  The construction of preamble sequences for channel estimation by
superposition of orthogonal pilots can improve performance of massive
grant-free uplink from machine-type devices. In this letter, a technique is
proposed to obtain full benefit from these "pilot mixtures" in presence of a
base station with a massive number of antennas. The proposed technique consists
of combining pilot mixtures with an intra-slot successive interference
cancellation (SIC) algorithm, referred to as inner SIC, to increase the number
of decoded messages per slot. In framed systems, the synergic effect of inner
SIC and of an outer SIC algorithm across slots, typical of coded random access
protocols, allows achieving a very high reliability with a low number of packet
replicas per active user.
