| One of the requirements of next generation mobile communications systems is to provide high speed and broadband access for multimedia services. Orthogonal Frequency Division Multiplexing (OFDM) can well solve the problem of multipath interference, which uses many low rate subcarriers to achieve a high data rate transmission. The cyclic prefix (CP) is introduced to eliminate the multipath interference. OFDM system has very high channel utilization ratio because of the orthogonal properties among its sub-carriers. OFDM applys in many communication standard, such as digital video broadcasting(DVB) system, wireless LAN, wireless MAN. This paper analyses the principle of OFDM, and focuses on OFDM-based multi-user access and synchronization technology, at last introduces one suggestion for emergency communication on physical layer.The main work of this paper are as follow.1. At first this paper analyses the advantages and disadvantages of multi-carrier technology, introduce the mathematic expression of OFDM, the application of IDFT/DFT, and the principle of cyclic prefix, then concludes the structure of OFDM transmitters and receivers.2. Summarizes the integration of OFDM and multi-user access technology, such as FDMA, TDMA, CDMA, etc. And then introduces the detection technology.3. The synchronization includes synchronization of time and frequency. Carrier frequency offset, OFDM symbol timing offset, sampling frequency offset will all introduce ICI and ISI, which can deteriorate the precision of receiver. In this paper the different algorithms for frame, symbol and frequency synchronization are compared, and also summarizes the scheme of synchronization for DVB-T.4. This thesis puts forward one solution for this service model for emergency communications, including multi-user access, channel coding, modulation, frame format and signal exchange flow, the physical layer of service channel is mostly referring to DVB-T. |
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