=Paper=
{{Paper
|id=Vol-2589/Paper2
|storemode=property
|title=An OFDMA MAC Protocol Aggregating Variable Length Data in the Next IEEE 802.11ax Standard
|pdfUrl=https://ceur-ws.org/Vol-2589/Paper2.pdf
|volume=Vol-2589
|authors=Mohand Moktfi,Mohand Yazid,Louiza Bouallouche-Medjkoune,Wissam Benlala
|dblpUrl=https://dblp.org/rec/conf/citsc/MoktfiYBB19
}}
==An OFDMA MAC Protocol Aggregating Variable Length Data in the Next IEEE 802.11ax Standard==
https://ceur-ws.org/Vol-2589/Paper2.pdf
Copyright © 2020 for this paper by its authors. Use permitted under Creative Commons License Attribution 4.0 International (CC BY 4.0)
An OFDMA MAC Protocol Aggregating Variable
Length Data in the Next IEEE 802.11ax Standard
Mohand MOKTEFI, Mohand YAZID, Louiza BOUALLOUCHE-MEDJKOUNE and Wissam BENLALA
LaMOS Research Unit, Faculty of Exact Sciences, University of Bejaia, 06000 Bejaia, Algeria
Emails: moktefi.mohand@gmail.com, yazid.mohand@gmail.com, louiza medjkoune@yahoo.fr, wissambenlala@gmail.com
Abstract—A new standard of the IEEE 802.11 standard is higher modulation rate 1024-QAM (1024-Quadrature Ampli-
behind the scenes whose ratification is planned for the end tude Modulation),Down-Link/Up-Link Multi-User Multiple-
of the current year (2019). It is the 802.11ax or HEW (High Input Multiple-Output (DL/UL MU MIMO) and spatial reuse.
Efficiency Wireless local area network) standard dedicated to the
future wireless networks. This standard promised better efficiency The implementation of these techniques poses many new
and throughput in more difficult use cases (dense environments) challenges to the scientists who are working to achieve.
by exploiting pre-existing physical and MAC (Medium Access Because the old Wi-Fi standards do not support these new
Control) capabilities and introducing new concepts, such as: features. In this work, we are interested to efficiently manage
OFDMA (Orthogonal Frequency-Division Multiple Access), 1024-
the OFDMA MAC communications in the next generation of
QAM (1024-Quadrature Amplitude Modulation) modulation or-
der, FD (Full-Duplex) communications and spatial reuse. The IEEE 802.11ax WLANs. In fact, various methods of access
new OFDMA modulation technology divides the transmission to the medium based on OFDMA have been proposed by
channel into sub-carrier groups (known as: RUs for Ressources the scientific community, each aimed at optimizing the use
Units) to which up to 9 users can simultaneously access a 20 of subcarriers and improving the transmission rate in a dense
MHz channel. In order to improve the user data rate in dense environment.
areas, an OFDMA-based MAC access method is proposed in
this paper by enabling the principle of aggregating frames of The remainder of this paper is outlined as follows. Section
variable lengths while ensuring synchronization. Our proposal is II introduces the OFDMA and OFDM technologies. Section
followed by simulation results to demonstrate the improvement III introduces the main research works about multi-user MAC
in throughput that it offers. protocols based on OFDMA technology. In Section IV, we
Keywords—IEEE 802.11ax HEW, OFDMA, Medium Access, describe our OFDMA MAC protocol. Simulation results are
Variable Length Data, Aggregation, Simulation and validation. given in Section VI, followed by Section VII which concludes
this paper.
