  This paper presents the first Network-Coded Multiple Access (NCMA) system
with multiple users adopting different signal modulations, referred to as
rate-diverse NCMA. A distinguishing feature of NCMA is the joint use of
physical-layer network coding (PNC) and multiuser decoding (MUD) to boost
throughput of multipacket reception systems. In previous NCMA systems, users
adopt the same modulation regardless of their individual channel conditions.
This leads to suboptimal throughput for many practical scenarios, especially
when different users have widely varying channel conditions. A rate-diverse
NCMA system allows different users to use modulations that are commensurate
with their channel conditions. A key challenge is the design of the PNC mapping
and decoding mechanisms in NCMA when different users adopt different
modulations. While there have been past work on non-channel-coded rate-diverse
PNC, this paper is the first attempt to design channel-coded rate-diverse PNC
to ensure the reliability of the overall NCMA system. Specifically, we put
forth a symbol-splitting channel coding and modulation design so that PNC/NCMA
can work over different modulations. We implemented our rate-diverse NCMA
system on software-defined radios. Experimental results show that the
throughput of rate-diverse NCMA can outperform the state-of-the-art
rate-homogeneous NCMA by 80%. Overall, the introduction of rate diversity
significantly boosts the NCMA system throughput in practical scenarios.
