  An effective way to realize non-orthogonal multiple access (NOMA) assisted
grant-free transmission is to first create multiple receive signal-to-noise
ratio (SNR) levels and then serve multiple grant-free users by employing these
SNR levels as bandwidth resources. These SNR levels need to be pre-configured
prior to the grant-free transmission and have great impact on the performance
of grant-free networks. The aim of this letter is to illustrate different
designs for configuring the SNR levels and investigate their impact on the
performance of grant-free transmission, where age-of-information is used as the
performance metric. The presented analytical and simulation results demonstrate
the performance gain achieved by NOMA over orthogonal multiple access, and also
reveal the relative merits of the considered designs for pre-configured SNR
levels.
