  In 6G, the trend of transitioning from massive antenna elements to even more
massive ones is continued. However, installing additional antennas in the
limited space of user equipment (UE) is challenging, resulting in limited
capacity scaling gain for end users, despite network side support for
increasing numbers of antennas. To address this issue, we propose an
end-user-centric collaborative MIMO (UE-CoMIMO) framework that groups several
fixed or portable devices to provide a virtual abundance of antennas. This
article outlines how advanced L1 relays and conventional relays enable device
collaboration to offer diversity, rank, and localization enhancements. We
demonstrate through system-level simulations how the UE-CoMIMO approaches lead
to significant performance gains. Lastly, we discuss necessary research efforts
to make UE-CoMIMO available for 6G and future research directions.
