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The new IEEE 802.11ax standard is aimed to serve many users while enabling every station to transmit a consistent stream of data without interruption. In this paper we evaluate the upper bound on the throughput of a Downlink IEEE 802.11ax channel using the Single User (SU) mode and using the Multi User Multiple-Input-Multiple-Output (MU-MIMO) and Orthogonal Frequency Division Multiple Access (OFDMA) mode. We compare between IEEE 802.11ax and IEEE 802.11ac for the case of 1, 4, 8, 16, 32 and 64 stations in different Modulation/Coding schemes (MCS) and different transmission windows’ sizes, 64 and 256 frames in IEEE 802.11ax. IEEE 802.11ax outperforms IEEE 802.11ac in the SU and MU modes by 52% and 74% in a reliable channel respectively, while in an unreliable channel the improvements are by 59% and 103% respectively. Also, in terms of the access delay, the advantage of IEEE 802.11ax increases as the number of stations increases.

IEEE 802.11 Standard (WiFi) [

In this paper we assume that the AP is communicating with a fixed set of stations in a Round Robin fashion, without collisions. We explore some of the Downlink (DL) and Uplink (UL) IEEE 802.11ax new mechanisms and we compare between the upper bounds on the unidirectional DL UDP throughputs of IEEE 802.11ax and IEEE 802.11ac in Single User (SU) and Multi User (MU) modes for 1, 4, 8, 16, 32 and 64 stations scenarios in reliable and unreliable channels. This is one of the aspects to compare between new amendments of the IEEE 802.11 standard [

In the SU transmission mode a single station transmits to the AP over the UL and the AP transmits to a single station over the DL in a given time. In the MU transmission mode the AP transmits to several stations simultaneously over the DL and several stations transmit simultaneously to the AP over the UL. In IEEE 802.11ac the MU mode is not possible over the UL. In IEEE 802.11ax up to 74 stations can transmit simultaneously over the UL [

The MU transmissions over the DL (DATA) and the UL (Acks) are done by MIMO and OFDMA. The IEEE 802.11ax standard enables new ways of multiplexing users using OFDMA and expends MIMO transmissions multiplexing format. In the IEEE 802.11ac the total channel bandwidth (20 MHz, 40 MHz, 80 MHz etc.) contains multiple OFDM sub-carriers while in IEEE 802.11ax OFDMA, different subsets of sub-carriers in the channel bandwidth can be used by different frame transmissions at the same time. Sub-carriers can be allocated for transmissions in Resource Units (RU) as small as 2 MHz.

The main contributions of this paper relate to the new OFDMA structure in 11ax. We suggest new scheduling strategies over the DL where the AP transmits UDP traffic to the stations and the stations reply with acknowledgments at the MAC layer. For each scheduling strategy we also evaluate upper bounds on the throughput and access delay which are influenced by the different PHY rates that are used in the different scheduling strategies. This paper deals with the DL and a companion paper deals with the UL [

In order to achieve its goals, one of the main challenges of IEEE 802.11ax is to enable simultaneous transmissions by several stations and to enable Quality-of-Service. Most of the research papers on IEEE 802.11ax thus far deal with these challenges and examine different access methods to enable efficient multi-user access to random sets of stations. In [

In [

The remainder of the paper is organized as follows: In Section 2 we mention some new features of IEEE 802.11ax that we later use in transmission scheduling strategies that are described in Section 3 for both the SU and MU modes. In our descriptions of the transmission scenarios we assume that the reader is familiar with the basics of the IEEE 802.11 systems, as can be found in e.g. [

The new IEEE 802.11ax standard is aimed to serve many users while enabling every station to transmit a consistent stream of data without interruption. Therefore, 11ax incorporates some new mechanisms in the PHY and MAC layers.

First, in 11ax there are two new Modulation/Coding schemes, 1024 QAM 3/4 and 1024 QAM 5/6, denoted MCS10 and MCS11 respectively. These MCSs can be used only in channels with bandwidths larger than 20 MHz. Also, 11ax uses larger OFDM FFT sizes, 4 times larger, and thus every OFDM symbol is 12.8 μs in 11ax, compared to 3.2 μs in 11ac.

In this paper we also use the two-level aggregation scheme, first introduced in IEEE 802.11n [

Another important new feature in 11ax is the ability to use a transmission window of 256 frames, compared to 64 frames only in 11ac.

Finally, in 11ax MU-MIMO or OFDMA are supported on both the UL and DL, while in 11ax only UL SU is possible. In 11ax it is possible to transmit to 74 stations simultaneously over the DL, while in 11ac this number is limited to 4.

As mentioned, one of the main goals of 11ax is to enable larger throughputs in the network when transmitting to several stations. In 11ax it is possible to transmit/receive simultaneously to/from 74 stations over the DL/UL while in 11ac the number of stations is limited to 4, and only over the DL. In this paper we compare the throughputs received in 11ac and 11ax when transmitting to S stations, S = 1 , 4 , 8 , 16 , 32 and 64 stations. Transmitting to one station only is done by using the SU mode of transmissions. The AP transmits to one station and receives a Block Ack (BAck) frame in return. In this mode the advantage of

11ax over 11ac is in its more efficient PHY layer and its new MCSs. The unscheduled SU traffic pattern in this case is shown in

Transmitting to several stations can be done in two ways. The first is by SU mode. When transmitting to S stations, the transmission cycle in

In 11ax,

A-MPDU frame that is transmitted to every station. In the following throughput computations we optimize the amount of overhead used due to the above methods by computing the minimum overhead needed as a function of the number of data MPDUs in the A-MPDU frame.

