| Orthogonal Frequency Division Multiplexing (OFDM) is a multicarrier transmissiontechnology which is capable of resisting the inter-symbol interference introduced by thefrequency selective fading channel. Compared with single-carrier technic, OFDM employsthe overlapped and orthogonal narrowband subchannel to bear the information data with highspectrally efficiency and it is able to compensate the channel by a simple single-tap frequencyequalizer. Therefore, it has been adopted by various wideband systems, such as ADSL andVDSL in the cable systems, WiFi in the wireless LAN, and LTE-Advance and WiMAX in thenext generation cellular systems.Nevertheless, OFDM suffers several drawbacks which degrades its performance. Forexample, its peak-to-average power ratio (PAPR) is relatively high and it is sensitive to thefrequency offset. Besides, based on recent literature, OFDM systems cannot take advantage ofthe multipath diversity provided by the multipath fading channel, resulting in degradation onsystem capacity. To counteract these issues, numbers of algorithms and schemes have beenproposed, e.g., linearly precoded OFDM. Moreover, in optical communication, a so called fastOFDM has been proposed to use a more compact spectrum and real modulate.This dissertation focuses on the background and development trend of OFDM to conductrelevant research hotspot. Then, it concentrates on the following topics, a) PAPR problem andPAPR reduction methods, b) multipath diversity and the linearly precoded OFDM with linearequalizer, and c) the recently proposed fast OFDM in optical systems. In respect to each topic,study has been conducted in-depth, and the mathematic models are formulated in order topresent corresponding algorithm and system scheme.In order to deal with the PAPR problem of OFDM signal, the characteristics of OFDMsignal are utilized to propose a partial shift mapping (PSM) algorithm. In PSM, differentcandidate signals with different PAPR are generated based on the original OFDM signal viamapping, and then the one with the lowest PAPR is chosen for transmission; at the receiver,the signal is recovered by reversing the mapping process. Compared with other methods, onlyone inverse discrete Fourier transform (IDFT) and complex addition operations are required to obtain these candidate signals, eliminating a mass of complex multiplication operations inthe algorithms of such kind. As a result, PSM relieves the computational complexity ofOFDM systems and achieves satisfied PAPR performance.As to the issue that the multipath diversity cannot be exploited by OFDM system undermultipath fading channel, we propose an efficient discrete Hartley transform (DHT) precodedOFDM system, and a simple precoding algorithm. Based on this algorithm, the cascaded DHTand IDFT operations are replaced by a one-level butterfly operation which contains onlyaddition operations, while the DFT operation at receiver is replaced by DHT to save DFTmodule in the system by using the one-level butterfly operation. Thus, the complexities ofboth computational and system design are reduced. Based on analysis and simulation, it isshown that the proposed DHT precoded OFDM system is able to exploit the multipathdiversity efficiently with a linear equalizer as other precoded OFDM systems. In addition, forthe intimate relation between DHT and DFT, the DHT precoded OFDM signal featuresextremely low PAPR which is similar to that of single-carrier signal.Further, this dissertation studies the most recently proposed fast OFDM system. In thissystem, the subcarrier space is half to the conventional OFDM systems, but inter-carrierinterference between the even and odd subcarriers exists if single-band modulation isemployed. A demultiplexing algorithm and system scheme have been proposed to resolve thisinterference. Compared with previous schemes, it completely avoids the interference withoutany data rate loss and thus gets better performance. |
