Edgewise Serial Message Passing Detection of Uplink SCMA Systems for Better User Fairness and Faster Convergence Rate

Sparse code multiple access (SCMA) is a promising non-orthogonal multiple access (NOMA) scheme proposed for the upcoming 5G system. Multiuser detection using the message passing algorithm (MPA) with parallel and serial schedules has been considered for SCMA systems. Although serial schedules provide faster convergence rates compared to the parallel schedule, the existing user-wise serial MPA detectors are found to have a user unfairness phenomenon in terms of BER performance, especially at small iteration numbers. To address this issue and to further accelerate the convergence rate, this letter proposes a class of edgewise serial schedules (ESSs). Simulations results demonstrate that optimized ESSs can not only improve the user fairness significantly but also achieve faster convergence rate compared to user-wise serial schedules.