Single User MAC Level Throughput Comparision: IEEE 802.11ax vs. IEEE 802.11ac

With the ever-increasing range of video and audio applications in portable handheld devices, demand for high throughput in Wi-Fi networks is escalating. In this paper we introduce several novel features defined in next generation WLAN, termed as IEEE 802.11ax standard, and compare between the maximum throughputs received in IEEE 802.11ax and IEEE 802.11ac in a scenario where the AP continuously transmits to one station in the Single User mode. The comparison is done as a function of the modulation/coding schemes in use. In IEEE 802.11ax we consider two levels of frame aggregation. IEEE 802.11ax outperforms IEEE 802.11ac by about 29% and 48% in reliable and unreliable channels respectively.


Introduction
The latest IEEE 802.11 Standard (Wi-Fi), created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802.11) [1] is currently the most effective solution within the range of Wireless Local Area Networks (WLAN). Since its first release in 1997 the standard provides the basis for Wireless network products using the Wi-Fi brand, and has since been improved upon in many ways. One of the main goals of these improvements is to increase the throughput achieved by users and to improve the Quality-of-Service (QoS) capabilities of the network.
To fulfill the promise of increasing IEEE 802.11 performance and QoS capabilities, a new amendment, IEEE 802.11ax, also known as High Efficiency (HE) WLAN, was introduced recently [2]. IEEE 802.11ax is a sixth generation of WLAN in the IEEE 802.11 set of WLANs [1] and it is a successor to IEEE O. Sharon, Y. Alpert 802.11ac [3] [4]. IEEE 802.11ax is predicted to have maximum capacity of around 9.5 Gbps in 2.4 and/or 5 GHz and has the goal of providing 4 times the throughput of IEEE 802.11ac [5] [6] [7] [8].
The performance of IEEE 802.11ax has been investigated in only few papers up to now. For example, in [9] the authors assume a network with legacy and IEEE 802.11ax stations and examine fairness issues between the two sets of stations. In [10] the authors suggest an access protocol over the Uplink of an IEEE 802.11ax WLAN based on Multi User Multiple-Input-Multiple-Output (MU-MIMO) and OFDMA PHY.
In this paper we compare between the throughputs of IEEE 802.11ax and IEEE 802.11ac in a scenario where the AP continuously transmits UDP like traffic to a single station using the Single User (SU) operation mode. The AP transmits without collisions using advanced modulation and coding (MCS) schemes and using frame aggregation. This is one of the aspects to compare between new amendments of the IEEE 802.11 standard [11].
The paper is organized as follows: In Section 2 we describe the new features of IEEE 802.11ax relevant to this paper. In Section 3 we describe the transmission scenario over which we compare between IEEE 802.11ax and IEEE 802.11ac in the SU mode. In this mode we assume that the AP transmits continuously to a single station. In Section 4 we analytically compute the throughput of the SU mode and in Section 5 we present the throughputs of various protocols for the SU mode and compare between them. In Section 6 we analytically compute the PHY rates from which using a 256 MAC Protocol Data Units (MPDU) window size in IEEE 802.11ax is better than using a 64 MPDUs acknowledgment window size. Section 7 summarizes the paper. In the Appendix we derive the optimal number of MPDUs for the aggregation used in Section 6. In the rest of the paper we denote IEEE 802.11ac and IEEE 802.11ax by 11ac and 11ax respectively.

Frame Aggregation in IEEE 802.11ax
In order to achieve the 4 times throughput compared to IEEE 802.11ac, the IEEE 802.11ax addresses several new features. We introduce some of these features below. We assume that the reader is familiar with the basics of the PHY and MAC layers of IEEE 802.11 described in previous papers, e.g. [12].
Assuming an OFDM based PHY layer, every OFDM symbol duration is extended from 3.2 μs in 11ac to 12.8 μs in 11ax. Since the same Guard Interval (GI) is added to every such symbol, the overhead in 11ax due to the GI is smaller than in 11ac. Second, in 11ax there are two new MCSs, 1024 QAM 3/4 and 1024 QAM 5/6, denoted MCS10 and MCS11 respectively, applicable for channels with bandwidth larger than 20 MHz. The above two features enhance the PHY rate of 11ax.
In this paper we focus on the two-level frame aggregation scheme, in which several MPDUs are transmitted within a single PHY Service Data Unit (PSDU). Every MSDU is rounded to an integral multiple of 4 bytes together with the sub-header field. Every MPDU is also rounded to an integral multiple of 4 bytes.
In 11ax and 11ac, the size of an MPDU is limited to 11,454 bytes. In 11ac, an A-MPDU is limited to 1,048,575 bytes and in 11ax it is limited to 4,194,304 bytes. In both 11ac and 11ax, the transmission time of the PPDU (PSDU and its preamble) is limited to ~5.4 ms (5400 μs) due to L-SIG (one of the legacy preamble's fields) duration limit [4].
In this paper we also assume that all the MPDUs transmitted in an A-MPDU frame are from the same Traffic Stream (TS). In this case up to 256 MPDUs are allowed in an A-MPDU frame of 11ax, while in 11ac up to only 64 MPDUs are allowed.

