Carrier Frequency Offset Estimation In OFDM

CP-aided block-wise transmission technique becomes more attractive,which has the advantages such as high frequency efficiency,outstanding performance in overcoming multipath and simple equalization.It is considered as the main technique for 4G mobile communications.There are two CP-aided block-wise transmission techniques:Orthogonal Frequency Division Multiplexing(OFDM) and Single Carrier with Frequency Domain Equalization(SC-FDE).AS two block-wise transmission techniques,OFDM and SC-FDE has duality.Besides those advantages described above,the main problem lied in block-wise system is that it's vulnerable to synchronization.Synchronization is a practical problem in any digital communication system.And it can be roughly divided into timing synchronization and frequency synchronization.Timing synchronization methods such as based on cyclic prefix(CP) and unique training sequences are mature.Timing unsynchronization doesn't affect the performance if the frame start is in the range of CP,as the cyclic characteristic of CP. However,the frequency synchronization methods have more limitation,for example, ML algorithm can be applied in AWGN channel basically,it works worse in multi-path fading channels.Also the performance of NL algorithm degenerates while M-QAM is used.And the property of SR algorithm has large fluctuation during in the time varing environment.Meanwhile,these methods are done without considering the channel estimation error or timing offset,which is not suitable for the practical case. The algorithm proposed by Minn & Letaief is a combined timing and frequency synchronization and channel estimation,but it isn't fit for IEEE802.11a.We propose the concept of post-reconstruction based on the SR algorithm,and combine it with iteration.And the original signal reconstruction method in SR algorithm is designated pre-reconstruction correspondlly.Compared with prereconstruction, the post-reconstruction has higher precision and steady convergence in time varying environment,under the premise of same computation.Also,it performs better than the pre-reconstruction while integrating the iteration in the consideration of channel estimation error and timing offset.MIMO is a new kind of technique in radio environments.However,it is a kind of narrow band transmission technique in essence,and it has to combine with other broadband communication technique to support higher rate,two of the combinations are MIMO-OFDM and MIMO-SCFDE.Finally,the method described above is applied in the frequency synchronization of MIMO-OFDM and MIMO-SCFDE systems.It shows good performance in the existence of frequency offset and decimal part of timing offset.However,it must be illuminated that the computation increases when combing with the iteration,but it will converge after about thirty or fourty times iteration.Besides,the estimation of both frequency offset and channel in OFDM or SC-FDE system can be obtained through only one training sequence,while the estimation of frequency offset and channel estimation of MIMO-OFDM or MIMO-SC-FDE system can be got by a set of training sequences.