AP-initiated multi-user transmissions in IEEE 802.11ax WLANs

Abstract

Next-generation IEEE 802.11ax Wireless Local Area Networks (WLANs) will make extensive use of multi-user communications in both downlink (DL) and uplink (UL) directions to achieve high and efficient spectrum utilization in scenarios with many user stations per access point. It will become possible with the support of multiuser (MU) multiple input, multiple output (MIMO) and orthogonal frequency division multiple access (OFDMA) transmissions. In this paper, we first overview the novel characteristics introduced by IEEE 802.11ax to implement AP-initiated OFDMA and multiuser MIMO (MU-MIMO) transmissions in both DL and UL directions. Namely, we describe the changes made at the physical layer and at the medium access control layer to support OFDMA, the use of trigger frames to schedule uplink multi-user transmissions, and the new multiuser RTS/CTS mechanism to protect large multi-user transmissions from collisions. Then, in order to study the achievable throughput of an IEEE 802.11ax WLAN, we use both mathematical analysis and simulations to numerically quantify the gains of MU transmissions and the impact of IEEE 802.11ax overheads on the WLAN saturation throughput. Results show the advantages of using MU transmissions in scenarios with many user stations. Additionally, we provide novel insights on the conditions in which IEEE 802.11ax WLANs are able to maximize their performance, such as the need to provide strict prioritization to AP-initiated MU transmissions to avoid collisions with transmissions from user stations.

Target Wake Time: Scheduled Access in IEEE 802.11ax WLANs

“…In summary, multiuser transmissions in IEEE 802.11ax, in both the downlink and the uplink, can be performed through OFDMA, MU-MIMO, or a combination of them. A detailed description of IEEE 802.11ax MU operation can be found in [6].…”

“…Packet aggregation is enabled, with a maximum Aggregate MAC Protocol Data Unit (AMPDU) size of 64 packets. The simulator used to obtain the results is written in C++ extending the work done in [6]. Figure 4 shows the average packet delay, average buffer occupancy, and the fraction of time the channel is idle, when either DCF or TWT is used for different traffic loads.…”

“…By assigning multiple stations to the same TWT session and awakening them at the same time, efficient scheduling of MU transmissions can be performed, using either MIMO or OFDMA capabilities, or both, while at the same time reducing the chances of collisions between the AP and stations, and so its negative effects [6]. Moreover, the periodic scheduling of MU transmissions may help to guarantee enough buffered traffic at the AP and at the stations to further exploit the use of packet aggregation.…”

Design of an Efficient OFDMA-Based Multi-User Key Generation Protocol

“…Similarly to IEEE 802.11ac, IEEE 802.11ax DL multi-user MIMO uses explicit feedback for feeding back the CSI. The transmit beamformer sends a sounding frame/null data packet (NDP), and the receive beamformee exploits it for estimating the CSI, which is then quantized and fed back to the beamformer [33]. However, it has been experimentally demonstrated that even when the CSI feedback is encrypted, attackers may still be able to infer the CSI [34].…”

Interference-Aware Adaptive Beam Alignment for Hyper-Dense IEEE 802.11ax Internet-of-Things Networks

“…In this section, previous research results and primary features of the 802.11ax standard are discussed. The research results in [6] demonstrated a downlink multi-user MIMO (DL MU-MIMO) along with new features of IEEE 802.11ax, i.e., uplink MU-MIMO (UL MU-MIMO) transmission in AP-initiated scenarios. In [6], the differences between legacy IEEE 802.11 wireless local area networks (WLANs) and IEEE 802.11ax were introduced.…”