NOMA-Assisted Grant-Free Transmission: How to Design Pre-Configured SNR Levels?

The key idea of non-orthogonal multiple access (NOMA) assisted grant-free transmission is to create pre-configured receive signal-to-noise ratio (SNR) levels that can be used to serve users in a way similar to conventional bandwidth resources, such as time slots and subcarriers. In the literature, there exist two different SNR-level designs, and the aim of this letter is to investigate their impact on the performance of grant-free transmission, where age-of-information is adopted as the performance metric as it captures not only failures at the physical layer, such as outages and bit errors, but also errors caused by user collisions. The presented analytical and simulation results illustrate 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.