  The extra-large multiple-input multiple-output (XL-MIMO) architecture has
been recognized as a technology for supporting the massive MTC (mMTC),
providing very high-data rates in high-user density scenarios. However, the
large dimension of the array increases the Rayleigh distance (dRayl), in
addition to obstacles and scatters causing spatial non-stationarities and
distinct visibility regions (VRs) across the XL array extension. We investigate
the random access (RA) problem in crowded XL-MIMO scenarios; the proposed
grant-based random access (GB-RA) protocol combining the advantage of
non-orthogonal multiple access (NOMA) and strongest user collision resolutions
in extra-large arrays (SUCRe-XL) named NOMA-XL can allow access of two or three
colliding users in the same XL sub-array (SA) selecting the same pilot
sequence. The received signal processing in a SA basis changes the dRayl,
enabling the far-field planar wavefront propagation condition, while improving
the system performance. The proposed NOMA-XL GB-RA protocol can reduce the
number of attempts to access the mMTC network while improving the average sum
rate, as the number of SA increases.
