A New Aggregation based Scheduling method for rapidly changing IEEE 802.11ac Wireless channels

In this paper we suggest a novel idea to improve the Throughput of a rapidly chang- ing WiFi channel by exploiting the standard aggregation schemes in IEEE 802.11ac networks, and by transmitting several copies of the same MPDU(s) in a single trans- mission attempt. We test this idea in scenarios where Link Adaptation is not used and show a significant improvement, in the order of tens of percents, in the achieved Throughput. Keywords


Introduction
The IEEE 802.11 Standard (WiFi), created and maintained by the IEEE LAN/MAN Standards Committee (IEEE 802) [1], is currently the most important solution within the range of Wireless Local Area Networks (LAN). Since its first release in 1997 the standard provides the basis for Wireless network products using the WiFi brand, and it has been improved in many ways. One of the main goals of these improvements is to optimize the Throughput of the MAC layer, and to improve its Quality-of-Service (QoS) capabilities.
To fulfill the promise of increasing IEEE 802.11 performance and QoS capabilities, and effectively supporting more client devices on a network, the IEEE 802.11 working group introduced the fifth generation in IEEE 802.11 networking standards; namely, the IEEE Video, in this paper we consider several methods to further improve the Throughput of the wireless channel by using aggregation. We consider methods in which some MPDU(s) are retransmitted several times in a lossy channel and in a single transmission attempt. We do not assume the model of Transmission opportunities (TXOP) [1], and a station transmits only one PSDU in every transmission attempt. We are not aware of any other research using aggregation to retransmit several copies of the same MPDU(s) in a single transmission attempt in order to increase Throughput.
Improving the quality of the wireless channel is also possible through Link Adaptation (LA) methods, in which a more robust Modulation/Coding scheme (MCS) is used, at the cost of reducing the available PHY rate. However, there are scenarios in which the Signalto-Noise-Ratio (SNR) is either rapidly changing, or it is changed in small amounts (dBs). In these cases LA is not used, either because the channel's SNR is not stable and it is changing faster than the LA tracing capability, or the changes are too small to trigger LA. For these scenarios we suggest methods to improve the Throughput of the wireless channel by using the new aggregation schemes.
Notice that by retransmitting MPDUs one actually reduces the PHY rate. However, we suggest in this paper to retransmit only few MPDUs, i.e. we reduce the PHY rate for only few MPDUs and not for all MPDUs as in LA. We show that such a change increases the Throughput considerably.
Another important point to mention is that we improve the Throughput of the current IEEE 802.11ac standard which has an upper bound of 64 MPDUs on the Transmission window size. We do not look for solutions that change the current standard as e.g. increasing the above upper bound.
Finaly, our proposal is not mandatory in the sense that it should or should not be used by all the stations all together in a givan time. For example, stations that are close to the AP and have a high SNR and so a low Packet Loss Rate should not use it. Stations that are located far from the AP and has a high Packet Loss Rate should use the proposal. In our later results we show when the proposal has high benefit and should be used.

Our work
We consider a single pair of transmitter/receiver, over a WiFi wireless channel. Such a scenario is possible when the WiFi channel is used as a Point-to-Point Backhaul (Usage Models 4a, 4b in [4]). The transmitter transmits MPDUs to the receiver using the above mentioned ARQ protocol, and using the A-MPDU/Two-Level aggregation schemes defined in the IEEE 802.11ac standard [1]. We assume a saturated scenario where the transmitter has an infinite number of MPDUs to transmit. We also assume UDP like traffic; the receiver does not transmit above Layer 2 acknowledgments such as TCP Acks to the transmitter.
It only transmits Layer 2 Acks. Therefore, the receiver does not contend on the channel and there are no collisions. As mentioned, we also do not assume the use in Transmission opportunities (TXOP) and only one PSDU is transmitted in every transmission event.
We investigate the performance of several methods to improve the Throughput by retransmitting several copies of the same MPDU(s) in a single transmission attempt, using aggregation. Blinded retransmission of several copies of the same MPDU(s) in a single transmission attempt has two advantages: First, the success probability of an MPDU that is retransmitted several times is improved. Second, the probability that the Transmission Window moves forward, therefore containing new MPDUs, is also increased. A disadvantage of this approach is increasing the transmission time of the PSDU frame by the same data bits. Investigated in this paper is whether this increase is beneficial.

