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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.

The latest IEEE 802.11 Standard (Wi-Fi), created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802.11) [

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 [

The performance of IEEE 802.11ax has been investigated in only few papers up to now. For example, in [

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 [

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.

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. [

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). Such a PSDU is denoted an Aggregate MAC Protocol Data Unit (A-MPDU) frame. In two-level aggregation every MPDU contains several MAC Service Data Units (MSDU). MPDUs are separated by an MPDU Delimiter field of 4 bytes and each MPDU contains MAC Header and Frame Control Sequence (FCS) fields. MSDUs within an MPDU are separated by a sub-header field of 14 bytes. 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 [

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.

In the SU operation mode every transmitted PPDU is destined to one user only. We assume the traffic pattern shown in

In

such field is composed of 2X LTF and therefore it is of duration 7.2 μs [

In

We assume the best effort Access Category in which

value of

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 [

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 [

Let X be the number of MPDU frames in an A-MPDU frame, numbered

Let

Then, the throughput in both 11ax and 11ac is given by Equation (1) [

where:

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 [

If one neglects the rounding of the denomination of Equation (1) the received throughput for every X and Y, the number of MSDUs in every MPDU, is as large as that received in Equation (1). The difference depends on the size of the deno- mination.

We therefore use the result in [

In

First, notice that in every figure the throughput is shown as a function of the MCSs in the x-axis. In every MCS 11ac and 11ax enable different PHY rates and so the comparison is done as a function of the MCSs in use. Also notice that

MCS10 and MCS11 are not possible in 11ac and so 11ac does not have results for these MCSs.

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. For

Notice that 11ax/256 outperforms 11ac in

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.

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 [

where

Notice from Equation (3) that given a number Y of MSDUs in an MPDU, it is worthwhile to contain as many MPDUs as possible in the A-MPDU frame, up to the limit on the PPDU transmission time.

Let MCS_{C} be the MCS from which 11ax/256 outperforms 11ax/64. Recall that

FCS fields in bytes. Also recall that

rate and let T be the limit on the transmission time of the PPDU (5400 μs in our

case). Finally, let

MSDUs per MPDU frame. For

Neglecting the rounding of

so

_{C} = MCS3 for any MSDU length

For positive BERs the optimal number of MSDUs per MPDU is not necessarily

Alternatively:

Using Equations (4) and (5) the search for the optimal A-MPDU can consider only the number X of MPDUs and the number Y of MSDUs per MPDU that maintain Equation (5). We can therefore re-write Equation (3) as:

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,

If we now substitute the parameters in Equation (7) by the values we use in this paper, and using_{C} is MCS0 as is shown in _{C} does not depend on the MSDUs’ sizes, as it is also observed from

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. Concerning IEEE 802.11ax two flavors are considered, using acknowledgment windows of 256 and 64 MPDUs respectively.

IEEE 802.11ax outperforms IEEE 802.11ac by 29% and 48% in reliable and unreliable channels respectively. The larger improvement in an unreliable channel is due to the larger number of MPDUs that IEEE 802.11ax enables. Also, a detailed analysis comparing between the two flavors of IEEE 802.11ax is given.

This paper is one of the first to evaluate the performance of IEEE 802.11ax. Other possible scenarios to consider are SU with TCP like traffic where the receiver generates Ack MSDUs such as TCP Ack segments, and MU-MIMO transmissions over the DL and the UL.

Sharon, O. and Alpert, Y. (2017) Single User MAC Level Throughput Comparision: IEEE 802.11ax vs. IEEE 802.11ac. Wireless Sensor Network, 9, 166-177. https://doi.org/10.4236/wsn.2017.95009

In this appendix we show how to derive

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

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