Research And Implementation On Key Technologies Of Broad Band Digital Multi-beamforming In Large Scale Low Frequency Radio Telescope Array

Large-scale low-frequency radio arrays are the key area of future radio astronomy research,and are also important parts of SKA(Square Kilometre Array)project.Compared with the future design goals SKA project,China's current 21CMA(21 Centimetre Array)system using analog beamforming and software signal processing has shortcomings as poor side lobe suppression,not adjustable beam pointing,low system operation efficiency,etc.In order to solve these problems,two novel broadband multi-beam DBF(Digital Beamforming)schemes based on21 CMA system architecture was designed,and further more,a digital signal pre-processing hardware module was designed and implemented.The broad band DBF method not only has adjustable sidelobe suppression,multi-beam and flexible beam pointing features,but also effectively improves the efficiency of the digital signal preprocessing of 21 CMA,thus enhancing the performance of 21 CMA system and make it more comparable to SKA future trends.The main contributions of this article are as follows:1.Combining the system architecture of 21 CMA,a constant directivity broadband digital multi-beamforming method based on precise digital delay was presented.In this method,the beam pointing control is implemented by precise broadband digital delay adjustment,and the sidelobe suppression is fulfilled by Taylor amplitude weighting.This implementation allows 21 CMA DBF have adjustable side lobe suppression,multi-beam and flexible beam pointing features,On the condition of generating 12 independent beams,the airspace coverage and surveys rate of 21 CMA can increase times,the sidelobe power of the beam is reduced by 10 decibels,thus enhancing the system performance by a whole.2.Considering the 21 CMA digital signal processing section as a whole,we designed a broad band digital multi-beamforming method based on digital channelizer which simultaneously achieves DBF and spectrum analysis capabilities.In this method,the broad band sampling signal is separated to several narrow bands by digial channelizer,and then achieves beamforming through the narrow band DBF technology of each sub-band signal.Compared to the method which DBF and spectrum analysis processed separately,the computational complexity can be reduced by87% on the condition of 12 independent beams.Because the digital signal preprocessing(including DBF and spectral analysis)is usually the most complex part of the 21 CMA system in computational complexity,and also the bottleneck of system resources consumption and processing speed,this DBF implementation scheme can reduce the implementation complexity of the system as a whole,and greatly improve the system efficiency.3.We design and implement a hardware platform for broad band digital multi-beamforming system which is suitable for large-scale low-frequency radio telescope arrays.The hardware platform is designed in a paralleled architecture which is suitable for broad band DBF processing.The hardware platform used a modular design approach,which has multi-channel and multi-beam expansion capabilities in hardware,and has good compatibility and scalability in software.It can achieve the two broad band digital multi-beamforming method proposed in this paper.The hardware platform is implemented with Xilinx's Virtex-5 FPGA(Field Programmable Gate Array)and Intersil's high performance ADC(Analog Digital Converter),which can provide broadband high-speed processing capability,therefore it can meet the broad band,massive data throughput,multi-beam requirements of large-scale low-frequency radio telescope arrays.