  Ever since the invention of Bell Laboratories Layer Space-Time (BLAST) in mid
1990s, the focus of MIMO research and development has been largely on pushing
the limit of spectral efficiency. While massive MIMO technologies laid the
foundation of high spectrum efficiency in 5G and beyond, the challenge remains
in improving energy efficiency given the increasing complexity of the
associated radio systems. With the substantial negative implications of climate
change looming ever closer, minimizing energy use is a key dimension of
achieving sustainability and is of paramount importance for any future
technology. Thus, every aspect of future extreme MIMO system design,
implementation, and operation will be scrutinized to maximize energy
efficiency. An analysis of the massive MIMO 5G radio energy consumption at
different loads leads to qualitative energy efficiency design goals for
emerging extreme MIMO systems. Following this, we focus on novel operational
and component technology innovations to minimize energy consumption.
