Realization Of OFDM Signal Based On FPGA

OFDM is an essential part in wireless communication.Due to its efficient modulation,its productive spectral efficiency because of carrier orthogonal and its advantage in combat dispersive channel on account of broad band,OFDM has been widely applied to a large amount of communication standard,along with standard in WLAN.Conventional baseband signal modulation in OFDM is always based on the structure of IFFT,along with imperfections of complex arithmetic operation and high transmission delay.With the rapid progress of the times,a new structure to produce baseband signal modulation of OFDM on basis of DDS has been proposed recent years.Different from the OFDM system using IFFT to synthesis modulated signals in the frequency domain,this method modulates sub carrier and accumulates all the sub carrier in the time domain,without many and diverse multiplications in IFFT and with less transmission delay.This thesis has a further study on the baseband signal modulation of OFDM based on DDS,to have a better optimization on the system strucure of OFDM.Furthermore,a corresponding circuit design is completed in this thesis.This thesis mainly chooses OFDM baseband signal modulation architecture based on CORDIC algorithm in DDS algorithm,because this algorithm can avoid the problem of complex storage resources in ROM structure compared with DDS based on ROM structure to realize OFDM baseband signal modulation.In order to further reduce the hardware resource consumption of the OFDM modulation structure based on CORDIC algorithm,two basic parameters of DDS,phase truncation and amplitude quantization and their influence on the OFDM structure is further studied.This main research contents of this thesis cover:First,this thesis uses CORDIC algorithm as a framework,and designs an OFDM transmitter based on IEEE 802.11 g protocol based on FPGA.In addition to the OFDM modulation structure based on CORDIC algorithm,it also includes channel coding,pilot insertion and cyclic prefix insertion modules.The typical transmission channel model of WLAN and the receiver model of OFDM are established through simulation at the same time.Then this thesis simulates the OFDM trasmitter's performance in actual channel through this two models.Then,phase truncation and amplitude quantization quantify the input phase and output amplitude of the DDS,their precision corresponds to the spectral purity of this OFDM structures.By analysing this two parameters' influence on the distribution of spur and the bit error rate of the OFDM system.By adjusting this two parameters,we can minimize the hardware structure of this system and the spur caused by this two parameters won't influence the system performance.Finally,this paper completes the circuit design of the OFDM transmitter,the transmitter circuit meets the basic requirements of IEEE 802.11 g protocol in WLAN.