-A new standard of the IEEE 802.11 standard is behind the scenes whose ratification is planned for the end of the current year (2019). It is the 802.11ax or HEW (High Efficiency Wireless local area network) standard dedicated to the future wireless networks. This standard promised better efficiency and throughput in more difficult use cases (dense environments) by exploiting pre-existing physical and MAC (Medium Access Control) capabilities and introducing new concepts, such as: OFDMA (Orthogonal Frequency-Division Multiple Access), 1024QAM (1024-Quadrature Amplitude Modulation) modulation order, FD (Full-Duplex) communications and spatial reuse. The new OFDMA modulation technology divides the transmission channel into sub-carrier groups (known as: RUs for Ressources Units) to which up to 9 users can simultaneously access a 20 MHz channel. In order to improve the user data rate in dense areas, an OFDMA-based MAC access method is proposed in this paper by enabling the principle of aggregating frames of variable lengths while ensuring synchronization. Our proposal is followed by simulation results to demonstrate the improvement in throughput that it offers.
Keywords—IEEE 802.11ax HEW, OFDMA, Medium Access, Variable Length Data, Aggregation, Simulation and validation.
Wireless technology has become widespread on virtually all user devices, as well as any inhabited deployment (homes, parks, airports, stadiums, etc.) since its arrival on the industrial market. However, users who are increasingly demanding, the number of connections and bandwidth intensive applications are growing. This increase will threaten the technology in its future growth to no longer serve these customers effectively. In addition to increased reliability, future networks will need to offer greater wireless capacity. This is where the sixth generation of Wi-Fi (Wireless-Fidelity) comes in. It’s the IEEE 802.11ax standard.
The new 802.11ax standard, also known as HEW (High
Efficiency Wireless local area network), has the ambitious
goal of quadrupling average throughput per user in dense
areas; it is an evolutionary improvement of the 802.11ac
standard. The standard has submitted three preliminary drafts
802.11ax since its launch in May 2014, namely D1.0, D2.0
and D3.0 and provides for finalization by the end of the
current year (2019) [
The implementation of these techniques poses many new challenges to the scientists who are working to achieve. Because the old Wi-Fi standards do not support these new features. In this work, we are interested to efficiently manage the OFDMA MAC communications in the next generation of IEEE 802.11ax WLANs. In fact, various methods of access to the medium based on OFDMA have been proposed by the scientific community, each aimed at optimizing the use of subcarriers and improving the transmission rate in a dense environment.
The remainder of this paper is outlined as follows. Section II introduces the OFDMA and OFDM technologies. Section III introduces the main research works about multi-user MAC protocols based on OFDMA technology. In Section IV, we describe our OFDMA MAC protocol. Simulation results are given in Section VI, followed by Section VII which concludes this paper.
II.
The main change in the 802.11ax standard is the introduction of OFDMA technology in both downlink and uplink transmissions. OFDMA makes it possible to multiplex more users in the same bandwidth. This is possible by allocating a contiguous subset of the sub-carriers (minimum 26, maximum 996) of the available spectrum for each user. This means that the existing 802.11ax channels (20, 40, 80 and 160 MHz wide) are divided into narrower subchannels with a predefined number of subcarriers. The allocated amount is referred to as the Resource Unit (RU) and it is allocated to users based on channel conditions and service requirements. The Figure 1 illustrates the allocation of RUs by the AP (Access Point). By using OFDM the entire channel is allocated to a single user, however using OFDMA several users can transmit simultaneously. The basic principle of OFDM and OFDMA is illustrated by Figure 2.
26 26 26
26 52
52 26 26 242 tones 26 52 26 106 26
26
52
Fig. 1. Configuration of RUs on a 20 MHz band [
Fig. 2. Configuration of RUs on a 20 MHz band
In downlink transmissions, an AP can increase the power
of some RUs while allowing weak users to maximize downlink
bit rates in the Basic Service Set (BSS), by diverting power
from powerful user. On the other hand, the uplink OFDMA
gains are mainly due to the aggregation of multiple users. Each
user transmitting on his assigned RU, which contributes to a
higher signal (SNR, signal-to-noise ratio) at the level of the
AP. In general, STAs (stations) have lower output power output
than APs, and this power asymmetry reduces uplink throughput
and may also limit the BSS range. Uplink OFDMA can be used
to compensate for such power asymmetry [
III.
Recently, several OFDMA MAC protocols for the next generation Wi-Fi have been proposed. A summary of some research on the IEEE 802.11ax future standard, based on OFDMA technology, is presented in this section. The research studies focused above using different concepts and principles to design OFDMA-based MAC access methods by adopting the different centralized and/or randomized scheduled access controls for multi-user downlink/uplink transmissions.
P. Nasiopoulos et al. [
G. Haile et al. [
T. Uwai et al. [
J. Lee et al. [
The operation of the H-MAC protocol is summarized as follows (with interframe expectations between transmissions): the access point sends the RMD message containing the requests for DL transmissions to the designated stations over the entire transmission channel, the stations designated by
HIFS Subchannel
RMD 20Mhz STA A
STA B STA C
Largest frame DL MPDU -> STA A CRU STA A CRU STA B CRU STA C CRU STA D CRU STA E CRU STA G
DL A-MPDU -> STA D CRU STA F
DL M-PDU ->STA C
ACK BA BA
RMU STA C STA D STA E STA F STA G
STA C - > UL DTA STA D -> UL DATA STA E -> UL DATA STA F -> UL DATA STA G -> UL DATA
BLOCK ACK
Time RMD and the stations having requests UL (random access) send the CRU message to the access point in a first phase named Control Period, the access point transmits its data packets to the stations designated by RMD and who responded by CRU and the stations acknowledge the good reception of DL data by sending ACKs each, during a phase qualified in H-MAC by Downlink Data Transmission Period. The access point then loads the UL requests through CRU, stacks them in the RMU message and transmits them to the designated stations, all over the channel during the trigger phase (Trigger Period) so that finally the stations having UL requests trigger the transmission of their UL data and the AP responds with a BA over the entire channel to acknowledge the UL data in the last phase Uplink Data Transmission Period.
