| Although multiple-carrier-modulation-based orthogonal frequency division multiplexing (OFDM) has been adopted as one core wireless-transmission technology in next-generation terrestrial wireless communication system (IMT-Advanced), the inter-carrier interference (ICI) and the descended spectrum efficiency resulted from cyclic prefix (CP) aiming to combat inter-symbol interference (ISI), are two crucial factors for limiting the transmission reliability and spectrum efficiency of OFDM systems. As to the above two problems, this dissertation investigates in two novel OFDM systems, the offset-quadrature-amplitude-modulation (OQAM)-based OFDM (OFDM/OQAM) system and cooperative OFDM system.Through introducing optimal prototype function of pulse-shaping filter and utilizing its well time-domain and frequency-domain localization property, the OFDM/OQAM systems can improve the spectrum efficiency, by discarding the cyclic prefix required in conventional OFDM systems. However, how to design the optimal preamble structure to achieve reliable channel estimation and synchronization is a key issue in OFDM/OQAM systems.On the other hand, cooperative communications achieve space-diversity gain, in which mobile terminal deployed with single antenna can form vitual multi-input-multi-output (MIMO) systems by sharing individual antenna in multiple-relay/-user environment. And a crucial problem for cooperative communications utilized in OFDM systems is to achieve accurate channel estimation and synchronization.As a result, this dissertation works on the channel estimation and synchronization problems in the above-two novel OFDM systems. In OFDM/OQAM systems, this dissertation proposes a ICI-cancellation-based channel-estimation scheme and a low-complexity efficient synchronization algorithm. As to the cooperative OFDM systems, this dissertation also proposes a novel interleaving-based synchronization scheme. Chapter two investigates the system structure and characteristic of OFDM/OQAM systems. After summarizing the conventional OFDM systems, the chapter addresses the signal model and fast implementation structure of OFDM/OQAM systems. And then by selecting extended gauss function (EGF) as pulse-shaping prototype function, the ability of combating time-and-frequency double-selective fading is studied.Chapter three discusses new channel estimation and synchronization scheme for OFDM/OQAM systems. Based on the description of the wireless channel model, the pilot-based channel estimation in conventional OFDM systems is introduced. And after analyzing the interference source of pilots in OFDM/OQAM systems, this dissertation gives a ICI-cancellation-based channel-estimation preamble and the corresponding algorithms. Next, two new time-frequency synchronization schemes are proposed effectively, which are based on frequency-domain preamble and time-domain preamble. The above algorithms are finally verified by computer simulation.In cheaper four, the channel estimation and synchronization problems in cooperative OFDM systems are studied. First and foremost, this cheaper summarizes the system and signal model of cooperative OFDM systems, and does research for the preamble-based channel estimation schemes by comparison. Then a novel interleaving-based synchronization scheme is proposed. Simulation is also finally utilized to verify the performance of channel estimation schemes and proposed synchronization technique.The research results in this dissertation provide effective approaches of channel estimation and synchronization in the above-two novel OFDM systems. Utilizing these novel techniques, the spectrum efficiency and transmission reliability are further improved, and the results can be applied in the next generation mobile communication systems, such as IMT- Advanced. |
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