  This article presents a novel enhancement to the random spreading based
coding scheme developed by Pradhan et al.\ for the unsourced multiple access
channel. The original coding scheme features a polar outer code in conjunction
with a successive cancellation list decoder (SCLD) and a hard-input soft-output
MMSE estimator. In contrast, the proposed scheme employs a soft-input
soft-output MMSE estimator for multi-user detection. This is accomplished by
replacing the SCLD based polar code with an LDPC code amenable to belief
propagation decoding. This novel framework is leveraged to successfully pass
pertinent soft information between the MMSE estimator and the outer code. LDPC
codes are carefully designed using density evolution techniques to match the
iterative process. This enhanced architecture exhibits significant performance
improvements and represents the state-of-the-art over a wide range of system
parameters.
