  We describe the motivation and design details of the "Phase II" upgrade of
the Murchison Widefield Array (MWA) radio telescope. The expansion doubles to
256 the number of antenna tiles deployed in the array. The new antenna tiles
enhance the capabilities of the MWA in several key science areas. Seventy-two
of the new tiles are deployed in a regular configuration near the existing MWA
core. These new tiles enhance the surface brightness sensitivity of the MWA and
will improve the ability of the MWA to estimate the slope of the Epoch of
Reionisation power spectrum by a factor of ~3.5. The remaining 56 tiles are
deployed on long baselines, doubling the maximum baseline of the array and
improving the array u,v coverage. The improved imaging capabilities will
provide an order of magnitude improvement in the noise floor of MWA continuum
images. The upgrade retains all of the features that have underpinned the MWA's
success (large field-of-view, snapshot image quality, pointing agility) and
boosts the scientific potential with enhanced imaging capabilities and by
enabling new calibration strategies.