Finally, we assume that the AP and the stations do not contend for the channel and so there are no collisions. The cycles in

There are several DL service scheduling strategies to transmit to a group of stations, and we compare between them. We now specify these scheduling strategies for every number S of stations, S = 1 , 4 , 8 , 16 , 32 , 64 . By x ⋅ S U A X ( 1 ) and x ⋅ S U A C ( 1 ) we denote a transmission to n stations in 11ax and 11ac respectively, using the transmission pattern in

The DL service scheduling strategies are as follows:

• S = 1 :

11ac: 1 ⋅ S U A C ( 1 ) .

11ax: 1 ⋅ S U A X ( 1 ) .

• S = 4 :

11ac: 4 ⋅ S U A C ( 1 ) , 1 ⋅ M U A C ( 4 ) .

11ax: 4 ⋅ S U A X ( 1 ) , 1 ⋅ M U A X ( 4 ) .

• S = 8 :

11ac: 8 ⋅ S U A C ( 1 ) , 2 ⋅ M U A C ( 4 ) .

11ax: 8 ⋅ S U A X ( 1 ) , 2 ⋅ M U A X ( 4 ) , 1 ⋅ M U A X ( 8 ) .

• S = 16 :

11ac: 16 ⋅ S U A C ( 1 ) , 4 ⋅ M U A C ( 4 ) .

11ax: 16 ⋅ S U A X ( 1 ) , 4 ⋅ M U A X ( 4 ) , 2 ⋅ M U A X ( 8 ) , 1 ⋅ M U A X ( 16 ) .

• S = 32 :

11ac: 32 ⋅ S U A C ( 1 ) , 8 ⋅ M U A C ( 4 ) .

11ax: 32 ⋅ S U A X ( 1 ) , 8 ⋅ M U A X ( 4 ) , 4 ⋅ M U A X ( 8 ) , 2 ⋅ M U A X ( 16 ) , 1 ⋅ M U A X ( 32 ) .

• S = 64 :

11ac: 64 ⋅ S U A C ( 1 ) , 16 ⋅ M U A C ( 4 ) .

11ax: 64 ⋅ S U A X ( 1 ) , 16 ⋅ M U A X ( 4 ) , 8 ⋅ M U A X ( 8 ) , 4 ⋅ M U A X ( 16 ) , 2 ⋅ M U A X ( 32 ) , 1 ⋅ M U A X ( 64 ) .

We assume the 5 GHz band, a 160 MHz channel, the AP has 4 antennas and every station has 1 antenna. In SU(1) and in the DL direction the entire channel is devoted to transmissions of the AP in both 11ac and 11ax. In UL SU the BAck frame is transmitted by using the legacy PHY basic rates. Therefore the UL Ack is sent at legacy mode where the station is transmitting in a 20 MHz primary channel and its transmission is duplicated 8 times in order to occupy the entire 160 MHz. 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.

The 160 MHz channel is divided in the MU mode into S 4 channels of 160 ⋅ 4 S MHz each. S can be 4, 8, 16, 32 or 64. 4 Spatial Streams are defined in

every channel and in every Spatial Stream the AP transmits to a different station. Notice for example that when S = 64 the AP transmits to 64 stations using 16 channels of 10 MHz each. For the case of S = 4 there is no need to divide the 160 MHz channel and only MU-MIMO is used. For S > 4 MU - MIMO + OFDMA is used. In the case of M U A C ,

For the UL Ack transmission in 11ax,

divided into S channels of 160 S MHz each, except in the case of S = 64 where

each station is allocated a channel of 2 MHz.

In

In

Notice also that the PSDU frame in 11ax contains a Packet Extension (PE) field. This field is mainly used in Multi-User (MU) mode and we assume it is not present in SU, i.e. it is of length 0 μs.

In

The PPDU format in

In

the PSDU additional time to move from a reception mode to a transmission mode. The largest duration of this field is 16ms which we assume in this paper.

In the appendix we show the PHY rates that are used in 11ac and 11ax in SU and MU, over the DL and UL and in the various MCSs.

Concerning non-legacy transmissions, we assume a GI of 0.8 μs for transmissions over the DL. For transmissions over the UL we assume a GI of 1.6 μs. Therefore, the OFDM symbols are of 13.6 μs and 14.4 μs over the DL and the UL respectively. Regarding legacy transmissions, the OFDM symbols are 4 μs.

We assume the Best Effort Access Category in which A I F S = 43 μ s , S I F S = 16 μ s and C W min = 16 for the transmissions of the AP. The BackOff interval is a random number chosen uniformly from the range [ 0 , ⋯ , C W min − 1 ] . Since we consider a very “large” number of transmissions from the AP and we assume that there are no collisions, we take the BackOff average value of

⌈ C W min − 1 2 ⌉ and the average BackOff interval is ⌈ C W min − 1 2 ⌉ ⋅ S l o t T i m e which

equals 67.5 μs for a S l o t T i m e = 9 μ s . We also assume that the MAC Header is of 28 bytes and the FCS is of 4 bytes. We use the above values for the various parameters since these are the default ones suggested by the WiFi Alliance [

Finally, we consider several channel conditions which are expressed by different values of the Bit Error Rate (BER) which is the probability that a bit arrives corrupted at the destination. We assume a model where these probabilities are bitwise independent [

Let X be the number of MPDU frames in an A-MPDU frame, numbered 1, ⋯ , X , and Y i is the number of MSDUs in MPDU number i. Let M a c H e a d e r , M p d u D e l i m i t e r and F C S be the length, in bytes, of the MAC Header, MPDU Delimiter and FCS fields respectively, and let

O M = M a c H e a d e r + M p d u D e l i m i t e r + F C S . Let L D A T A be the length, in bytes, of

the MSDU frames. Also, let L e n = 4 ⋅ ⌈ L D A T A + 14 4 ⌉ and C i = 8 ⋅ 4 ⋅ ⌈ O M + Y i ⋅ L e n 4 ⌉ .

