摘要(英) |
With the requirements of higher resolution of Synthetic Aperture Radar (SAR) imaging, real-time computations including the large-size FFT and intensive data communications become a challenging task. Two-dimensional (2D) Fourier transform is widely used in SAR imaging algorithms, like Range Doppler Algorithm (RDA) and Chirp Scaling Algorithm (CSA). The 2D Fourier transform accounts for more than 70% of the computations in the algorithms. In this thesis, the imaging efficiency in the different conditions is first discussed. The decomposition of azimuth FFT is adopted, which increases the efficiency of communications between local memory and DRAM by the longer burst length of the burst mode. The 8 sets of radix -23 single-delay feedback architecture are used due to its regularity and pipeline nature with less complex multipliers. The in-place bit-reversal buffer and data access between DRAM and local buffer save the memory requirements. Besides, the look-up ROM tables are shared for decomposed FFT/IFFT in the azimuth direction by simple control logic. The 2D FFT/IFFT is designed with the customized floating-point format on FPGA of Xilinx ultrascale VCU128. The 2D FFT/IFFT supports 8192, 16384, and 32768 points in the range direction, and support 8192 points in azimuth direction. The frequency of the FPGA approach 111 MHz. The signal-to-quantization-noise (SQNR) of after successive 2D FFT and IFFT is above 37.9dB. Beside 2D FFT/IFFT, the phase compensation is also required in RDA. Range compression, azimuth compression and second range com-pression (SRC) are employed. We evaluate and compare the performance and utili-zation of the complex multipliers and Coordinate rotation Digital Computer (CORDIC) operations and select to use complex multipliers. The azimuth compression is com-pleted by mixed datapath formats, which consist of the double-precision float-ing-point, the customized floating-point, and the fixed-point representations. The piecewise Taylor series expansion replaces the square root operations for complexity reduction. The azimuth compression supports the radar squint angle from 0 to 9 de-grees. The signal-to-noise ratio is 52.7dB. The designs of piecewise Taylor series ex-pansion, the index computation, and the ROM tables are shared by SRC and azimuth compression for hardware reduction. |
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