Studies On Real Time Signal Processing Techniques For Multibeam System

With the constant revolution of multibeam echo sounding techniques, researches of modern multibeam swath bathymeters are developing in the directions of miniaturization, multi-function, high-precision, high-integration and standardization. This thesis focuses on the beamforming algorithms and the FPGA implementation.Firstly, CBF algorithm, DFT beamforming algorithm and FIM beamforming algorithm are introduced based on uniform linear array and horizontal planar array. Simulations and comparisons are carried out, the results show that the mainlobe beamwidth of FIM algorithm is much narrower than that of CBF algorithm, and DFT algorithm has the highest operation efficiency.Secondly, a plan of transmit beamforming is given. The coarse transmission phase delay based on D/A clock is 0.098μs , while the phase delay combined with DDS technique achieves 0.09 phase resolution, which corresponds to 4.88 ns time resolution at 50 kHz transmission frequency.Thirdly, a design for optimizing floating-point units based on pipeline structures is explored. The results show that the maximum frequency of floating-point adder and floating-point multiplier compared with that of IP core on Xilinx Virtex-4 series FPGA is increased by 10% and 30.7% respectively.Then, an FPGA implementation of receiving beamforming system based on v-type horizontal planar array is discussed in detail, which includes data buffer, Hilbert converter and conventional beamformer. Simulations show that the maximum frequency of the system achieves 203.75 MHz. The whole programs can finish beam scanning in 2.5μs which meets the requirements of real-time.Next, an FPGA implementation of DFT beamforming technique based on shortcut calculation is presented, containing buttfly calculator, address generator and data storage. Simulations indicate the maximum frequency reaches 132.24MHz, which makes the operation time decrease by 24.5%, compared with DSP implementation.Finally, an FPGA implementation of FIM technique based on uniform linear array is shown, which includes covariance matrix, the average of Toeplitz matrix and FFT beamforming. Simulation results verify its feasibility.