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This paper proposes and demonstrates a PHY-layer design of a real-timeprototype that supports Ultra-Reliable Communication (URC) in wirelessinfrastructure networks. The design makes use of Orthogonal Frequency DivisionMultiple Access (OFDMA) as a means to achieve URC. Compared with Time-DivisionMultiple Access (TDMA), OFDMA concentrates the transmit power to a narrowerbandwidth, resulting in higher effective SNR. Compared with Frequency-DivisionMultiple Access (FDMA), OFDMA has higher spectrum efficiency thanks to thesmaller subcarrier spacing. Although OFDMA has been introduced in 802.11ax, thepurpose was to add flexibility in spectrum usage. Our Reliable OFDMA design,referred to as ROFA, is a clean-slate design with a single goal ofultra-reliable packet delivery. ROFA solves a number of key challenges toensure the ultra-reliability: (1) a downlink-coordinated time-synchronizationmechanism to synchronize the uplink transmission of users, with at most $0.1us$timing offset; (2) an "STF-free" packet reception synchronization method thatmakes use of the property of synchronous systems to avoid packet misdetection;and (3) an uplink precoding mechanism to reduce the CFOs between users and theAP to a negligible level. We implemented ROFA on the Universal Software RadioPeripheral (USRP) SDR platform with real-time signal processing. Extensiveexperimental results show that ROFA can achieve ultra-reliable packet delivery($PER<10^5$) with $11.5dB$ less transmit power compared with OFDM-TDMA whenthey use $3$ and $52$ subcarriers respectively.

ROFA: An OFDMA system for Ultra-Reliable Wireless Industrial Networking