Channel Equalization Of FrFT-OFDM Communication Systems

Due to the advantages of high spectrum efficiency and anti-multipath fading, orthogonal frequency division multiplexing(OFDM) has becomes the core technology of the downlink in 4G mobile communication, which greatly enhance the speed of data transmission. However,OFDM applications may suffers from some performance degradation when the speed of relative motion is much faster between communication terminals, and meanwhile the Doppler frequency shift of channel will seriously affect data transmission in OFDM system. Fractional Fourier transform(FrFT) is applied in fractional Fourier transform orthogonal frequency division multiplexing(FrFT-OFDM) system, which uses chirp type of orthogonal base as subcarrier signal to achieve modulation and demodulation process, instead of the conventional Fourier transform in OFDM system. Because of the carrier frequency of sub-channels changed over time, in this paper, appropriate fractional Fourier transform order is determined for the transmission signal according the active change in time-varying channel; and for the interference decrease between subcarriers, which will improves performance of the FrFT-OFDM system.The performance of anti-fading and channel equalization of FrFT-OFDM communication systems are investigated in this paper, and the system performance under different channel environment is simulatated. Meanwhile, hardware experiment platform of FrFT-OFDM base band system is built. The specific content can be summarized as follows:First of all, the mathematical model is established through learning the basic principles of FrFT-OFDM system, and the FrFT discrete numerical computation algorithms, which is suitable for chirp type sub-carrier signal to complete the modulation and demodulation, is studied In addition, the baseband system of MATLAB simulation program is written to simulate the BER performance under white Gaussian noise, and the bandwidth efficiency rate of the system is deduced.Then, wireless channel model is established, and the ability to resist channel fading in FrFT-OFDM system is analyzed. Besides, the equalization technology at the receiving terminals is studied in this paper, and the fractional Fourier domain Zero-Forcing(ZF)equalization is proposed. The performance of system in the multipath channel, the Doppler frequency shift channel and the double selective fading channel is simulated and analyzed.Finally, the FrFT-OFDM hardware implementation scheme is design by using FPGA as the core digital processing chip to build system's sending and receiving platform. Additionally,Verilog HDL is applied for the programming and debugging for sender, receiver and channel,and tested on the hardware platform. It is concluded that the system bit error rate under different signal-to-noise ratio verify the feasibility of scheme.