C i is the length, in bits, of MPDU number i.

In the entire analysis ahead we assume that the Ack frames’ transmissions are all successful because Ack frames are short and in most cases are transmitted in legacy mode.

The throughput in both 11ax and 11ac for the traffic pattern in

T h r = ∑ i = 1 X 8 ⋅ Y i ⋅ L D A T A ⋅ ( 1 − B E R ) C i A I F S + B O ( a v e r a g e ) + P D L + T ( D A T A ) + S I F S + P U L + T ( B A c k ) (1)

where:

T ( D A T A ) = T S y m D L ⋅ ⌈ ∑ i = 1 X C i + 22 T S y m D L ⋅ R D L ⌉ T ( B A c k ) = T S y m U L ⋅ ⌈ ( 30 × 8 ) + 22 T S y m U L ⋅ R U L ⌉ (2)

The term B O ( a v e r a g e ) refers to the average value of the BackOff interval, as given in Section 3.5. As was explained in Section 3.5 we use an average value for this interval since there are no collisions.

T ( D A T A ) and T ( B A c k ) are the transmission times of the data A-MPDU frames and BAck frames respectively. T ( B A c k ) is based on the BAck frame’s lengths given in

R D L and P D L are the PHY rate and preamble used over the DL respectively while R U L and P U L are similarly defined for the UL (see

The term in Equation (1) is not continuous, 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 neglecting the rounding of the denominator of Equation (1), the received throughput for every X and Y (Y is the equal number of MSDUs in MPDUs) is as large as that received in Equation (1). The difference depends on denominator size.

We therefore use the result in [

The throughputs of 11ac and 11ax are given in Equations (3)-(6) and their derivation can be found in [

The throughput of 11ac for the traffic pattern in

T h r A C = 4 ⋅ ∑ i = 1 X 8 ⋅ Y i ⋅ L D A T A ⋅ ( 1 − B E R ) C i A I F S + B O ( a v e r a g e ) + P D L + T ( D A T A ) + 7 ⋅ ( S I F S + P U L ) + 4 ⋅ T ( B A c k ) + 3 ⋅ T ( B A R ) (3)

where:

T ( D A T A ) = T S y m D L ⋅ ⌈ ∑ i = 1 X C i + 22 T S y m D L ⋅ R D L ⌉ T ( B A c k ) = T S y m U L ⋅ ⌈ ( 30 × 8 ) + 22 T S y m U L ⋅ R U L ⌉ T ( B A R ) = T S y m U L ⋅ ⌈ ( 24 × 8 ) + 22 T S y m U L ⋅ R U L ⌉ (4)

are the transmission times of the data A-MPDU frames, the BAck frames and the BAR frames respectively. The transmission times of the BAck and BAR frames are based on their lengths given in

The throughput of 11ax for the traffic pattern in

T h r A X = S ⋅ ∑ i = 1 X 8 ⋅ Y i ⋅ L D A T A ⋅ ( 1 − B E R ) C i A I F S + B O ( a v e r a g e ) + P D L + T ′ ( D A T A ) + P E + S I F S + P U L + T ′ ( B A c k ) + P E (5)

where:

T ′ ( D A T A ) = T S y m D L ⋅ ⌈ ∑ i = 1 X C i + ( ( O M + 72 ) ⋅ 8 ) + 22 T S y m D L ⋅ R D L ⌉ T ′ ( B A c k ) = T S y m U L ⋅ ⌈ ( 30 × 8 ) + 22 T S y m U L ⋅ R U L ⌉ (6)

P D L and P U L are again the preambles in the DL and UL respectively.

In the term for T ′ ( D A T A ) we assume the case of a Trigger Frame which holds for X data MPDUs in the A-MPDU frame such that 19 ≤ X ≤ 64 . For 1 ≤ X ≤ 18 it is more efficient to use the HE Control Element of 4 bytes added to every data MPDU, and the term ( ( O M + 72 ) ⋅ 8 ) is therefore replaced by ( X × 4 × 8 ) . Notice that the 72 bytes come from 33 bytes of the TF frame, 28 bytes of the MAC Header, 4 bytes of the FCS field, 4 bytes of the MPDU Delimiter and rounding to an integral number of 4 bytes. For the BAck frame, T ′ ( B A c k ) is based on a BAck frame acknowledging 64 MPDUs. In 11ax it is also possible to acknowledge 256 MPDUs and in this case the 30 bytes in T ′ ( B A c k ) are replaced by 54 bytes. See

Again, the terms in Equation (3) and Equation (5) are not continuous and therefore we again use the result in [

We verified the analysis results of 11ax by simulation using the ns3 simulator [

In this section we show an approximation to the value of X O P T , the optimal number of MPDUs in an A-MPDU, i.e. the number of MPDUs that maximizes the throughput, as a function of the BER. We concentrate on 11ax although the computation is valid for 11ac as well.

