# A Survey of Wi-Fi 6: Technologies, Advances, and Challenges
## 4 OFDMA in Wi-Fi 6
### Related Work
#### 4.3.2 Centralized Approaches
Then, other stations sense the channel to find free RUs. They resume the backoff counter and try to select one of these resources. The procedure continues until all the stations successfully allocate resources or there are no more RUs. The authors of [63] grouped the stations in a BSS to reduce collision and interference. They clustered the geographically close stations. Each cluster has a leader station that contends to access the channel for the whole cluster. The access point schedules these leaders and subsequently, leaders schedule their member stations based on their available resources. However, it needs to modify the Wi-Fi 6 standard. [PERSON] et al. [64] improved the throughput by making the network capable of sharing a single RU among multiple stations. Virtual timeslots within an RU distinguish the timing for each station's access to the medium. The authors of [65] leveraged OFDMA to enable unidirectional full-duplex transmissions. The access point has self-interference cancellation antennas so that it can both transmit and receive different signals. However, the stations do not have this kind of complex hardware and they are not full duplex. Thus, the access point must identify the hidden terminals. They use two OFDM symbol times for signaling. In the first OFDM symbol time, the access point gathers the uplink transmission demands of the stations. In the second OFDM symbol time, it compares these demands with its downlink traffic and finds the hidden stations accordingly. Then, transmissions can take place, full-duplex for the access point and half-duplex for the receiving stations. [PERSON] et al. [66] adapted EDCA to OFDMA to enable priority access. The access point executes a scheduler that assigns different contention window sizes to each station. More precisely, the EDCA algorithm is executed virtually by the access point to decide the station that will use each RU within a PPDU. Because of the _virtual_ execution, no collisions are actually created; the access point selects a single winner. The station receives an RU if its backoff value is smaller than the available RUs. The RU size that the station allocates is a factor of 26-tone RU. It defines a limit for the number of 26-tone RUs participating in a single transmission using the Min-Max and Z-score normalization functions. [PERSON] et al. [67] proposed a new multi-user OFDMA frame format to maximize transmission efficiency. They proposed to find the optimal length of the multi-user frame, considering the status of the buffer and the bitrate of the stations. They proved that the problem is NP-complete, and decomposed it into subproblems that they solved sequentially. CC-MAC [68] is a centralized contention-based MAC that exploits uplink OFDMA random access. Firstly, the access point sends a Contention Period Announcement (CPA) frame. Then, each active station selects a random slot after the CPA to send an association ID signal. Thus, the access point knows the list of pending frames and announces the winners in a Contention Resolution (CR) frame. If the number of winners is strictly larger than the number of RUs, the access point must select a subset to announce in its CR frame. Finally, the winning stations can start their transmission synchronously. This mechanism is an alternative to BSR and TF concepts.