I. I NTRODUCTION
Wireless technology has become widespread on virtually II. BACKGROUND
all user devices, as well as any inhabited deployment (homes,
parks, airports, stadiums, etc.) since its arrival on the industrial The main change in the 802.11ax standard is the intro-
market. However, users who are increasingly demanding, the duction of OFDMA technology in both downlink and uplink
number of connections and bandwidth intensive applications transmissions. OFDMA makes it possible to multiplex more
are growing. This increase will threaten the technology in its users in the same bandwidth. This is possible by allocating a
future growth to no longer serve these customers effectively. contiguous subset of the sub-carriers (minimum 26, maximum
In addition to increased reliability, future networks will need 996) of the available spectrum for each user. This means
to offer greater wireless capacity. This is where the sixth that the existing 802.11ax channels (20, 40, 80 and 160
generation of Wi-Fi (Wireless-Fidelity) comes in. It’s the IEEE MHz wide) are divided into narrower subchannels with a pre-
802.11ax standard. defined number of subcarriers. The allocated amount is referred
to as the Resource Unit (RU) and it is allocated to users
The new 802.11ax standard, also known as HEW (High based on channel conditions and service requirements. The
Efficiency Wireless local area network), has the ambitious Figure 1 illustrates the allocation of RUs by the AP (Access
goal of quadrupling average throughput per user in dense Point). By using OFDM the entire channel is allocated to a
areas; it is an evolutionary improvement of the 802.11ac single user, however using OFDMA several users can transmit
standard. The standard has submitted three preliminary drafts simultaneously. The basic principle of OFDM and OFDMA is
802.11ax since its launch in May 2014, namely D1.0, D2.0 illustrated by Figure 2.
and D3.0 and provides for finalization by the end of the
current year (2019) [1]. While the new 802.11ax standard 26 26 26 26 26 26 26 26 26
is designed to maximize network efficiency, it also provides
a better experience for traditional wireless LANs and more 52 52 26 52 52
predictable performance for advanced applications, such as: 4K 106 26 106
video, Ultra HD, wireless office, Internet of Things (IoT), etc. 242 tones
IEEE 802.11ax will achieve its goals taking into account the
following key features: Orthogonal Frequency-Division Multi- Fig. 1. Configuration of RUs on a 20 MHz band [2].
ple Access (OFDMA), adoption of Full-Duplex transmissions,
current OFDMA) for the new generation WLAN. The C-
OFDMA method provides improvements to the H-OFDMA
method of the authors [4], [5] in terms of throughput. In fact,
the H-OFDMA method for a transmission of k data results
in a total of k acknowledgments transmitted sequentially to
the k stations, plus an overload in transmission planning. For
that, the authors to [6] have opted for a method allowing
simultaneous transmissions in order to reduce this overload
in data and in acknowledgments. The C-OFDMA method
takes place in three phases: the Sub-channel Request (SR)
phase, the Sub-channel Assignment (SA) phase and the Data-
Transmission (DT) phase.
T. Uwai et al. [7] highlighted on an adaptive Backoff
Fig. 2. Configuration of RUs on a 20 MHz band mechanism for OFDMA random access with a finite service
period in IEEE 802.11ax. Based on OFDMA random access
research in the IEEE 802.11ax standard, backoff parameters
In downlink transmissions, an AP can increase the power should adjust to network conditions. In 802.11ax, the service
of some RUs while allowing weak users to maximize downlink period (SP) defines the operating time of the OFDMA random
bit rates in the Basic Service Set (BSS), by diverting power access and the access point is supposed to initiate random
from powerful user. On the other hand, the uplink OFDMA access. When SP ends, the stations should wait for the next
gains are mainly due to the aggregation of multiple users. Each SP to transmit their packages. In dense environments, this leads
user transmitting on his assigned RU, which contributes to a to a discontinuity, including a probability of packet loss due
higher signal (SNR, signal-to-noise ratio) at the level of the to high latency, and a degradation in performance. For this,
AP. In general, STAs (stations) have lower output power output the authors of [7] introduced an adaptive Backoff algorithm
than APs, and this power asymmetry reduces uplink throughput for OFDMA random access using an analytical performance
and may also limit the BSS range. Uplink OFDMA can be used model. They show that maximum throughput can be achieved
to compensate for such power asymmetry [3]. even in dense environments by adjusting the parameters of
the Backoff. The experiment is carried out on a UL-MU
transmission protocol based on the OFDMA random access
III. R ELATED W ORK and divided into three phases: the transmission request phase
Recently, several OFDMA MAC protocols for the next (TR), the UL-MU frame transmission phase (UL-MU frame
generation Wi-Fi have been proposed. A summary of some Transmission).
