  Massive multiple-input-multiple-output (M-MIMO) is a key technology for 5G
networks. Within this research area, new types of deployment are arising, such
as the extremely-large regime (XL- MIMO), where the antenna array at the base
station (BS) has extreme dimensions. As a consequence, spatial non-stationary
properties appear as the users see only a portion of the antenna array, which
is called visibility region (VR). In this challenging transmission-reception
scenario, an algorithm to select the appropriate antenna-elements for
processing the received signal of a given user in the uplink (UL), as well as
to transmit the signal of this user during downlink (DL) is proposed. The
advantage of not using all the available antenna-elements at the BS is the
computational burden and circuit power consumption reduction, improving the
energy efficiency (EE) substantially. Numerical results demonstrate that one
can increase the EE without compromising considerably the spectral efficiency
(SE). Under few active users scenario, the performance of the XL-MIMO system
shows that the EE is maximized using less than 20% of the antenna-elements of
the array, without compromising the SE severely.
