| Since the wireless communication becomes the main way of people day by day,the frequency spectrum becomes a more and more scarce resource.The conventional single carrier transmission methods can't meet the needs of users.OFDM(Orthogonal Frequency Division Multiplexing),as a multi-carrier transmission way,which could be reckoned a modulation or multiplexing technique.It has many advantages over other wireless communication technologies, one major reason lies in the way that it has better robustness to frequency-selective fading and multipath.In addition, it is also a parallel technology which a high-speed data stream is divided into a number of low-speed data stream. Since the rate of each modulation signal is low, so that a relatively long cycle, and thus with a stronger resistance to delay spread, coupled with another element of each symbol using cyclic prefix as a guard interval, so the ISI(inter symbol interference) can be significantly reduced. The difference between ofdm and other multi-carrier transmission is that OFDM supports frequence-spectrum overlapping only if the sub-carriers are orthogonal each other,so it has high band-width efficiency. Therefore,it's a high speed transmission technology on wireless communications which the resources are scarce.Are two sides to everything, OFDM techniques also have shortcomings, mainly the following two points:(1) The high Peak-to-Average Power Ratio(PAPR) issue has been the difficulty and key of OFDM technology,thus become a major obstacle toward practical. Since the OFDM signals non-constant envelope, as we all know, that any non-linear RF amplification will lead to inter-modulation components, thus affecting the neighboring signals and system's performance. In order to improve transmission efficiency and system performance, the signal's peak and mean factor must be taken into account in the transmitter.(2) High sensitivity to phase noise, timing and frequency drift. When need precise timing, removing noise and reducing the frequency offset become particularly important.If the two points can't be achieved,the orthogonal among the subcarriers will be destroyed,then the ICI and ISI will be seriously.Chapter 1 of this thesis put forward the investigation background and the development situation of the OFDM technic, pointed out the objectives of the research,innovation points and structural arrangements, because of its superior performance, has been selected as the main technologie of the next generation mobile communication; Chapter 2 describes the the basic principle of OFDM. First of all, the multipath of the conventional single carrier transmission has been discussed, and then putting forward the OFDM technology, representing the orthogonal principles,advantages and disadvantages of OFDM, the last describing the shortcomings of OFDM,that is,its high PAPR,proposing it's definition and resolvent; chapter 3 discusses the distortion technology and coding techniques to reduce PAPR and proposed an improved clipping method and a block coding method; chapter 4 discusses the non-distortion technology to reduce the PAPR,which mainly contains selective mapping and partial transmit sequence, and proposed improved methods and devices, the last part, proposing the realization of syncretic methods and carry out a novel method which is suitable for multi-antenna OFDMA system; chaper 5 of the thesis are summarized and the expectation on the furture. |
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