  This paper presents the first network-coded multiple access (NCMA) system
prototype operated on high-order modulations up to 16-QAM. NCMA jointly
exploits physical-layer network coding (PNC) and multiuser decoding (MUD) to
boost throughput of multipacket reception systems. Direct generalization of the
existing NCMA decoding algorithm, originally designed for BPSK, to high-order
modulations, will lead to huge performance degradation. The throughput
degradation is caused by the relative phase offset between received signals
from different nodes. To circumvent the phase offset problem, this paper
investigates an NCMA system with multiple receive antennas at the access point
(AP), referred to as MIMO-NCMA. We put forth a low-complexity symbol-level NCMA
decoder that, together with MIMO, can substantially alleviate the performance
degradation induced by relative phase offset. To demonstrate the feasibility
and advantage of MIMO-NCMA for high-order modulations, we implemented our
designs on software-defined radio. Our experimental results show that the
throughput of QPSK MIMO-NCMA is double that of both BPSK NCMA and QPSK MUD at
SNR=10dB. For higher SNRs at which 16-QAM can be supported, the throughput of
MIMO-NCMA can be as high as 3.5 times that of BPSK NCMA. Overall, this paper
provides an implementable framework for high-order modulated NCMA.
