| Nowadays OFDM (Orthogonal Frequency Division Multiplexing) technique is widely used in wireless communication because of its high-speed data transmission and effectiveness in combating the frequency selective fading channel. The idea of OFDM was proposed in mid-1960s and was first used in several high-frequency military systems. But it's only in the last decade, with the development of Digital Signal Processors (DSP) and Discrete Fourier Transform (DFT) that applications of OFDM become possible. Now, OFDM technique has been adopted as the new European DAB standard, and HDTV standard. It is also a candidate technique of 4G mobile communication. The other standards include IEEE 802.11a (U.S.A), HIPERLAN2 (European), OFDM/UWB (802.15.3a), etc.OFDM can be viewed as either a modulation technique or a multiplex technique. As a multi-carrier transmission technique, OFDM divides the available spectrum into many subcarriers, each one being modulated by a low data rate stream, so that each user can employ a number of carriers to transmit data simultaneously. The difference between OFDM and the traditional frequency division multiplexing (FDM) is that the subcarriers in OFDM to be modulated are orthogonal with each other. As a matter of fact, OFDM achieves a higher bandwidth efficiency. The other characteristic of OFDM is the employment of discrete Fourier transform (DFT) to replace the banks of sinusoidal generator and the demodulation which significantly reduces the implementation complexity of OFDM modems.The main advantages of OFDM include immunity to delay spread and multipath,resistance to frequency selective fading and efficient bandwidth usage. The disadvantages of OFDM behave in two main aspects: synchronization and peak-to-average power ration(PAPR).The main theme of this paper is study on carrier frequency synchronization in OFDM system. Synchronization is an important task for any communication system. Without accurate synchronization the transmitted data can not be received reliably. How to overcome the sensitivity to timing and frequency offset is one of the main research directions on OFDM. In OFDM system, synchronization tasks can be classified into three parts: symbol (timing) synchronization, frequency synchronization and sampling rate synchronization. The timing error is due to the propagation delay. The frequency offset arises from two sources: the mismatch between transmitter and receiver oscillators, Doppler effects in mobile radio channels. The sampling rate offset comes from different sampling frequency between receiver and sender.The frequency offset estimation is an important problem in the OFDM system. The main problem arising from frequency offset is that it introduces inter-carrier interference (ICI) and destroys the orthogonalization between subcarriers. Frequency synchronization algorithms for existing OFDM systems can be divided in two main categories. First group of algorithms based on insertion of the cyclic prefix (CP).The advantage of this algorithm: the aid of the pilot symbols is not required, efficient bandwidth usage. The disadvantages: 1.The maximum frequency offset which can be corrected is limited to half the inter-carrier spacing. 2. These schemes are only effective when a large number of subcarriers are used, commonly more than 100. Second group of frequency synchronization algorithms uses train symbols or pilot. It can rectify the frequency offset in a larger scope, but the efficiency of bandwidth usage is limited.This dissertation focuses on the main algorithms of the carrier frequency offset estimation. Two main algorithms have been discussed. The first one is non-data-aided blind estimation based on CP, the other is data-aided estimation based on training symbols. The content is divided into six chapters:Chapter one has introduced the background of this theme, the study purpose and the developing trend nowadays. In the end of this chapter the main content of this paper will be put forward.Chapter two has introduced the basic principle of OFDM. The main point is the arithmetic theory and the technique feature. In the meanwhile the real waveform and spectrum of OFDM signal are simulated. One of the main disadvantages in OFDM is the synchronization problem. It will be brought up emphatically in order to prepare for the next chapter.Chapter three is a whole introduction and review on OFDM synchronization technique. It includes the influence that the synchronization offset brings to OFDM system. It also explains the main algorithms and their principles.Chapter four has studied a main category on OFDM synchronization, frequency synchronization estimation using cyclic prefix. This method is also available within the algorithm of symbol synchronization using cyclic prefix. It will not waste the system's bandwidth. But the obvious disadvantage is its low precision. There are other disadvantages for this algorithm, such as: the system must have enough numbers of subcarriers; the estimate effect is good enough in the AWGN environment, but in the frequency selective fading channel environment the effect will become worse because the relativity between signals is destroyed. In this case there must be improved algorithms to be put forward. In the end of this chapter several improved algorithms will be discussed.Chapter five has mainly studied the train-symbol-aided frequency synchronization algorithm. This algorithm can be carried through both in time domain and frequency domain. Commonly the time domain approach is coarse synchronization with a limit estimation range; the frequency domain approach is fine synchronization or integer times estimation. Although this algorithm will lead some loss of bandwidth and increase the complexity in system realization, it has many advantages such as high estimate precision, flexible algorithm and bigger estimate range. So this algorithm has become a main study direction at the present time. In this dissertation the train symbol structure and the synchronization implement question of IEEE 802.11a will be studied. A low complex algorithm will be combined with the PN-aided frequency synchronization algorithm. The simulation result indicates the validity of it.Chapter six has summarized the whole content of this dissertation, and made further prospects.Carrier frequency synchronization is a hotspot in nowadays OFDM study domain, and the correlative algorithms and theories emerge in endlessly. This paper has just chosen several representative algorithms to study and simulate. Each of them has advantages and disadvantages. In actual application of OFDM we need analyze the practical situation. |
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