We re-write Equation (5) by ignoring the rounding of T ′ ( D A T A ) and T ′ ( B A c k ) , ignoring the 22 bits in the numerators of T ′ ( D A T A ) and T ′ ( B A c k ) , settings O p = A I F S + B O + S I F S + P U L + T ′ ( B A c k ) + P E , assuming that every MPDU has the same number Y of MPDUs, O M = M a c H e a d e r + M p d u D e l i m i t e r + F C S and ignoring the overhead due to the TF frame:

T h r = S ⋅ X ⋅ Y ⋅ 8 ⋅ L D A T A ⋅ ( 1 − B E R ) 8 ⋅ ( Y ⋅ L e n + O M ) O p + P D L + X ⋅ 8 ⋅ ( Y ⋅ L e n + O M ) R D L (7)

Notice that given a number Y of MPDUs in an A-MPDU, the throughput increases as X increases. Therefore, it is worthwhile to transmit as large A-MPDUs as possible, up to the limit on the transmission time of the A-MPDU frame. Let T be this limit, 5484 μs in our case. Then, the following approximation on the relation between X and Y can be written:

T = X ⋅ 8 ⋅ ( Y ⋅ l e n + O M ) R D L + P D L (8)

or:

X = R D L ⋅ ( T − P D L ) 8 ⋅ ( Y ⋅ L e n + O M ) (9)

In Equation (8) and Equation (9) we approximate that the sum of the A-MPDU transmission time plus the DL preamble is T.

We now substitute the term for X in Equation (7) by the term in Equation (9) and receive:

T h r = S ⋅ R D L ⋅ ( T − P D L ) 8 ⋅ ( Y ⋅ L e n + O M ) ⋅ Y ⋅ 8 ⋅ L D A T A ⋅ ( 1 − B E R ) 8 ⋅ ( Y ⋅ L e n + O M ) T + O p − P D L (10)

Notice that the denominator of Equation (10) is a constant and so to find the maximum throughput as a function of Y one needs to find the maximum of the following function:

Y 8 ⋅ ( Y ⋅ L e n + O M ) ⋅ ( 1 − B E R ) 8 ⋅ ( Y ⋅ L e n + O M ) (11)

The optimal Y, Y O P T , is given in Equation (12):

Y O P T = O M ⋅ ( 1 − 4 8 ⋅ O M ⋅ ln ( 1 − B E R ) − 1 ) 2 ⋅ L e n (12)

Notice that by Equation (9) we can now write the optimal X, X O P T , as:

X O P T = R D L ⋅ ( T − P D L ) 8 ⋅ O M ( ( 1 − 4 8 ⋅ O M ⋅ ln ( 1 − B E R ) − 1 ) 2 ⋅ L e n + 1 ) (13)

Notice that we look for an integer Y O P T and that Y O P T must be at least 1. Therefore, Equation (13) is only an approximation for X O P T .

Consider now

receive that Y O P T = 653 L e n . For L e n = 1516, 528 and 80 bytes we receive

Y O P T = 0.43 , 1.23 and 8.16 respectively. For Y O P T = 0.43 we need to round up to 1 and receive X O P T = 21.72 . It turns out that X O P T = 21 yields a larger throughput than 22 MPDUs. For Y O P T = 1.23 we can take either ⌊ Y O P T ⌋ = 1 or ⌈ Y O P T ⌉ = 2 . For the two cases we receive ⌊ X O P T ⌋ = 59 and 30 respectively where the first case yields a larger throughput. We handle the case for L e n = 80 similarly, where the X O P T is now 50. All these values for X O P T appear in

In

over a wide range of BER values. This is because as the BER increases it is worthwhile transmitting short MPDUs, but one MSDU must be included in an MPDU. For MSDUs of 512 bytes there is more flexibility in the number of MSDUs per MPDU and so the optimal number of MPDUs is more flexible. For MSDUs of 60 bytes the number of MSDUs per MPDU varies according to the BER in the most flexible way and so does the number of MPDUs. The number of optimal MPDUs is smaller than in MSDUs of 512 bytes because the smaller size of the MSDUs enables using the MPDUs more efficiently, the MPDUs are little longer than in the case of 512 bytes MSDUs and due to the limit on the A-MPDU transmission time, a smaller number of MPDUs is needed.

For BER = 0 Equation (7) becomes:

T h r = S ⋅ X ⋅ Y ⋅ 8 ⋅ L D A T A O p + P D L + X ⋅ 8 ⋅ ( Y ⋅ L e n + O M ) R D L (14)

and one needs to optimize the function:

Y 8 ⋅ ( Y ⋅ L e n + O M ) (15)

which reveals that in every MPDU it is worthwhile to contain the maximum number of MSDUs, Y M A X , which is ⌊ 11454 − O M L e n ⌋ .

