Latency-Sensitive Networked Control Using 802.11ax OFDMA Triggering

Orthogonal Frequency-Division Multiple Access (OFDMA) is a feature implemented in IEEE 802.11ax (WiFi 6) that aims to improve performance by sending packets simultaneously to multiple users. This paper highlights performance issues when using OFDMA with a real-time control application based on the commonly used transmission control protocol (TCP). OFDMA is designed to send data to multiple users simultaneously with reduced latency by grouping many subcarrier frequencies into resource unit (RU) blocks for each user. However, we found that when using a TCP data stream subject to a variable update rate from late packets, the OFDMA algorithm under test was not sufficient to improve performance, and instead increased latency. This surprising finding may pose a concern for industrial applications that rely on TCP transport. Experiments were performed to compare the latency and physical performance results of OFDMA enabled, OFDMA disabled, and our previous works which utilized wireless time-sensitive networking (TSN) features implemented in software. The previous wireless TSN results demonstrated close to deterministic latencies for a TCP link subject to inter-network interfering traffic.