Abstract 802.11ax introduces OFDMA to WiFi. It thus enables multiplexing users/user groups in the frequency domain. WiFi networks usually operate in a multipath environment which generates a frequency selective channel. Hence, the capacity of a user/user group changes over different subcarriers. A good scheduling and resource allocation scheme can maximize user rates by allocating users and user groups on subcarriers based on their CSI and other system considerations. In this paper, we investigate how to optimally assign users and user groups to subcarriers with the goal of maximizing the throughput in the context of 802.11ax. We introduce a novel divide and conquer based algorithm which we prove to be optimal under the assumption that a user can be assigned to more than one resource unit. We also introduce two practical algorithms, a greedy and a recursive one, which split the bandwidth into resource units and schedule users on them while observing all 802.11ax protocol constraints. Extensive simulations and experiments comparing the performance of the aforementioned algorithms establish that our practical schemes achieve very good performance in all studied scenarios while handling a plethora of real-world constraints.