Network-Coded Multiple Access with Higher-Order Modulations

This paper presents the first network-coded multiple access (NCMA) system operated on higher- order modulations beyond BPSK. NCMA allows multiple nodes to transmit simultaneously to an access point (AP) to boost throughput of wireless local area networks (WLAN): the key idea is to jointly exploit multiuser decoding (MUD) and physical-layer network coding (PNC). High- order modulations are commonly adopted in WLAN systems when the signal-to-noise ratio (SNR) is medium or high. However, direct generalization of the existing NCMA decoding algorithm, originally designed for BPSK, to higher-order modulations will lead to huge performance degradation. We find that the throughput degradation is caused by the relative phase offset between received signals from different nodes. To circumvent the throughput degradation, this paper investigates an NCMA system with multiple receive antennas at the AP, referred to as MIMO-NCMA. We have implemented MIMO-NCMA on software-defined radios. Our experimental results show that, at SNR of 10dB, the throughput of MIMO-NCMA outperforms single-antenna NCMA and conventional distributed MIMO-MUD, respectively. We believe that MIMO-NCMA throughput can be further improved with modulations beyond QPSK (e.g., 64-QAM).