Multiuser Access via OFDMA Technology in High Density IEEE 802.11ax WLANs: A Survey

IEEE 802.11ax known as HEW (High Efficiency WLANs: Wireless Local Area Networks) is the new standard designed to meet the objectives of the next generation of high density wireless networks, such as: airports, bus stations, stadiums, etc. The key innovative feature introduced in IEEE 802.11ax is the OFDMA (Orthogonal Frequency Division Multiple Access) technology. The basic principle of OFDMA at PHY (physical) layer is dividing a radio channel into smaller sub-channels, and hence enabling multiple and independent transmissions between multiple wireless devices. At MAC (Medium Access Control) layer, the OFDMA technology is thus intended to allow a massive connection of users, while reducing access delay and increasing individual throughput. This is why, several OFDMA MAC protocols, based on different approaches, have been designed and implemented for the purpose of efficiently using the radio resources supplied by the OFDMA technology at PHY layer. In this paper, we aim at: (i) reviewing the main existing OFDMA MAC protocols available in the literature with the goal of highlighting the strengths and weaknesses of each protocol, (ii) proposing a new classification of OFDMA MAC protocols, and (iii) drawing some perspectives for future research.