Researches On ICI Mitigation In Wideband Wireless OFDM Systems

The orthogonal frequency division multiplexing (OFDM), is one of the keytechnologies of physical layer (PHY) in many wide-band wireless systems, such aswireless local area network (WLAN), wireless metropolitan area network (WMAN),and the fourth generation mobile communication systems (4G). The OFDM is also oneof the fundamental technologies for future wireless communication, due to itsintegration with the emerging techniques such as cooperative communication andcognitive radio. However, OFDM is sensitive to inter-carrier interference (ICI), whichdestroys the orthogonality among the subcarriers and degrades the system performance.There are many reasons to cause ICI in practical wideband wireless OFDM systems,such as the impairment of the RF circuits, the imperfection of the baseband processingand the time-variation of wireless channels.The main idea of the dissertation is to study ICI mitigation in the physical layer forwideband wireless OFDM systems, including the phase noise (PHN) mitigation and ICIequalization for time-varying channels. Besides, the improved schemes are presented.The contributions of the dissertation are as follows:Firstly, based on the basic structure of the PHY in wireless communications, thesources of ICI in OFDM are analyzed comprehensively, which can be described by thethree aspects: the residual error in DSP algorithms, the impairment of analog front-end,and the time-varying characteristics of the channels. The ICI effects from the sources onOFDM are also analyzed.Secondly, the frequency-domain joint estimation algorithm of the carrier frequencyoffset (CFO), channel and PHN, is proposed for the preamble stage in burst OFDMsystems with PHN. Using PHN covariance prior information and two repeat trainingsymbols in preamble, the fine CFO is estimated in frequency domain with high accuracy,which isn’t effected by the PHN and channel. Then a low complexity algorithm withoutmatrix inversion and the PHN prior is given, and it’s proven that the simplicity has alittle influence on the performance. In order to get the joint estimation of channel andPHN, a quadratic programming problem with equality constraint is formatted andsolved, and then the lowpass character of PHN is utilized to reduce the computationalcomplexity. In all, the proposed algorithm is finished in frequency domain, so thefrequency domain character of PHN and channel can be used, and no prior information about the channel length and PHN covariance is required.Thirdly, through the interpolation of the common phase error (CPE) of PHN, thejoint estimation of data symbol and PHN is proposed for the data transmission stage inburst OFDM systems with PHN. In order to compensate the interference from the PHNof local oscillators, an iterative algorithm for PHN estimation and equalization isproposed. Firstly, the slow-going PHN is approximated by the interpolation of theadjacent CPE, and then the interpolation points of the PHN are considered as the initialvalues for the following iteration. Finally, the phase noise and de-mapping symbols canbe obtained jointly by the iterative estimation and equalization in the frequency domain.Computer simulation illustrates that the proposed scheme with lower complexitysignificantly outperforms the conventional method based on CPE coefficient in the biterror ratio (BER) performance when the number of iterations is the same. In addition,the proposed scheme without iterations has much better BER performance than thesimple non-iteration CPE correction method, especially for the environments withsevere PHN.Fourthly, the amplify-and-forward (AF) cooperative OFDM system model andreceiver structure is proposed for AF cooperative OFDM system with PHN and CFO.By the approximate signal model, the scheme of synchronization and channelestimation for inner receiver is designed. And for the data transmission stage, adecision-directed joint PHN mitigation and decoding algorithms is proposed, which issuperior to the approach without PHN suppression. The proposed scheme andalgorithms can be extended to the AF cooperative OFDM systems with multiple relays.Finally, based on the space alternating generalized expectation-maximization(SAGE) algorithm, and the ordered serial interference cancellation (OSIC) approach, anICI mitigation algorithm, named as the SAGE-OSIC, is proposed for OFDM systemsover multipath fading and rapidly time-varying channels. Computational complexityinvestigations and simulation results indicate that the proposed SAGE-OSIC, with noincrease in the computational complexity, significantly outperforms the existingmethods based on the SAGE, especially for large Doppler shifts. In addition, theperformance of the proposed SAGE-OSIC is very close to that of the VBLASTalgorithm, where the latter demands huge computation. Hence, the proposed algorithmis particularly well suited for practical highly mobile systems.