  The aim of this paper is to characterize the impact of non-orthogonal
multiple access (NOMA) on the age of information (AoI) of grant-free
transmission. In particular, a low-complexity form of NOMA, termed
NOMA-assisted random access, is applied to grant-free transmission in order to
illustrate the two benefits of NOMA for AoI reduction, namely increasing
channel access and reducing user collisions. Closed-form analytical expressions
for the AoI achieved by NOMA assisted grant-free transmission are obtained, and
asymptotic studies are carried out to demonstrate that the use of the simplest
form of NOMA is already sufficient to reduce the AoI of orthogonal multiple
access (OMA) by more than 40%. In addition, the developed analytical
expressions are also shown to be useful for optimizing the users' transmission
attempt probabilities, which are key parameters for grant-free transmission.
