  We describe an hierarchical, frequency-domain beamforming architecture for
synthesising a sky beam from the wideband antenna feeds of digital aperture
arrays. The development of densely-packed, all-digital aperture arrays is an
important area of research required for the Square Kilometre Array (SKA) radio
telescope. The design of real-time signal processing systems for digital
aperture arrays is currently a central challenge in pathfinder projects
worldwide. In particular, this work describes a specific implementation of the
beamforming architecture to the 2-Polarisation All-Digital (2-PAD) aperture
array demonstrator.