research on the IEEE 802.11ax future standard, based on
J. Lee et al. [8] suggested a new hybrid MAC protocol (H-
OFDMA technology, is presented in this section. The research
MAC) designed to increase channel utilization in OFDMA.
studies focused above using different concepts and principles
It is a protocol that relies mainly on a centralized approach
to design OFDMA-based MAC access methods by adopting
that also allows random access in a game. Access to the
the different centralized and/or randomized scheduled access
medium is controlled by three types of messages defined by
controls for multi-user downlink/uplink transmissions.
these authors [8] and which are: (1) Request-to-Multiple-DL
P. Nasiopoulos et al. [4], [5] have proposed a hybrid (RMD), this message is sent on the channel to identify each
MAC protocol based on OFDMA and CSMA/CA (H-OFDMA, candidate station for DL transmission,(2) Clear-to-Receive-
Hybrid OFDMA) to increase the throughput of the next with-UL-Request (CRU), when receiving the RMD message,
WLAN generation. H-OFDMA uses two frame transmission the stations designated in the DL transmissions can respond by
phases: the Transmission Opportunity Request(TR) phase and sending a CRU message in their corresponding sub-channels,
the Scheduled Transmission (ST) phase. The RTS packet is they may also carry information on UL requests for transmis-
sent randomly by applying a CSMA/CA scheme in the TR sions, (3) Request-to-Multiple-UL (RMU), the RMU control
phase. The adoption of OFDMA in the H-MAC method can message is only a trigger for UL transmissions sent by the
cause a conflict between the users who are candidates for access point to the stations having requests for transmissions in
a transmission when the number of sub-channels is smaller UL. As a result, the H-MAC protocol allows a random access
than the number of users. Thus, several stations compete for portion for stations not designated by the RMD message in
each subchannel which produces a probability of collision. the network and the RMD message is sent over the entire
As a result, H-OFDMA prompts the use of CSMA/CA in transmission channel, stations not designated by the latter can
the TR phase to send the RTS packet to solve the collision access randomly to one of the subchannels of the transmission
problem. The access point then sequentially schedules the data channel by decoding the available free sub-channel addresses
transmissions of the stations having sent their RTS packets defined in RMD and transmit their UL requests by sending the
correctly and responds to them by sending the CTS packet CRU message.
in the broadcast. The stations transmit their data sequentially
according to the planning of the AP and each station occupies The operation of the H-MAC protocol is summarized as
all the sub-channels during the transmission of its data. follows (with interframe expectations between transmissions):
the access point sends the RMD message containing the
G. Haile et al. [6] proposed a competing MAC protocol requests for DL transmissions to the designated stations over
based on OFDMA and CSMA/CA named C-OFDMA (Con- the entire transmission channel, the stations designated by
Largest frame
HIFS
RMD and the stations having requests UL (random access) SIFS SIFS SIFS SIFS SIFS SIFS
Sub-
send the CRU message to the access point in a first phase channel CRU STA A
STA C - >
UL DTA
named Control Period, the access point transmits its data CRU STA B
DL MPDU
ACK
-> STA A STA D ->
packets to the stations designated by RMD and who responded CRU STA C
RMU UL DATA
by CRU and the stations acknowledge the good reception of RMD STA C
20 Mhz CRU STA D STA E ->
DL data by sending ACKs each, during a phase qualified in STA A STA D UL DATA
DL A-MPDU BLOCK
H-MAC by Downlink Data Transmission Period. The access STA B -> STA D
BA
STA E ACK
point then loads the UL requests through CRU, stacks them STA C CRU STA E STA F STA F ->
UL DATA
in the RMU message and transmits them to the designated CRU STA F DL M-PDU ->STA C
STA G
stations, all over the channel during the trigger phase (Trigger BA STA G ->
Period) so that finally the stations having UL requests trigger CRU STA G
UL DATA
the transmission of their UL data and the AP responds with a
BA over the entire channel to acknowledge the UL data in the Random Access STAs’ CRU Time
last phase Uplink Data Transmission Period. Control Period DL aggregated-Data Transmission Period
Trigger
Period
Uplink Data
Transmission Period
Padding bits
The following table summarizes the main features of the
studied protocols:
Fig. 3. Operation steps of the A-VLD-MAC protocol.
