# A Survey of Wi-Fi 6: Technologies, Advances, and Challenges
## 4 OFDMA in Wi-Fi 6
### Related Work
#### 4.3.7 Performance Evaluation  
Most of the Wi-Fi 6 performance evaluations have focused on throughput. The authors of [94] and [95] presented an analytical model to evaluate both the downlink and uplink throughputs. They considered single-user and multi-user scenarios with UDP-like traffic and different network sizes. In particular, they proved the throughput superiority of this standard compared to Wi-Fi 5. Wi-Fi 6 also provides a shorter delay when the number of stations increases. [PERSON] et al. [96] analytically evaluated UORA performance using a bidimensional Markov chain model. This model considered the number of contending users and the transmission probability for throughput evaluation. Similarly, [PERSON] et al. [97] proposed an analytical model based on a Markov chain to improve the throughput for saturated flows. The model was based on the backoff counter value and the number of available RUs. It then calculated the transmission and collision probability. [PERSON] et al. [98] focused on the AP-initiated OFDMA and MU MIMO features. In particular, the different A-MPDU must be aligned when several stations transmit in parallel through different sub-channels. Matlab simulations validated this analytical model. Network simulations have also been conducted to measure the performance of Wi-Fi 6. The authors of [99] validated OFDMA performance using NS-3 simulations. [PERSON] et al. [100] analyzed the performance of uplink multi-user OFDMA. They introduced the BSR delivery rate metric to measure a network's performance in dense deployments. They concluded that the throughput depends on the distribution of random access and scheduled access. Additionally, a tradeoff exists between this metric and the throughput as they have an inverse relationship. [PERSON] et al. [101] focused on concurrent transmissions and per-user throughput in OPNET. They indicated that Wi-Fi 6 users outperform legacy Wi-Fi by four times in throughput mainly because of less collisions even with a large number of transmissions in parallel. The authors in [102] developed a lightweight simulator to evaluate uplink/downlink OFDMA performance. They concluded that increasing the number of scheduled access RUs improves the throughput, whereas increasing the number of random access RUs degrades the throughput. In addition, the OFDMA contention window has to be carefully adjusted to bound the number of contending stations to reduce collisions. [PERSON] et al. [103] developed an analog/digital mixed large-scale integration (LSI). They used an evaluation board to validate their approach and to measure the throughput achieved by Wi-Fi 6. In particular, the novel standard offers a 2.1 times higher throughput than Wi-Fi 5. Latency is another essential criterion for time-critical applications. The authors of [104] considered the same assumptions as [96; 97] but quantified the average access delay, which is particularly important for real-time networks. The authors in [105] investigated Wi-Fi 6 performance with and without multi-user EDCA channel access mode in a dense network. A low multi-user EDCA timer can significantly improve the throughput and reduce the real-time application latency compared to legacy EDCA. Furthermore, an upper bound on the network size is obtained for different multi-user EDCA periods and MCSs to achieve optimal performance. [PERSON] et al. [106] evaluated downlink OFDMA in a testbed and concluded that downlink OFDMA does not considerably improve latency. Its performance depends on the access point's proper scheduling. [PERSON] et al. [107] implemented OFDMA in NS-3 to evaluate the improvements compared to legacy Wi-Fi. They observed that OFDMA reduces the latency, has more overheads in acknowledgment transmission, minimizes the padding necessity, and extends range coverage. The authors of [108] investigated the coexistence of Dedicated Short Range Communications (DSRC) as a vehicular communication protocol in IEEE 802.11p and uplink/downlink transmissions in Wi-Fi 6.