| With IMT-2000 3G services still on the horizon, research and development are takingplace all over the world to define the next generation of wireless broadband multimediacommunication systems, which is called the Beyond three Generation (B3G, or the FourthGeneration, 4G) Wireless Communication Systems. 4G will integrate various functions andapplications that need large data rates and efficient bandwidth schemes. Orthogonal Frequency Division Multiplexing (OFDM) is a promising candidatemodulation technique for B3G systems. It is a multicarrier modulation method whichprovides efficient bandwidth utilization and the robustness against Multipath delay spread.The basic principle of OFDM is to divide a wideband frequency channel into severaloverlapping subchannels. Despite spectral overlap, the subchannels can be perfectlyseparated at the receiver as long as the subchannels are orthogonal. In these systems, thereceiver must keep synchronization with the transmitter in both time and frequencydomains to maintain the orthogonality. In this dissertation, many systematic and deepresearches have been done for the time and frequency synchronization of the receivers inOFDM systems. This dissertation researches on the synchronization technology in theOFDM cellular wireless communication systems. Firstly, in order to define the demand for receivers' synchronization module in anOFDM cellular wireless communication system, this dissertation has analyzed thedistribution situation of Signal-to-Noise-Ratio (SNR) of the uplink and downlink in suchsystems. It is concluded that the synchronization must have a good performance evenSNR<5dB. Secondly, since Doppler frequency is important for both frequency synchronization inOFDM systems and the implementation of the adaptive channel resource allocation scheme, IIIthe dissertation move to study for the maximum Doppler frequency in the wireless channel.Methods based on autocorrelation, power spectrum of the channel estimates and the levelcrossing rate of the average signal level have proposed for the maximum Doppler frequencyestimation. For the autocorrelation and power spectrum based methods, the results of thechannel estimation will have an impact on the estimation precision, while for the levelcrossing rate based methods, it falls to estimate the maximum Doppler frequency when thechannel condition is hostile. In this dissertation, instead of using channel estimates, wepropose an algorithm which uses the transform domain pilot data to estimate the maximumDoppler frequency information in an OFDM system. Simulations demonstrate that thealgorithm performs well both in single- and multi- path channel when SNR is low. It issuitable for single carrier and multicarrier (such as OFDM) transmission systems. Thirdly, considering the burst characteristic of the data packet service in OFDMcellular wireless communication systems and based on the OFDMA multiple access schemein 802.16 protocols, the uplink available spectrum is subdivided into several blocks calledas "Time-Frequency blocks". A whole synchronization scheme is proposed for suchTime-Frequency block structure. It consists of coarse symbol and fractional frequencysynchronization, fine frequency synchronization and fine symbol synchronization. Since allthe estimation algorithms uses the information of a Time-Frequency block instead of onlyone symbol, the scheme performs well even in a lower SNR (such as SNR<0dB). The finefrequency offset estimation algorithm uses continual pilots to get both integral and residualfractional part of the frequency offset. As a result, the frequency synchronization precisionof signals will improve greatly. The fine symbol estimation also uses the pilots. It is basedon the Least-Squares (LS) criterion and uses a size-changing power window to search thesymbol start point. For the whole synchronization scheme, there is no extra overheadingneeded in the system...
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