With its recent developments, Wi-Fi is becoming one of the key technologies to provide flexible yet deterministic communication in Industrial IoT Scenarios. IEEE 802.11ax employs a paradigm shift from a distributed to a centralized coordination strategy with its new channel access mechanism called Orthogonal Frequency Division Multiple Access (OFDMA). Using OFDMA, the Access Point (AP) always has to win contention against its associated Stations (STA) before it can initiate a trigger-based transmission. However, contention-based, distributed channel access mechanisms are still supported in IEEE 802.11ax networks, interfering with any hard real-time communication. Multi-user Enhanced Distributed Channel Access (MU EDCA), introduced in IEEE 802.11ax, was designed to influence when to use the distributed or the centralized coordination strategy. It allows to shift the probability of STAs accessing the channel towards the AP, such that the AP can initiate an OFDMA-based transmission. OFDMA is a promising starting point for real-time transmissions in Wi-Fi. We propose a modification of MU EDCA that guarantees worst-case channel access times for the AP in networks which only consist of Wi-Fi 6 capable devices (e.g., in the 6-GHz band). Simulations result in a worst-case channel access delay for the AP of 55 mu s independent of the number of STAs.
