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.

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In this survey we attempt to describe the Quality of Service (QoS) mechanisms employed by Medium Access Control (MAC) protocols designed for ad-hoc networks. We begin with background information: an overview of the related work, the definition of QoS and QoS-related metrics, a general description of contention-free and contention-based protocols for wireless networks, a discussion of issues affecting QoS provisioning in ad-hoc networks, as well as a novel classification of the QoS mechanisms. Then, each mechanism is briefly explained and implementation examples from different protocols are provided.Furthermore, a separate section is devoted to the completed and ongoing standardization work in the field. Afterwards, an extensive comparison of salient features, advantages and disadvantages of all described MAC mechanisms is given in order to guide future protocol designers. Finally, we comment on the most probable future research directions. Based on the presented survey, we observe that QoS provisioning is not only challenging but also a significant contemporary research problem. The protocol designs presented in the literature usually involve trade-offs between certain metrics, and currently there is no ideal solution which deals with all the issues affecting ad-hoc networks. Therefore, we trust that this survey will be of great help to designers of future QoS-aware protocols.

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