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Abstract: 3D mapping of matter distribution in the Universe through the 21 cm radio emission of atomic hydrogen (HI) is a complementary approach to optical surveys for the study of the Large Scale Structures, in particular for measuring the BAO (Baryon Acoustic Oscillation) scale up to redshifts , and therefore constraining dark energy parameters. We propose a novel method to map the HI mass distribution in three dimensions in radio, without detecting or identifying individual compact sources. This method would require an instrument with a large instantaneous bandwidth () and high sensitivity, while a rather modest angular resolution () should be sufficient. These requirements can be met by a dense interferometric array or a phased array (FPA) in the focal plane of a large primary reflector, representing a total collecting area of a few thousand square meters with few hundred simultaneous beams covering a 20 to 100 square degrees field of view. We describe the development and qualification of an electronic and data processing system for digital radio interferometry and beam forming suitable for such instruments with several hundred receiver elements. [Copyright &y& Elsevier]