200 IEEE COMMUNICATIONS SURVEYS & TUTORIALS, VOL. 21, NO. 1, FIRST QUARTER 2019
the decoding of at least some packets in case of short noise
bursts, at the expense of slightly increased overhead.
Since contention-based channel access inevitably leads to
collisions, from the very beginning IEEE 802.11 tried to add
various contention-free channel access mechanisms to the stan-
dard. Both the “historical” Point coordinated function (PCF,
obsolete now) and the subsequent Hybrid Controlled Channel
Access (HCCA) allow an AP to access the channel without
contention. Channel access coordination is accomplished by
introducing an Interframe Space called PIFS (PCF InterFrame
Space) which, being shorter than the DIFS (Distributed coor-
dination function InterFrame Space) used by the remaining
STAs, permits the AP to acquire the channel access with-
out any contention, so as to transmit data or poll the STAs
and grant them channel access. In practice, contention-free
access techniques have seen a very marginal deployment,
especially because of their inefficiency in scenarios when sev-
eral APs work in the same area. Indeed, if several APs use
PIFS, their transmissions will start simultaneously and col-
lide. This problem is partially addressed in the HCCA TXOP
Negotiation mechanism introduced in 802.11aa. The mecha-
nism allows various APs to use different time intervals for
transmission. Unfortunately, HCCA TXOP Negotiation can
only avoid collisions between APs which can communicate
with each other. Moreover, it does not reduce the collision
probabilities between an AP and the alien STAs, which still
can use random access.
The IEEE 802.11 Working Group has historically put a sig-
nificant effort to improve the Quality of Service (QoS) in
Wi-Fi networks. Specifically, the 802.11e amendment intro-
duces Enhanced Distributed Channel Access (EDCA) and
HCCA which distinguish voice, video, best effort and back-
ground traffic and serve them differently. While EDCA just
assigns different priorities to these types of traffic, the sophis-
ticated HCCA allows an AP to schedule transmissions taking
into account specific QoS requirements, like the delay bound,
the packet loss ratio, or the required bandwidth. However,
determining exact requirements is a non trivial task, and
arguably another key reason behind the scarce deployment of
the contention-free HCCA.
For many devices which use Wi-Fi (e.g., laptops and smart-
phones) power consumption is an important issue. In 802.11
networks, power management is based on alternating between
two states: awake and doze. In the awake state, a STA can
transmit and receive frames, while in the doze state, its radio
is switched off. An active STA is always awake, while a
power-saving (PS) STA alternates between these states. The
AP buffers data destined for PS STAs until the STA wakes
up and retrieves it. Many amendments introduce new power-
saving features, but most of them are related to switching off
the radio for a rather long time, i.e., for hundreds of mil-
liseconds or even for seconds. Some of them require a PS
STA to contend for the channel if it wants to retrieve data
from the AP. Such methods are inefficient in dense environ-
ments because of collisions, huge overhead and large delays.
Some other methods allow an AP and a PS STA to schedule
a series of times when the STA retrieves data from the AP.
The period of the series depends on the QoS requirements.
The tight dependence of these methods with HCCA function-
ality — specifically with the Traffic Specification (TSPEC)
information element which parametrizes QoS requirements —
prevents their usage in consumer electronics.
Finally, the 802.11ac amendment [17]–[19] was introduced
mainly with the purpose of significantly increasing the data
rate of a 10x factor with respect to 802.11n. Besides increas-
ing the number of spatial streams up to 8, 802.11ac addresses
the problem of how to cope with terminals that, for obvious
manufacturing reasons, could not deploy more than 1 or 2
antennas. To this purpose, the 802.11ac first introduces the
DL MU-MIMO, which allows an AP to assign various DL
spatial streams to different STAs — the UL MU transmis-
sion was postponed to subsequent standards owing to the tight
synchronization requirements which would have required a
significant re-design. Additionally, 802.11ac widens the trans-
mission bands up to 160 MHz (also exploting non-contiguous
80+80Mhz channels) and increases the constellation order to
256-QAM, which raises data rates up to 7 Gbps. To reduce
the header-induced overhead at such high data rates, the
amendment increases the maximal length of a frame from
65 535 (802.11n) to 4 692 480 octets. Nevertheless, for
short packets, such as instant messages, Web requests, TCP
acknowledgments, etc. the channel is still used inefficiently.
B. Main Features of 802.11ax
Similarly to the previous amendments that improve the
nominal bit rates, 802.11ax contains a new PHY protocol
with higher modulation and coding schemes. In contrast to
802.11ac, 802.11ax does not increase the number of the
MIMO spatial streams and does not widen the channel. Thus
the nominal data rates are increased up to 9.6 Gbps, which is
just 37% higher than that of 802.11ac (rather small compared
to the 10x growth of 802.11n or 802.11ac!) [20]. The desired
increase of the user throughput is achieved by more efficient
spectrum usage.
The key feature of 802.11ax is the adoption of an OFDMA
approach, an approach widely used in cellular networks, but
brand new in Wi-Fi. The rationale is that the very wide chan-
nels (80 MHz, 80+80 MHz and 160 MHz) introduced by
802.11ac suffer from frequency selective interference, which
significantly impairs the practically achievable rates. With
OFDMA, adjacent subcarriers (tones) are grouped together
into a resource unit (RU) and a sender can choose the best RU
for each particular receiver, which actually results in higher
Signal-to-Interference-plus-Noise Ratio (SINR), Modulation
and Coding Scheme (MCS) and throughput. Moreover, since
the efficiency of high data rates degrades when a STA has
only few data to transmit, advanced aggregation techniques
aimed to reduce channel access, acknowledgment (ACK) and
preamble-induced overhead become useless. Allocating nar-
row RUs for such STAs is an efficient remedy. According to
the latest TGax investigations, OFDMA provides a 6 times
higher throughput than legacy DCF [21], see Fig. 1.
OFDMA makes Wi-Fi radio access closer to the LTE one.
However in contrast to LTE, OFDMA works on top of the
legacy DCF and is coordinated by the AP. It means that having
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