Therefore:

X O P T = R D L ⋅ ( T − P D L ) 8 ⋅ ( ⌊ 11454 − O M L e n ⌋ ⋅ L e n + O M ) (16)

For example, for

For MSDUs of 1500, 512 and 64 bytes one receives L e n = 1516, 528 and 80 bytes respectively, which gives X O P T = 3.166, 3.031, 2.958 respectively. Since we look for an integer X O P T one needs to choose between 3 or 4 MPDUs for the first two cases and between 2 or 3 MPDUs for the third case. It turns out that 3,3,3 are the optimal number of MPDUs respectively, as appears in

We compare between all applicable configurations and DL service scheduling flavors of the AP transmissions to up to 64 stations. The service scheduling flavors are as follows:

Concerning 11ac:

・ DL SU, UL SU Back transmission in legacy mode, up to 64 MPDUs in an A-MPDU frame, denoted previously as S U A C ( 1 ) .

・ DL 4 users MU-MIMO, UL 4 times SU BAck transmission in legacy mode, up to 64 MPDUs in an A-MPDU frame, denoted previously as M U A C ( 4 ) .

Concerning 11ax:

・ DL SU, UL SU BAck transmission in legacy mode, up to 64 or 256 MPDUs in an A-MPDU frame, denoted previously as 11ax/64 and 11ax/256 S U A X ( 1 ) respectively.

・ DL 4 users MU-MIMO, UL MU-MIMO or OFDMA BAck transmission, up to 64 or 256 MPDUs in an A-MPDU frame, denoted previously as 11ax/64 and 11ax/256 M U A X ( 4 ) respectively.

・ DL S = 8, 16, 32, 64 users DL MU-MIMO + OFDMA, UL MU-MIMO + OFDMA or OFDMA BAck transmission, up to 64 or 256 MPDUs in an A-MPDU frame, denoted previously as 11ax/64 and 11ax/256 M U A X ( S ) respectively.

For every number S of stations we checked what is the best transmission scheduling strategy, the best MCS and the best A-MPDU frame structure. In doing so we checked for every number S of stations all the applicable scheduling strategies, e.g. for 64 stations and 11ac these are 64 ⋅ S U A C ( 1 ) and 16 ⋅ M U A C ( 4 ) ,

Every transmission flavor is checked over all applicable MCSs. For 11ac these are MCS0-MCS9. For 11ax these are MCS0-MCS11 except in the case of 64 stations, where only MCS0-MCS9 are applicable. We also check for every transmission flavor and MCS the optimal working point by optimizing the number of MPDUs and number of MSDUs in every MPDU that yields the maximum throughput, i.e. we look for the optimal A-MPDU frame structure. We checked all the above for MSDUs of 64, 512 and 1500 bytes and B E R = 0 , 10 − 6 , 10 − 5 .

There are three sets of results shown in

Recall that in

In

We see from

In M U A X ( 16 ) the PHY rates are less than half of those in M U A X ( 8 ) and together with the larger preamble this explains why M U A X ( 16 ) has a smaller throughput than M U A X ( 8 ) and M U A X ( 4 ) . The explanation for the throughputs of M U A X ( 32 ) and M U A X ( 64 ) is similar to those given above for M U A X ( 8 ) and M U A X ( 16 ) . Notice that the PHY rates in M U A X ( 64 ) are less than half of those of M U A X ( 32 ) and also that MCS10 and MCS11 are not applicable for M U A X ( 64 ) , which is a main factor in the sharp decrease in the throughput of M U A X ( 64 ) compared to M U A X ( 32 ) .

Notice also that for all stations 11ax outperforms 11ac due to larger PHY rates and simultaneous transmissions of BAck frames in the UL compared to sequential transmissions in legacy mode in 11ac. For 4, 8, 16, 32 and 64 stations and using MU-MIMO, 11ax outperforms 11ac by 59%, 4470 vs. 2808 Mbps, the throughputs in M U A X ( 4 ) and M U A C ( 4 ) respectively. In SU when transmitting to 1 station only, 11ax outperforms 11ac by 52%, 1133 vs. 742 Mbps.

Although the throughput metric is important, so is the access delay metric, defined in this paper as the time elapsed between two consecutive transmissions from the AP to the same station. Notice for example that in the case of M U A X ( 4 ) that achieves the largest throughput, the access delay in the case of 64 stations is 16 times the cycle of

In

In

In

When comparing between the throughputs of M U A X ( 8 ) and M U A C ( 4 ) , 11ax outperforms 11ac by 103%, 3872 vs 1902 Mbps respectively. For SU(1) 11ax outperforms 11ac by 74%, 940 vs. 540 Mbps respectively.

In

Also worth mentioning is the relation between the access delays of M U A X ( 4 ) and M U A X ( 8 ) . For B E R = 10 − 5 they are about the same because the maximum throughput in both DL service scheduling flavors is received when an A-MPDU frame contains 255 MPDUs of 1 MSDU each. Since the PHY rates in M U A X ( 8 ) are about half of those in M U A X ( 4 ) , the cycle length in M U A X ( 8 ) is about double in length than in M U A X ( 4 ) . However, this is compensated by double the number of stations to which the AP transmits in M U A X ( 8 ) compared to M U A X ( 4 ) ; overall the access delays are similar in both DL service scheduling flavors.

In BER = 0 the cycle length in both M U A X ( 4 ) and M U A X ( 8 ) are about the same, around 5.5 ms, transmitting as many MSDUs as possible. The access delay in M U A X ( 4 ) is now twice than that of M U A X ( 8 ) because of the 4 vs. 8 stations to which the AP transmits in M U A X ( 4 ) and M U A X ( 8 ) respectively.

