commercial applications that include wireless local area networks (IEEE 802.11a), terrestrial digital audio broadcasting (DAB-T), and terrestrial digital video broadcasting (DVB-T). OFDM can combat inter-symbol interference (ISI) using the computationally efficient fast Fourier transform (FFT) instead of a complex equaliser. Despite its many advantages, OFDM is well known for its sensitivity to the time and frequency offset.In the wireless environment, the OFDM data is transmitted as one symbol, so in order to implement the correct demodulation, the correct beginning of symbol should be got from the received signals to reduce the effect of ISI. Furthermore, the high frequency utilization and the robust transmission of OFDM are founded on the orthogonal properties of the sub carriers, so if the frequency offset is generated and the orthogonal properties among sub carriers is lost, that will cause the inter-carrier interference (ICI) and high BER of system. As a result, in the OFDM system, how to synchronize in a quick and accurate manner is quite an important issue, which affects the performance of the whole system. So the corresponding time and frequency synchronization algorithms should be proposed and realized in the OFDM receiver design. This paper studies the synchronization technique in the OFDM system, which is to fulfill efficient estimation and compensation of time and frequency offset to enhance the system performance.In this paper, firstly the history of OFDM technique is introduced. So are the advantage and the disadvantage of OFDM. According to the basic OFDM theory, the OFDM singals peculiarity and the model of common OFDM system is given, followed by explaining the basic characteristic of wireless channel and typical channel model.Secondly, the effect of carrier frequency offset, sampling clock offset, and symbol timing offset on OFDM signals are exploited. For the symbol timing synchronization problem, two methods are introduced: based on the cyclic prefix and training sequences respectively. A few classical methods using the training sequences of OFDM frame are investigated. Performances under AWGN channel and frequency-selective channel are analyzed and compared by simulation. Then one of the widely used synchronization algorithms named maximum likelihood (ML) estimation is fully exploited, which makes use of cyclic prefix statistical properties instead of additional training sequences. After performance analysis, a simplified ML algorithm based on cyclic prefix is proposed. It also makes use of sufficient information provided by cyclic prefix. Not only avoids complicated correlation coefficient operation, but also simplifies hardware implement. Simulations show that the simplified ML algorithm is not so sensitive to the length of the cycfic prefix and has a better performance.This paper finally discusses carrier frequency synchronization methods, including two types of which are based on cyclic prefix and training sequence respectively. We analyze and compare the advantages and limits of each method. At last, synchronization flow of OFDM is summarized and transformed into flow chart.