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