Overall it can be concluded from

In

The maximum throughput is always received in M U A X ( 4 ) in MCS11 ( MCS9 in M U A X ( 64 ) ) due to the highest PHY rates in this MCS. Considering M U A X ( 4 ) notice that for B E R = 0 11ax/256 outperforms 11ax/64 only in MCS10 and MCS11 while in B E R = 10 − 5 11ax/256 outperforms 11ax/64 starting from MCS2 (starting from MCS5 in B E R = 10 − 6 ). In B E R = 0 it is efficient to transmit large MPDUs. Therefore, the limit on the A-MPDU frame size is imposed by the limit of 5.484 ms on the transmission time of the PPDU. Only in larger PHY rates there is room for more than 64 MPDUs and in these cases 11ax/256 has an advantage over 11ax/64. In B E R = 10 − 5 it is efficient to transmit short MPDUs. In this case the significant limit is the number of MPDUs. 11ax/256 outperforms 11ax/64 from MCS2 because it enables transmitting more short MPDUs than 11ax/64. A detailed analysis of this phenomenon can be found in [

Another interesting phenomenon is the relation between UL MU-MIMO and UL OFDMA. When using UL OFDMA the UL PHY rates are much smaller than those in UL MU-MIMO (see

In

The results for M U A X ( 64 ) are shown in

In this paper we compare between DL service scheduling flavors to optimize throughputs of 11ac and 11ax over the DL when considering UDP like traffic and several DL service scheduling stations are transmitting in the system. We also consider several transmission flavors in 11ac and 11ax using MU-MIMO and OFDMA. We look for upper bounds on the throughput received at the MAC layer after neutralizing any aspects of the PHY layer as the relation between the BER and the MCSs in use, the number of Spatial Streams (SS) in use, channel correlation when using MU-MIMO, the sounding protocol etc.

IEEE 802.11ax outperforms 11ac by the order of several tenths of percentage because it enables simultaneous transmissions on both the DL and the UL while 11ac has this capability over the DL only, and for 4 stations only. Also, 11ax has larger PHY rates which also improve its efficiency compared to 11ac.

In 11ax there is not one best DL service scheduling transmission flavor. M U A X ( 8 ) achieves good results in terms of throughout, but M U A X ( 16 ) and M U A X ( 32 ) also achieve good throughput results, but with significantly smaller access delay. 11ax achieves its best throughputs in MCS11 in the case of up to 32 stations, and in MCS9 in the case of 64 stations.

There is an optimal A-MPDU frame structure. In M U A X ( 4 ) it is sufficient to transmit around 70 MPDUs and 256 MPDUs in an A-MPDU frame for B E R = 0 and B E R = 10 − 5 respectively. For M U A X ( 64 ) these numbers of MPDUs are smaller, around 3 for B E R = 0 and 21, 58 and 50 for MSDUs of 1500, 512 and 64 bytes respectively, due to smaller PHY rates.

Finally, using up to 256 MPDUs in an A-MPDU frame outperforms the case of using up to 64 MPDUs in the cases where the PHY rates are large and/or the channel is unreliable, i.e. B E R = 10 − 5 .

Sharon, O. and Alpert, Y. (2017) Scheduling Strategies and Throughput Optimization for the Downlink for IEEE 802.11ax and IEEE 802.11ac Based Networks. Wireless Sensor Network, 9, 355-383. https://doi.org/10.4236/wsn.2017.910020

In this appendix we show two tables containing PHY rates. In

1 | 2 | 3 | 4 | ||||||
---|---|---|---|---|---|---|---|---|---|

SU DL data transmission rate in 11ax | SU DL data transmission rate in 11ac | UL BAck transmission rate in 11ax | UL BAck transmission rate in 11ac | ||||||

MCS | PHY Rate (Mbps) GI = 0.8 μs | Preamble (μs) | PHY Rate (Mbps) GI = 0.8 μs | Preamble (μs) | PHY Rate (Mbps) | Preamble (μs) | PHY rate (Mbps) | Preamble (μs) | |

1 station IEEE 802.11 ax | 1 station IEEE 802.11 ac | ||||||||

0 | 72.1 | 43.2 | 58.5 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

1 | 144.1 | 43.2 | 117.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

2 | 216.2 | 43.2 | 175.5 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

3 | 288.2 | 43.2 | 234.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

4 | 432.4 | 43.2 | 351.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

5 | 576.5 | 43.2 | 468.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

6 | 648.5 | 43.2 | 526.5 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

7 | 720.6 | 43.2 | 585.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

8 | 864.7 | 43.2 | 702.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

9 | 960.7 | 43.2 | 780.0 | 36.0 | 48.0 | 20.0 | 48.0 | 20.0 | |

10 | 1080.9 | 43.2 | N/A | N/A | 48.0 | 20.0 | N/A | N/A | |

11 | 1201.0 | 43.2 | N/A | N/A | 48.0 | 20.0 | N/A | N/A |

1 | 2 | 3 | 4 | 5 | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|

DL MU data transmission rate in 11ax | UL MU-MIMO BAck transmission rate in 11ax | UL OFDMA BAck Transmission rate in 11ax | DL MU-MIMO data transmission rate in 11ac | UL BAck transmission rate in 11ac | |||||||