Single User Model
In the SU operation mode every transmitted PPDU is destined to one user only.
We assume the traffic pattern shown in Figure 1 where the AP continuously transmits Data MSDUs to a station, and the station responds with the Block Ack (BAck) control frame [1]. A transmission of a PPDU from the AP followed by a BAck control frame from the station is denoted Transmission Cycle and such a cycle repeats itself continuously, as shown in Figure 1 The BAck frame is transmitted in the UL using the legacy mode (i.e. the mode used in the first generation of IEEE 802.11 WLANs) by using the legacy PHY basic rates' set. The UL PHY rate is set to the largest possible PHY rate in the set that is smaller or equal to the DL data rate.
In Figure 2 we show the PPDU formats used. In Figure 2  such field is composed of 2X LTF and therefore it is of duration 7.2 μs [2].
Notice that in SU mode and when using the same number S of SS, the preamble in 11ax is longer than that in 11ac by In Figure 2(b) we also show the legacy preamble format used by both 11ac and 11ax over the UL.
We assume the best effort Access Category in which 43 s AIFS = µ ,  which equals 67.5 μs for a 9 s SlotTime = µ . We also assume that the MAC Header is of 28 bytes and the FCS is of 4 bytes.
In the 5 GHz band we assume a 160 MHz channel BW and the AP has 4 antennas and every station has 1 antenna. Therefore, we assume 4 Spatial Streams (SS) and in this case, the PHY rates in the various MCSs and the preambles can be found in [2].
Finally, we consider several channel conditions that are expressed by different values of the Bit Error Rate (BER). We assume a model where these probabilities are bit-wise independent [13].

Throughput Analysis: Single User Mode
Let X be the number of MPDU frames in an A-MPDU frame, numbered Then, the throughput in both 11ax and 11ac is given by Equation (1) [12] where BER is the Bit Error Rate, and i C defined above is the length of MPDU number i in bits: where: are added to every transmission by the PHY layer convolutional protocol [4]. DL R and UL R are the DL and UL PHY rates respectively and DL P and UL P are the preambles used in the DL and UL respectively, see Figure 2.
The term in Equation (1) is not continuous and so it is difficult to find the optimal X and Y i (s), i.e. the values for X and Y i (s) that maximize the throughput.
However, in [12] it is shown that if one neglects the rounding in the denomination of Equation (1)  We therefore use the result in [12] and look for the maximum throughput as follows: We check for every X, 1 for 11ax) and for every Y,

Throughput Results: Single User Mode
In Figure 3 and Figure 4 we show the maximum throughput of 11ax and 11ac for two different channel conditions, 0 BER = and  In all the figures the performance of 11ax is better than that of 11ac. This is due to the larger PHY rates that 11ax enables in every MCS compared to 11ac.

AIFS BO P SIFS P T BAck
the throughput is received when using not "too" many MPDUs. The larger number of MPDUs that 11ax/256 enables is therefore less significant than in 5 10 BER − = and so is the relative improvement in throughput between 11ax/256 and 11ac.
A summarizing conclusion from the above is that 11ax outperforms 11ac in an unreliable channel more than in a reliable channel because 11ax enables more MPDUs in a transmission. These MPDUs can be short in order to maintain a large success probability; thus, 11ax enables many short MPDUs, with a relatively large success probability, a feature that is not possible in 11ac.

Acknowledgment Window Size Analysis-Single User Mode
One can conclude the following from the results in Section 5: First, as the BER is larger, 11ax/256 outperforms 11ax/64 from smaller PHY rates. Second, the MCS from which 11ax/256 outperforms 11ax/64 is not dependent on the MSDU size.
We want to investigate these phenomena further.
In the following analysis, we use the above mentioned approximation from [12] where we neglect the rounding in the denomination of Equation (1) Figure 3 the difference between 11ax/64 to 11ax/256 in MCS3 is too small to be noticed, however from MCS4 the difference is noticeable.

Unreliable Channel, BER > 0
For positive BERs the optimal number of MSDUs per MPDU is not necessarily max Y . Therefore, we use the following approximation. Given that it is worthwhile to transmit as long PPDUs as possible, then let opt X and opt Y be the number of MPDUs and the number of MSDUs per MPDU respectively in the optimal A-MPDU, i.e. the A-MPDU that achieves the largest throughput. Then, Equations (4) and (5) can give a relation between opt X and opt Y : Alternatively: Using Equations (4) and (5) Notice that the denomination of Equation (6) is a constant because we use the outcome that it is most efficient that the transmission time of the PPDU will be the largest possible.
To find the largest throughput we derive Equation (6) according to X and find that the optimal X is the single positive solution of a quadratic equation, which reveals that Equation (6) is unimodal. The optimal X, opt X , is given by Equation (7) If we now substitute the parameters in Equation (7) Figure 4. Notice that by the above in turns out that the MCS C does not depend on the MSDUs' sizes, as it is also observed from Figure 4.

Summary
In this paper we compare between the maximum throughputs of IEEE 802.11ax and IEEE 802.11ac in a single user operation mode and in UDP like traffic. In our SU mode the AP transmits continuously to a station using two-level aggregation.

Appendix
In this appendix we show how to derive opt X in Equation (7) from the term for the throughput in Equation (6).
After some arrangement of Equation (6) we receive the following equation: The first two terms of Equation (8) are constants and so we need to find the X that maximizes Equation (9): In order to find this X we derive Equation (9) and compare the derivative to 0.
We receive: Solving the quadratic equation, Equation (10), we get that the optimal X is given in Equation (11): We choose the "−" alternative before the square root because we look for a positive X and the term ( ) ( )  (7).
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