WFRFT And Partial FFT-Based Transmission Methods Under Rapidly Time-Varying Channels

Under the rapidly development of modern information age, the high mobility ofcommunication equipment and the fast transmission of communication signal are moreand more challenging for the whole communication industry. However, the communi-cation environment is more complexity than before. And, there will be not only pro-duce high Doppler interference due to the high mobility, but also generate multiple pathinterference due to the complex transmission paths, such as the long term evolution ad-vanced(LTE-A), worldwide interoperability for microwave access(Wi MAX), underwateracoustic(UWA) communication and digital video broadcasting(DVB) communication.These rapidly time-varying multipath(or doubly selective) fading channels will inducethe inter-carrier interference(ICI), inter symbol interference(ISI) and carrier frequencyoffset(CFO), which will severely impair the communication quality. Therefore, the re-search technologies, on the suppression of ICI, ISI and CFO under rapidly time-varyingmultipath fading channels, are important for wireless communications.The traditional methods, to overcome the interference from the rapidly time-varyingmultipath channels, are based on the orthogonal frequency division multiplexing(OFDM)system. However, the orthogonality between different subcarriers has been destroyed dueto the significant time variations under rapidly time-varying multipath channels, whichwill induce significant ICI. Some other works, proposed to mitigate the ICI, are based onthe single carrier modulation(SCM) system with frequency domain equalization(FDE).Unfortunately, SCM system will be easily plagued by the inter-symbol interference(ISI)caused by the large time-delay spread under the rapidly time-varying fading channels,which will also deteriorate the performance of wireless communications. To this end,a weighted type fractional Fourier transform(WFRFT) based hybrid carrier modulation(HCM) system has been exploited to suppress the ISI and ICI under rapidly time-varyingmultipath channels. Specially, HCM system can be as a convergence of SCM and OFDMsystems according to its constitution. Thus, it can be expected to inherit the advantage ofSCM and OFDM system, and to achieve a better performance than both SCM and OFDMsystems under rapidly time-varying multipath channels. Besides, from the demodulationmode, the traditional OFDM system employ the traditional Fourier transform that cannotanalysis the local characteristics of interference. It is a crucial to find a local analysis toolto mitigate the ICI/ISI and CFO under doubly selective channels with the high Dopplerspread. Fortunately, a novel demodulation mode–partial FFT demodulation will outper-form the traditional Fourier transform at the local interference analysis. The aims to thisdissertation, utilizing the novel HCM with partial FFT demodulation system, are to fur-ther suppress the ICI and CFO under typical rapidly time-varying multipath channels.Furthermore, a low complexity equalization algorithm based on HCM with partial FFTdemodulation system will be derived in the dissertation. The main research work can beexpressed as follows:This dissertation firstly develops the theorem of multiple weighted type fractionalFourier transform(m-WFRFT). To the vague relationship of different m-WFRFT, this dis-sertation builds the relationship of modulation order between different m-WFRFT basedon the weight fractional Fourier transform matrix. Furthermore, the dissertation providesits implementation structure and explains its physical meaning. Based on the modulationorder relationship between different m-WFRFTs, this dissertation proposes the general-ized hybrid carrier system, which can be the general form of hybrid carrier system basedon 4-WFRFT. At last, the dissertation also analysis its applications on wireless commu-nications, which can provide the significant theorem basis of other chapters.To the inter-carrier interference(ICI) suppression under rapidly time-varying multi-path channels, this dissertation also proposes the hybrid carrier with partial FFT demod-ulation system based ICI mitigation algorithm, and designs the implement structure ofthis algorithm. The research interest stems from the superiority of the WFRFT basedHCM system over the traditional SCM and OFDM systems as previously explained. Be-sides, the partial FFT demodulation can redistribute the interference on the useful signalinto many other useless signal points. Therefore, the power of ICI over the useful signalpoints will be less than before. However, this cannot be implemented by the traditionalFourier transform. The optimal division number can be derived by the average power ofinterference under the rapidly time-varying multipath channels. Moreover, the optimalmodulation order can be selected by previously design a simple dictionary according tonormalized Doppler frequency. It can be demonstrated from the numerical simulation thatthe superiority of HCM with partial FFT demodulation system involving ICI suppressionalgorithm over doubly selective channels.To the carrier frequency offset(CFO) mitigation under rapidly time-varying multi-path channels, this dissertation has proposed the CFO suppression algorithm based on theHCM with partial FFT demodulation system previously mentioned. Also the disserta-tion has analyzed the generating mechanism of CFO, and obtained that the time-varyingof multipath delay spread is the primary reason to produce CFO under the rapidly time-varying multipath channels. Moreover, the dissertation finally applies the proposed al-gorithm into the typical rapidly time-varying multipath channels, DVB communicationchannels and UWA channels. Furthermore, numerical simulation has been presented, un-der the typical rapidly time-varying multipath channels, to demonstrated the superiorityof HCM system with the CFO suppression algorithm over SCM and OFDM system at thesame division number.Considering the high computational complexity of the partial FFT demodulationbased interference suppression algorithm, the dissertation, employing the banded charac-teristic of frequency channel matrix, has proposed that the HCM with partial FFT demod-ulation system based banded MMSE equalization algorithm. The interests stem from thebanded characteristic of the frequency channel matrix. Also, the dissertation finds thatthe frequency channel matrix after partial FFT demodulation, will also keep that char-acteristic. Therefore, associated with the banded MMSE equalization, a low complexityequalization algorithm has been proposed. Moreover, it achieves a good trade-off betweencomputational complexity and performance.