  We describe the design, validation, and commissioning of a new correlator
termed "MWAX" for the Murchison Widefield Array (MWA) low-frequency radio
telescope. MWAX replaces an earlier generation MWA correlator, extending
correlation capabilities and providing greater flexibility, scalability, and
maintainability. MWAX is designed to exploit current and future Phase II/III
upgrades to MWA infrastructure, most notably the simultaneous correlation of
all 256 of the MWA's antenna tiles (and potentially more in future). MWAX is a
fully software-programmable correlator based around an ethernet multicast
architecture. At its core is a cluster of 24 high-performance GPU-enabled
commercial-off-the-shelf compute servers that together process in real-time up
to 24 coarse channels of 1.28 MHz bandwidth each. The system is highly flexible
and scalable in terms of the number of antenna tiles and number of coarse
channels to be correlated, and it offers a wide range of frequency / time
resolution combinations to users. We conclude with a roadmap of future
enhancements and extensions that we anticipate will be progressively rolled out
over time.