MCS | PHY Rate (MBps) GI = 0.8 μs | Preamble (μs) | PHY Rate (MBps) GI = 1.6 μs | Preamble (μs) | PHY Rate (MBps) GI = 1.6 μs | Preamble (μs) | PHY Rate (MBps) GI = 0.8 μs | Preamble (μs) | PHY Rate (MBps) | Preamble (μs) | |

4 stations IEEE 802.11 ax | 4 stations IEEE 802.11 ac | ||||||||||

0 | 72.1 | 72.8 | 68.1 | 64.8 | 16.3 | 64.8 | 58.5 | 48.0 | 48.0 | 20.0 | |

1 | 144.1 | 72.8 | 136.1 | 64.8 | 32.5 | 64.8 | 117.0 | 48.0 | 48.0 | 20.0 | |

2 | 216.2 | 68.8 | 204.2 | 64.8 | 48.8 | 64.8 | 175.5 | 48.0 | 48.0 | 20.0 | |

3 | 288.2 | 68.8 | 272.2 | 64.8 | 65.0 | 64.8 | 234.0 | 48.0 | 48.0 | 20.0 | |

4 | 432.4 | 68.8 | 408.3 | 64.8 | 97.5 | 64.8 | 351.0 | 48.0 | 48.0 | 20.0 | |

5 | 576.5 | 68.8 | 544.4 | 64.8 | 130.0 | 64.8 | 468.0 | 48.0 | 48.0 | 20.0 | |

6 | 648.5 | 68.8 | 612.5 | 64.8 | 146.3 | 64.8 | 526.5 | 48.0 | 48.0 | 20.0 | |

7 | 720.6 | 68.8 | 680.6 | 64.8 | 162.5 | 64.8 | 585.0 | 48.0 | 48.0 | 20.0 | |

8 | 864.7 | 68.8 | 816.7 | 64.8 | 195.0 | 64.8 | 702.0 | 48.0 | 48.0 | 20.0 | |

9 | 960.7 | 68.8 | 907.4 | 64.8 | 216.7 | 64.8 | 780.0 | 48.0 | 48.0 | 20.0 | |

10 | 1080.9 | 68.8 | 1020.8 | 64.8 | 243.8 | 64.8 | N/A | N/A | N/A | N/A | |

11 | 1201.0 | 68.8 | 1134.2 | 64.8 | 270.8 | 64.8 | N/A | N/A | N/A | N/A | |

8 stations IEEE 802.11 ax | 4 stations IEEE 802.11 ac | ||||||||||

0 | 36.0 | 76.8 | 34.0 | 64.8 | 8.1 | 64.8 | 58.5 | 48.0 | 48.0 | 20.0 | |

1 | 72.1 | 76.8 | 68.1 | 64.8 | 16.3 | 64.8 | 117.0 | 48.0 | 48.0 | 20.0 | |

2 | 108.1 | 72.8 | 102.1 | 64.8 | 24.4 | 64.8 | 175.5 | 48.0 | 48.0 | 20.0 | |

3 | 144.1 | 72.8 | 136.1 | 64.8 | 32.5 | 64.8 | 234.0 | 48.0 | 48.0 | 20.0 | |

4 | 216.2 | 68.8 | 204.2 | 64.8 | 48.8 | 64.8 | 351.0 | 48.0 | 48.0 | 20.0 | |

5 | 288.2 | 68.8 | 272.2 | 64.8 | 65.0 | 64.8 | 468.0 | 48.0 | 48.0 | 20.0 | |

6 | 324.3 | 68.8 | 306.3 | 64.8 | 73.1 | 64.8 | 526.5 | 48.0 | 48.0 | 20.0 | |

7 | 360.3 | 68.8 | 340.3 | 64.8 | 81.3 | 64.8 | 585.0 | 48.0 | 48.0 | 20.0 | |

8 | 432.4 | 68.8 | 408.3 | 64.8 | 97.5 | 64.8 | 702.0 | 48.0 | 48.0 | 20.0 | |

9 | 480.4 | 68.8 | 453.7 | 64.8 | 108.3 | 64.8 | 780.0 | 48.0 | 48.0 | 20.0 | |

10 | 540.4 | 68.8 | 510.4 | 64.8 | 121.9 | 64.8 | N/A | N/A | N/A | N/A | |

11 | 600.4 | 68.8 | 567.1 | 64.8 | 135.4 | 64.8 | N/A | N/A | N/A | N/A | |

16 stations IEEE 802.11 ax | 4 stations IEEE 802.11 ac | ||||||||||

0 | 17.2 | 84.8 | 16.3 | 64.8 | 8.1 | 64.8 | 58.5 | 48.0 | 48.0 | 20.0 | |

1 | 34.4 | 84.8 | 32.5 | 64.8 | 16.3 | 64.8 | 117.0 | 48.0 | 48.0 | 20.0 | |

2 | 51.6 | 76.8 | 48.8 | 64.8 | 24.4 | 64.8 | 175.5 | 48.0 | 48.0 | 20.0 | |

3 | 68.8 | 76.8 | 65.0 | 64.8 | 32.5 | 64.8 | 234.0 | 48.0 | 48.0 | 20.0 | |

4 | 103.2 | 72.8 | 97.5 | 64.8 | 48.8 | 64.8 | 351.0 | 48.0 | 48.0 | 20.0 | |

5 | 137.6 | 72.8 | 130.0 | 64.8 | 65.0 | 64.8 | 468.0 | 48.0 | 48.0 | 20.0 | |

6 | 154.9 | 72.8 | 146.3 | 64.8 | 73.1 | 64.8 | 526.5 | 48.0 | 48.0 | 20.0 | |

