Gemini FPGA hardware platform for the SKA low correlator and beamformer

In this paper the hardware designed for the SKA Low (Square Kilometre Array) correlator and beamformer (CBF) is discussed. SKA-Low is a low frequency aperture array (LFAA) to be located in remote Western Australia. The array is collecting radio signals in the frequency range from 50 to 350 MHz. The large number of dual polarization antennas (131072) are distributed over a total of 512 stations with a maximum spacing of 65 km. Each station forms a single dual polarization beam with a bandwidth of 300 MHz. The bandwidth is split into 384 coarse channels of 781250 Hz in width by LFAA station beamformers. The CBF hardware receives these station beam signals and splits them into finer channels using a polyphase filterbank. Thereafter the fine channels from all stations are aligned in time, correlated and simultaneously beamformed. There are two independent beamformers, one for pulsar search (PSS), that creates 500 array beams, and another for pulsar timing (PST), that creates 16 array beams. A variety of additional functionality and modes is required in the correlator and beamformers to meet the requirements. The core of the CBF hardware is a board with a Field Programmable Gate Array (FPGA), called Gemini, that can implement all functions of the CBF. The amount of input data and processing for CBF is that large, that it requires several hundreds of these FPGA boards. The current estimate is that the SKA Low CBF will need 288 FPGAs. The same board can be used for all functions, because it is the interconnect between boards and the FPGA firmware that determine the function. This versatile hardware design could therefore potentially be used for other radio telescopes or data processing systems as well.