  new generation of Wireless Local Area Networks (WLANs) will make its
appearance in the market in the forthcoming years based on the amendments to
the IEEE 802.11 standards that have recently been approved or are under
development. Examples of the most expected ones are IEEE 802.11aa (Robust Audio
Video Transport Streaming), IEEE 802.11ac (Very-high throughput at < 6 GHz),
IEEE 802.11af (TV White Spaces) and IEEE 802.11ah (Machine-to-Machine
communications) specifications. The aim of this survey is to provide a
comprehensive overview of these novel technical features and the related open
technical challenges that will drive the future WLAN evolution. In contrast to
other IEEE 802.11 surveys, this is a use case oriented study. Specifically, we
first describe the three key scenarios in which next-generation WLANs will have
to operate. We then review the most relevant amendments for each of these use
cases focusing on the additional functionalities and the new technologies they
include, such as multi-user MIMO techniques, groupcast communications, dynamic
channel bonding, spectrum databases and channel sensing, enhanced power saving
mechanisms and efficient small data transmissions. We also discuss the related
work to highlight the key issues that must still be addressed. Finally, we
review emerging trends that can influence the design of future WLANs, with
special focus on software-defined MACs and the internet-working with cellular
systems.
