The Murchison Widefield Array is a dipole-based aperture array synthesis telescope designed to operate in the 80-300 MHz frequency range. It is capable of a wide range of science investigations but is initially focused on three key science projects: detection and characterization of three-dimensional brightness temperature fluctuations in the 21 cm line of neutral hydrogen during the epoch of reionization (EoR) at redshifts from six to ten; solar imaging and remote sensing of the inner heliosphere via propagation effects on signals from distant background sources; and high-sensitivity exploration of the variable radio sky. The array design features 8192 dual-polarization broadband active dipoles, arranged into 512 ldquotilesrdquo comprising 16 dipoles each. The tiles are quasi-randomly distributed over an aperture 1.5 km in diameter, with a small number of outliers extending to 3 km. All tile-tile baselines are correlated in custom field-programmable gate array based hardware, yielding a Nyquist-sampled instantaneous monochromaticuvcoverage and unprecedented point spread function quality. The correlated data are calibrated in real time using novel position-dependent self-calibration algorithms. The array is located in the Murchison region of outback Western Australia. This region is characterized by extremely low population density and a superbly radio-quiet environment, allowing full exploitation of the instrumental capabilities.

This paper describes a correlator that is optimized for the Xilinx Virtex-4 SX FPGA, and its application in the SKAMP radio telescope at the Molonglo Radio Observatory. The digital backend of the SKAMP telescope consists of more than 800 Virtex-4 FPGAs. Correlation is performed between each and every pairing of antenna inputs, so the SKAMP telescope, with its 384 inputs, has approximately 74,000 antenna correlations; with 100 MHz of input bandwidth from each antenna this requires real-time processing of more than 7 tera complex multiply-accumulates per second. The correlation cell described takes advantage of the hard IP blocks found within the Virtex-4 FPGA to perform one 4+4-bit complex correlation per cycle at a clock rate exceeding 256 MHz. At the core of each cell is an efficient 4-bit signed complex multiplier, implemented using the 18-bit signed multiplier of the Virtex-4 DSP slice, and a short term accumulator, implemented using the adjacent Block RAM. Nearly 30,000 correlation cells are instantiated across 192 Virtex-4 SX35 devices in order to process all the data from the SKAMP telescope.