TABLE I. C HARACTERISTICS OF THE STUDIED PROTOCOLS .
Articles CTS/RTS DL UL MU-OFDMA Centralised Distributed
to the different stations are all of the same length
[9] X X X X
[4], [5] X X X X induces us to H-MAC. Thus no aggregation can be
[6] X X X X X done.
[7] X X X X
[10] X X X X • Case 2: ∃i 6= M AX, M AX − LON G1i > 0 for
[8] X X X X X X all stations there is at least one packet of length less
than M AX. The occurrence of this case allows us to
introduce the aggregation mechanism. To do this, the
IV. AGGREGATED VARIABLE L ENGTH DATA FOR
AP is looking at this point in the rest of the data of
OFDMA COMMUNICATIONS
each queue, the longest packet can be inserted into
It is found that the adoption of H-MAC under variable X, where X is the difference between M AX and
length data is inefficient at all, given the time lost to transmit LON G1i (see the equation eqref eq1), within the
stuffing bits instead of raw data. However, an improvement limit of not exceeding the transmission time of the
in the efficiency of the H-MAC method is demonstrated longest frame M AX (see the equation (2)), where
by adopting our proposal called Aggregated-Variable Length tsM AX is the transmission time of the data M AX,
DATA-MAC (A-VLD-MAC). The illustration of the A-VLD- tsLON G1i is the transmission time of the first packet
MAC method is detailed in the next subsection. LON G1i and tsLON G2i is the transmission time of
the second packet LON G2i that could be inserted into
A. A-VLD-MAC X for the station i.
The proposed A-VLD-MAC multiple access method makes M AX − LON G1i = X. (1)
changes in the DL data transmission phase, with the aim of
tsM AX ≥ tsLON G1i + tsLON G2i . (2)
improving the transmission rate by exploiting the OFDMA
RUs effectively. The improvement in fact consists of intro- • The AP encapsulates the MPDU packets, aggregated
ducing the aggregation mechanism of the data frames to the A-MPDU packets if they exist, and forwards the
H-MAC method, by considering the variable length of data. packets to the specified stations.
The following diagram shows the different frames sent in A-
VLD-MAC. • The stations having received the frames sent by the
AP, answer by ACK or BA for the MPDU and A-
The sending of MPDU and A-MPDU frames is done by MPDU respectively.
management in the access point. This management is detailed
as follows: The remainder of the H-MAC method is unchanged for the
remaining period (control period, trigger period, and UL data
• After receiving the CRU frames, the access point loads transmission period). The different conditions for the sequence
the first data to be sent in each queue for each station of the phases are exposed in the flowchart given in the Figure
and searches for the data having the maximum length. 4.
• The AP verifies if the data to be transmitted for the
different stations are not all of the same length, one V. S IMULATION R ESULTS AND A NALYSIS
of the following two cases will be applied:
We have used the C programming language under Linux
• Case 1: ∀i 6= M AX, M AX − LON G1i = 0 where operating system for implementing and assessing the perfor-
M AX is the maximum length and LON G1i is the mance of the proposed A-VLD-MAC method designed for
length of the packet to send to the station i. Falling in optimizing the OFDMA MAC communications under variable
the case where the different packets to be transmitted length data. The choice of programming language is made in
packets in both methods, a simulation is implemented. The
Wit HIFS and start
A-VLD-MAC results of this simulation are given in Figure 5. According
H- MAC A-VLD-MAC
Has data Tx RMD
to send? for UL 500
450
Tx RMD 400
with Rx CRU?
DL STA info. 350
300
Mbits/S
Rx Tx MRU Rx UL
CRU? with UL data & 250
STA info. Tx ACK
200
DL 150
STA’s
CRU? 100
50
LONGi = MAX ? 0
1 2 3 4 5 6 7 8 9
Stations
Tx DL DATA & CRU with
LONG2i < X
Rx ACK UL Info?
Fig. 5. Average throughput.
