Compensation Of Synchronization Offset In OFDM System And Its Implementation On FPGA

Efficient spectrum utilization,high transmission rate and high reliability are the development trends in the digital field.It is very meaningful to study the reliability reception of OFDM,a multi-carrier technology with high transmission efficiency and high spectrum utilization.While it has many advantages,it also has the disadvantage of being sensitive to offset.In this thesis,the source and influence of synchronization error in OFDM system are analyzed,and then a compensation scheme of synchronization error is given.For the purpose of verifying the practicability of the design,the thesis is designed and verified according to the IEEE802.11 a standard,and a complete digital demodulation scheme which includes frame synchronization,carrier synchronization,symbol synchronization,channel estimation and equalization,and sampling frequency offset synchronization,demodulation and other modules is proposed,completing the synchronization offset compensation and the FPGA implementation of the whole system.Through the research and analysis of the timing offset,it is found that the timing offset will cause inter symbol interference,resulting in weakened demodulation performance.The timing synchronization can be achieved by frame synchronization and symbol synchronization.In terms of reliability,the frame synchronization algorithm with delay correlation and length preservation is selected.For the peak detection fuzzy problem,the symbol synchronization algorithm based on cross-correlation is selected for offset compensation.To reduce resource consumption,sliding window accumulation and quantification are used in the hardware design.The carrier frequency offset causes inter subcarrier interference and phase shift,which seriously affect the demodulation performance.On the basis of comparing the accuracy and complexity of the algorithm,the time domain algorithm based on short training sequence is selected as the carrier synchronization algorithm.To obtain higher carrier synchronization accuracy,the offset parameter is ensured by the average value of several estimate.In hardware implementation,the frequency offset error is calculated by accumulation instead of multiplication,which simplifies the hardware structure.The sampling frequency offset will cause inter symbol interference and inter subcarrier interference,which has a great impact on the system.Starting from the error model of the received signal,the main factors causing the offset are analyzed.The data-assisted tracking is used to find the offset information and the least-squares method is used to obtain the best estimation.Considering the complexity of the implementation,the least squares estimation algorithm has been improved.Due to the need of synchronization offset compensation,the channel estimation and equalization are performed to eliminate the influence of phase difference.After synchronization,the demodulation module is designed to realize the conversion of constellation points to the binary data stream,compensating the magnitude by dynamic adjustment of the threshold.After doing that the bit error rate is calculated and the synchronization effect is verified.Each module of the system is verified by Matlab simulation,Isim simulation and ChipScope logic test.The results show that the design can support frequency offset range of 0-200 kHz,sampling frequency offset of 0-200 ppm and Doppler offset of 0-300 Hz.The compensated error vector magnitude is less than 10% rms which meets the design requirements.