7 | 172.1 | 72.8 | 162.5 | 64.8 | 81.3 | 64.8 | 585.0 | 48.0 | 48.0 | 20.0 | |

8 | 206.5 | 72.8 | 195.0 | 64.8 | 97.5 | 64.8 | 702.0 | 48.0 | 48.0 | 20.0 | |

9 | 229.4 | 72.8 | 216.7 | 64.8 | 108.3 | 64.8 | 780.0 | 48.0 | 48.0 | 20.0 | |

10 | 258.1 | 72.8 | 243.8 | 64.8 | N/A | N/A | N/A | N/A | N/A | N/A | |

11 | 286.8 | 72.8 | 270.8 | 64.8 | N/A | N/A | N/A | N/A | N/A | N/A |

1 | 2 | 3 | 4 | 5 | |||||||
---|---|---|---|---|---|---|---|---|---|---|---|

DL MU data Transmission rate in 11ax | UL MU-MIMO BAck transmission rate in 11ax | UL OFDMA BAck transmission rate in 11ax | DL MU-MIMO data transmission rate in 11ac | UL BAck transmission rate in 11ac | |||||||

MCS | PHY Rate (MBps) GI = 0.8 μs | Preamble (μs) | PHY Rate (MBps) GI = 1.6 μs | Preamble (μs) | PHY Rate (MBps) GI = 1.6 μs | Preamble (μs) | PHY Rate (MBps) GI = 0.8 μs | Preamble (μs) | PHY Rate (MBps) | Preamble (μs) | |

32 stations IEEE 802.11 ax | 4 stations IEEE 802.11 ac | ||||||||||

0 | 8.6 | 104.8 | 8.1 | 64.8 | 1.7 | 64.8 | 58.5 | 48.0 | 48.0 | 20.0 | |

1 | 17.2 | 104.8 | 16.3 | 64.8 | 3.3 | 64.8 | 117.0 | 48.0 | 48.0 | 20.0 | |

2 | 25.8 | 84.8 | 24.4 | 64.8 | 5.0 | 64.8 | 175.5 | 48.0 | 48.0 | 20.0 | |

3 | 34.4 | 84.8 | 32.5 | 64.8 | 6.7 | 64.8 | 234.0 | 48.0 | 48.0 | 20.0 | |

4 | 51.6 | 80.8 | 48.8 | 64.8 | 10.0 | 64.8 | 351.0 | 48.0 | 48.0 | 20.0 | |

5 | 68.8 | 80.8 | 65.0 | 64.8 | 13.3 | 64.8 | 468.0 | 48.0 | 48.0 | 20.0 | |

6 | 77.4 | 80.8 | 73.1 | 64.8 | 15.0 | 64.8 | 526.5 | 48.0 | 48.0 | 20.0 | |

7 | 86.0 | 80.8 | 81.3 | 64.8 | 16.7 | 64.8 | 585.0 | 48.0 | 48.0 | 20.0 | |

8 | 103.2 | 80.8 | 97.5 | 64.8 | 20.0 | 64.8 | 702.0 | 48.0 | 48.0 | 20.0 | |

9 | 114.7 | 80.8 | 108.3 | 64.8 | 22.2 | 64.8 | 780.0 | 48.0 | 48.0 | 20.0 | |

10 | 129.0 | 80.8 | 121.9 | 64.8 | N/A | N/A | N/A | N/A | N/A | N/A | |

11 | 143.4 | 80.8 | 135.4 | 64.8 | N/A | N/A | N/A | N/A | N/A | N/A | |

64 stations IEEE 802.11 ax | 4 stations IEEE 802.11 ac | ||||||||||

0 | 3.8 | 136.8 | 3.5 | 64.8 | 0.8 | 64.8 | 58.5 | 48.0 | 48.0 | 20.0 | |

1 | 7.5 | 136.8 | 7.1 | 64.8 | 1.7 | 64.8 | 117.0 | 48.0 | 48.0 | 20.0 | |

2 | 11.3 | 100.8 | 10.6 | 64.8 | 2.5 | 64.8 | 175.5 | 48.0 | 48.0 | 20.0 | |

3 | 15.0 | 100.8 | 14.2 | 64.8 | 3.3 | 64.8 | 234.0 | 48.0 | 48.0 | 20.0 | |

4 | 22.5 | 88.8 | 21.3 | 64.8 | 5.0 | 64.8 | 351.0 | 48.0 | 48.0 | 20.0 | |

5 | 30.0 | 88.8 | 28.3 | 64.8 | 6.7 | 64.8 | 468.0 | 48.0 | 48.0 | 20.0 | |

6 | 33.8 | 88.8 | 31.9 | 64.8 | 7.5 | 64.8 | 526.5 | 48.0 | 48.0 | 20.0 | |

7 | 37.5 | 88.8 | 35.4 | 64.8 | 8.3 | 64.8 | 585.0 | 48.0 | 48.0 | 20.0 | |

8 | 45.0 | 88.8 | 42.5 | 64.8 | 10.9 | 64.8 | 702.0 | 48.0 | 48.0 | 20.0 | |

9 | 50.0 | 88.8 | 47.2 | 64.8 | 11.1 | 64.8 | 780.0 | 48.0 | 48.0 | 20.0 | |

10 | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | |

11 | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A | N/A |