I. Introduction
Wireless uplink non-orthogonal multiple access (NOMA) is able to significantly improve the throughput of a random access system [1]. Using the uplink NOMA scheme, signals from different users are allowed to be superimposed in the power domain and decoded at the base station through successive interference cancellation (SIC). Therefore, the uplink NOMA scheme supports connectivity for a large number of users with sparse activities [1], [2].