TsMAX ≥ tsLONG1i+ tsLONG2i ?
to the results of the simulation, the A-VLD-MAC method
considerably improves the average throughput compared to the
H-MAC method. The average bit rate can reach 453 Mbps in
Tx MPDU +A-MPDU & A-VLD-MAC compared to 319 Mbps in H-MAC, a difference
Rx ACK + BA Resp
of 134 Mbps. These results show the effectiveness of the A-
VLD-MAC method in dense networks.
Fig. 4. Flowchart of the transitions of A-VLD-MAC.
B. Average throughput per user
Responding to user requirements while providing better
relation to the simplicity, flexibility and speed of the language. service in high density areas is one of the goals of the new
We have used a simulation environment with a 20 MHz trans- IEEE 802.11ax standard, and providing better throughput is
mission channel, supporting 9 simultaneous users in OFDMA one of the important services. The Figure 6 represents the
(RU = 26 subcarriers). The PHY and MAC parameters that results of the simulation of the average flows per user in each
we have put forward to evaluate the performance are defined of the two H-MAC and A-VLD-MAC methods.
in the Table II.
TABLE II. IEEE 802.11 AX PHY AND MAC PARAMETERS . H-MAC A-VLD-MAC
70
Parameters Signification Value
Channel bandwidth bandwidth width 20 MHz 60
HIFS inter-frame time H-MAC 25 s
SIFS inter-frame time 16 s 50
PHY header PHY header transmission time 36 s
MAC header length of the MAC header 320 Bits 40
Mbits/S
Data rate Data rate transmission 65 Mbps
Basic rate Data rate of the overhead 6 Mbps 30
Del Pad Delimiter size and padding of A-MPDU packets 56 Bits
ACK ACK size 112 Bits 20
BA size of ACK block 320 Bits
10
The performance metrics that we have computed to eval- 0
1 2 3 4 5 6 7 8 9
uate the performance of our A-VLD-MAC proposal are: the Stations
average throughput, the average throughput per user, and the
bit loss rate of the DL transmissions. Given the proposed
Fig. 6. Average throughput per user.
method A-VLD-MAC is an improvement of the H-MAC
method with variable packet lengths, the simulation results
obtained for the A-VLD-MAC method are compared to those We can see clearly in the figure 6 above, that the average
of the H-MAC. throughput per user in the A-VLD-MAC method is consider-
ably improved compared to the results of the H-MAC method.
A. Average throughput Apart from the average throughput per user offered for a single
station that is equal in both methods (specific case A-VLD-
The average rate determines the speed of data transmission. MAC), the average throughput per maximum user achieved and
In order to evaluate the rate of DL transmissions in the network which is almost stable by increasing the number of stations in
by varying the number of stations and the lengths of the A-VLD-MAC is 56 Mbps against 50 Mbps for H-MAC and
can be downgraded to 33 Mbps by increasing the number of R EFERENCES
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We can see in Figure 7 that the loss rate varies according
to the length of the packets and the number of stations in the
H-MAC method, it can reach a rate of '44% bit loss stuffing.
On the other hand, the maximum loss rate in stuffing bits in A-
VLD-MAC does not exceed ' 13 %, so a gain of '31%(44%-
13%=31% ) in data is offered using A-VLD-MAC. We also
note, the stability of the loss rates in A-VLD-MAC, despite the
variation in number of stations and lengths of the data, which
guarantees the efficiency of A-VLD-MAC in the reduction of
loss rates in stuffing bits.
VI. C ONCLUSION
The efficient use of transmission channel in the IEEE
802.11ax standard allows for increased throughput. The pur-
pose of the proposed A-VLD-MAC method is to optimize
the use of OFDMA transmissions by improving the H-MAC
method while introducing the data frame aggregation mech-
anism defined in the IEEE 802.11e standard. In this paper,
we have detailed the operation steps of the proposed method
A-VLD-MAC which is an improvement of the H-MAC [8]
method, while underlining the differences between the two
methods. A simulation part is also implemented in this work
to demonstrate the effectiveness of the proposed solution.
Obviously, the results given by the simulation of the two
methods assert that the proposed method A-VLD-MAC is more
efficient than H-MAC by increasing the transmission rate and
reducing the rate of loss in stuffing bits.