| In high-speed broadband wireless communication systems,orthogonal frequency division multiplexing(OFDM)is a kind of multi-carrier transmission technique that has wide application prospects.OFDM has lots of advantages such as high frequency utilization,strong abilities against inter symbol interference and channel fading,so it has been applied to many fields,such as the fourth generation mobile communication technology and so on.However,the OFDM system has a high peak-to-average power ratio(PAPR),and it is very sensitive to the frequency offset and carrier phase offset.Especially,the high PAPR requires large dynamic range of the power amplifier in OFDM transmitter,which greatly increases the complexity of the system.Based on the traditional methods of reducing PAPR,the constant envelope orthogonal frequency division multiplexing(CE-OFDM)is studied,which is a combination of traditional OFDM and constant envelope modulation.It modulates the OFDM signal into the frequency part or phase part of a constant envelope carrier by using frequency modulation or phase modulation,which effectively reduces its PAPR.Besides,nonlinear power amplifier can be used in CE-OFDM system,which evidently reduces the power consumption compared to OFDM system.Therefore,it is of great significance to study on CE-OFDM system and the design of its waveform.Firstly,traditional OFDM system is studied,including its mathematical model,cyclic prefix structure,bandwidth,etc.OFDM's high PAPR and some existing methods of reducing PAPR are also considered.Then,fundamental of CE-OFDM and its system model are studied,including its mathematical model,subcarrier modulation pattern and subcarrier waveform.Then,some fundamental issues of carrier synchronization are studied for further research,such as synchronization mode,frame structure and estimate performance bound.Then,the traditional arctangent receiver and two improved receivers,the basic linear receiver and the enhanced receiver,for CE-OFDM system are studied respectively.Simulations of the three receivers under different modulation index and modulation order show that the arctangent receiver can be applied in most situations but must have a complicated unwrapping algorithm,and two improved receivers have better performance under small modulation indexes but have the problem of deterioration of performance under high modulation order.Then,a symmetry decoupling technique for joint frequency-phase estimate is proposed,which can effectively eliminate the effect of residual frequency in phase estimate.Also,the concepts of serial/parallel joint frequency-phase estimators are proposed.Then,after studying the phase estimate of CE-OFDM system,a carrier synchronization scheme for the system is proposed,which uses the rotational periodogram averaging algorithm with the pilot symbols as coarse synchronization and the cyclic prefix cross correlation algorithm as fine synchronization.Simulations show that the concatenation algorithm has a fine performance and can achieve the performance of ideal synchronization by adjusting the number of pilot symbols.Finally,under the development environment of Quartus ? 9.0 and Modelsim SE 10.1c,the implementations of CE-OFDM modulator and demodulator are designed based on Verilog hardware design language in field-programmable gate array(FPGA).The occupation of resources and information processing speed for the two FPGA projects are also analyzed respectively.And the correctness of the designs is verified by waveform simulation. |
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