The following table summarizes the main features of the studied protocols:
It is found that the adoption of H-MAC under variable length data is inefficient at all, given the time lost to transmit stuffing bits instead of raw data. However, an improvement in the efficiency of the H-MAC method is demonstrated by adopting our proposal called Aggregated-Variable Length DATA-MAC (A-VLD-MAC). The illustration of the A-VLDMAC method is detailed in the next subsection.
A. A-VLD-MAC
The proposed A-VLD-MAC multiple access method makes changes in the DL data transmission phase, with the aim of improving the transmission rate by exploiting the OFDMA RUs effectively. The improvement in fact consists of introducing the aggregation mechanism of the data frames to the H-MAC method, by considering the variable length of data. The following diagram shows the different frames sent in AVLD-MAC.
The sending of MPDU and A-MPDU frames is done by management in the access point. This management is detailed as follows:
After receiving the CRU frames, the access point loads the first data to be sent in each queue for each station and searches for the data having the maximum length. The AP verifies if the data to be transmitted for the different stations are not all of the same length, one of the following two cases will be applied: Case 1: 8i 6= M AX; M AX LON G1i = 0 where M AX is the maximum length and LON G1i is the length of the packet to send to the station i. Falling in the case where the different packets to be transmitted Control Period
Random Access STAs’ CRU
DL aggregated-Data Transmission Period to the different stations are all of the same length induces us to H-MAC. Thus no aggregation can be done.
Case 2: 9i 6= M AX; M AX LON G1i > 0 for 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 AP is looking at this point in the rest of the data of each queue, the longest packet can be inserted into X, where X is the difference between M AX and LON G1i (see the equation eqref eq1), within the limit of not exceeding the transmission time of the longest frame M AX (see the equation (2)), where tsMAX is the transmission time of the data M AX, tsLONG1i is the transmission time of the first packet LON G1i and tsLONG2i is the transmission time of the second packet LON G2i that could be inserted into X for the station i.
M AX
LON G1i = X: tsMAX tsLONG1i + tsLONG2i : (1) (2) The AP encapsulates the MPDU packets, aggregated A-MPDU packets if they exist, and forwards the packets to the specified stations.
The stations having received the frames sent by the AP, answer by ACK or BA for the MPDU and AMPDU respectively.
The remainder of the H-MAC method is unchanged for the remaining period (control period, trigger period, and UL data transmission period). The different conditions for the sequence of the phases are exposed in the flowchart given in the Figure 4.
V. SIMULATION RESULTS AND ANALYSIS
We have used the C programming language under Linux operating system for implementing and assessing the performance of the proposed A-VLD-MAC method designed for optimizing the OFDMA MAC communications under variable length data. The choice of programming language is made in Wit HIFS and start
A-VLD-MAC relation to the simplicity, flexibility and speed of the language. We have used a simulation environment with a 20 MHz transmission channel, supporting 9 simultaneous users in OFDMA (RU = 26 subcarriers). The PHY and MAC parameters that we have put forward to evaluate the performance are defined in the Table II.
The performance metrics that we have computed to evaluate the performance of our A-VLD-MAC proposal are: the average throughput, the average throughput per user, and the bit loss rate of the DL transmissions. Given the proposed 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 of the H-MAC.
The average rate determines the speed of data transmission. In order to evaluate the rate of DL transmissions in the network by varying the number of stations and the lengths of the packets in both methods, a simulation is implemented. The results of this simulation are given in Figure 5. According
H- MAC A-VLD-MAC 500 450 400 350 300 S / its250 b M200 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 A-VLD-MAC compared to 319 Mbps in H-MAC, a difference of 134 Mbps. These results show the effectiveness of the AVLD-MAC method in dense networks.
Responding to user requirements while providing better service in high density areas is one of the goals of the new IEEE 802.11ax standard, and providing better throughput is one of the important services. The Figure 6 represents the results of the simulation of the average flows per user in each of the two H-MAC and A-VLD-MAC methods.
H-MAC A-VLD-MAC 1 2 3 4 6 7 8
9 5 Stations
We can see clearly in the figure 6 above, that the average throughput per user in the A-VLD-MAC method is considerably improved compared to the results of the H-MAC method. Apart from the average throughput per user offered for a single station that is equal in both methods (specific case A-VLDMAC), the average throughput per maximum user achieved and which is almost stable by increasing the number of stations in A-VLD-MAC is 56 Mbps against 50 Mbps for H-MAC and can be downgraded to 33 Mbps by increasing the number of stations.
A large amount of bits are sent using the H-MAC method with lengths of varying sizes to synchronize transmissions. The amount of stuffing bits sent in a DL transmission varies according to the lengths of the transmitted data for each user. The charge in stuffing bits is reduced by adopting our proposition A-VLD-MAC. The following Figure 7 shows the difference between the rate of loss in stuffing bits using the H-MAC method and the method A-VLD-MAC for the same lengths of data sent to the same number of stations in both methods.
H- MAC
A-VLD-MAC 60 50 10 0
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 AVLD-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.
CONCLUSION
The efficient use of transmission channel in the IEEE
802.11ax standard allows for increased throughput. The
purpose 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
mechanism 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 [