  Sparse regression codes with approximate message passing (AMP) decoding have
gained much attention in recent times. The concepts underlying this coding
scheme extend to unsourced access with coded compressed sensing (CCS), as first
pointed out by Fengler, Jung, and Caire. More specifically, their approach uses
a concatenated coding framework with an inner AMP decoder followed by an outer
tree decoder. In the original implementation, these two components work
independently of each other, with the tree decoder acting on the static output
of the AMP decoder. This article introduces a novel framework where the inner
AMP decoder and the outer tree decoder operate in tandem, dynamically passing
information back and forth to take full advantage of the underlying CCS
structure. The enhanced architecture exhibits significant performance benefit
over a range of system parameters. Simulation results are provided to
demonstrate the performance benefit offered by the proposed access scheme over
existing schemes in the literature.
