A New Frequency Offset Correction Method For OFDM Using Autocorrelation

Along with the gradual maturation of the third generation of wireless personalcommunications, large amount of professionals are now interested in the keytechnologies about the forth generation wireless personal communications. Currently,the most stand out problem in wireless personal communication is the conflictbetween the scarcity of frequency resource and the people's infinite requirement.Large amount of research works are now focus on MIMO, OFDM, Turbo codes,LDPC codes, Multi-User Detection and adaptive modulation to increase the capacityof the system of 4G.In this thesis several solutions are presented to solve the problems in MIMO,OFDM and LDPC codes. A shift autocorrelaton frequency offset corrction algorithmis presented to solve the synchronization of sub-carriers in OFDM system. Theessential idea of this algorithm is: when an orthogonal multi-carrier system is ofperfect frequency synchronize, the demodulated signals of different sub-carriers areindependent of each other. Whereas when frequency offset exists, intercarrierinterference will distort the demodulated signal, i.e. every demodulated signal is thesum of several modulated signals' projects on the demodulating frequency. So theadjacent demodulated signals consist of the element of the same modulated signal,and these demodulated signals are correlated with each other. The degree that theycorrelated with each other depends on sub-carrier relative frequency offset. Thisalgorithm can be also use in MIMO-OFDM, PCC-OFDM and DWMT systems.The work in LDPC decoding can be devided into three aspects: thirst, a newparameter is introduced to enhance the decoding performance of bit-flipping decodingalgorithm. The reliability performance of every bit can be described much more clearbased on this parameter;second, a token-based algorithm is presented to solvethe "optimal taboo length" problem in Taboo-based algorithm. Combined with thenew parameter, the token based algorithm can result in an improved performance thatcan be compared with BP algorithm, while still maintaining the simplicity inherent insoft bit-flipping algorithm;third, the new parameter can be used to identify every bit'sre-liability performance in belief propagation decoding process, and reduce the effectof the bits which are very reliable, so the calculation amount of belief propagation canbe largely reduced.Another important work of the thesis is that a spatial-time-frequency LDPCstructure is builded in MIMO-OFDM system to make every LDPC code make full useof the diversity. A parameter decision soft interference cancellation schema ispresented for this LDPC-MIMO-OFDM structure.