Our results
We consider the A-MPDU and Two-Level aggregation schemes over several PHY rates: We show that the Throughput is improved using our methods, especially in low PERs, and the improvement can sometimes be in the order of tens of percentages ! Another important aspect of our proposed methods is that they are simple to implement and fully comply with the IEEE 802.11 standard [1].

Previous works
The performance of the IEEE 802.11 protocol has been investigated in dozens of papers over the years. We only mention a few of those that relate to our current research. The first set of papers deals with the basic access scheme of IEEE 802.11 . In [5,6] the Throughput and Delay performance of the legacy transmission mode (no aggregation) are investigated, with upper and lower limits set on the Throughput and Delay achievable [5]. In [7,8,9] an analytical study of the Throughput of the basic IEEE 802.11, together with collisions, is performed, taking into account the RTS/CTS control mechanism [1]. In [10,11,12,13,14] the performance of the legacy transmission mode using Block Ack and RTS/CTS is investigated.
In [15,16,17,18,19,20,21,22,23,24,25,26,27] the Throughput and Delay performance of the A-MSDU, A-MPDU and Two-Level aggregation schemes is investigated. Several papers assume an error-free channel with-no collisions, several papers assume an error-prone channel and some papers also assume collisions. In [28,29,30,31,32] the performance of IEEE 802.11ac is investigated. Papers [29,32] consider the performance of the aggregation schemes in IEEE 802.11ac and compare the performance of IEEE 802.11ac to that of IEEE 802.11n.
Another set of papers, e.g. [33,34,35,36,37,38], deals with QoS together with the aggregation schemes. In particular, in [38] the use of the ARQ protocol of the IEEE 802.11 standard [1], together with the aggregation schemes, is investigated in relation to QoS guarantee. In this paper we also investigate the use of the ARQ protocol with the aggregation schemes, but this time we investigate another aspect of the aggregation: Blinded retransmission of several copies of the same MPDU(s) in the same transmission attempt, in order to improve the Throughput when Link Adaptation is not used. As far as we know, such an aspect of the aggregation schemes has not previously been investigated.
The rest of the paper is organized as follows: In Section 2 we describe the network model used. In Section 3 we show the performance of our methods. Section 4 is a summary of the paper.

Successful transmissions
In Figure 1 Figure 1 repeats itself.
We assume that the transmitter is using the Best Effort Access Category and the following values are taken from the WiFi Alliance (WFA) publications [39]. The WFA is an organization that performs certification tests. The tests ensure reliability of the WiFi brand, and certification programs can be seen from the certified products.
In the Best Effort Access Category the AIFS is 43µs. The BackOff is a multiple of the SlotT ime size, which for the OFDM PHY layer is 9µs. We assume that there are no collisions and so, on average, one half of the minimum BackOff interval is used, i.e. 7.5 · 9 = 67.5µs.
The duration of the PHY preamble, preceding the PSDU transmission, is changed according to the number of spatial streams in use [1]. In this paper it is 43µs for the most part, corresponding to 3 spatial streams. The SIFS is 16µs. The Block Acknowledgment (BAck) frame is 32 bytes long. Its transmission time, denoted BAckTime, is 32µs, using the Basic PHY Rate of 24Mbps, and including the legacy PHY Preamble of 20µs. If the PHY rate R used for data frame transmissions is lower than 24Mbps then R is also used for the BAck transmission. However, in this paper we use Rs with higher values than 24Mbps.

The A-MPDU aggregation scheme
The A-MPDU aggregation scheme is shown in Figure 2. Several MAC Protocol Data Units

The Error model
We assume that the process of frame loss in a Wireless fading channel can be modeled with a good approximation by a low order Markovian chain, such as the two state Gilbert  [40,41].
In this model the state diagram is composed of two states, "Good" and "Bad", meaning successful or unsuccessful reception of every bit arriving at the receiver, respectively. Bit-Error-Rate (BER) is the probability of moving from the Good state to the Bad state. (1 − BER) is the probability of remaining at the Good state. According to the above model, the success probability of a frame of length B bits is (1 − BER) B and the failure probability p is given by Eq. 1: By the above model one can see that as the frame length B increases, so does its failure probability.
Notice that errors in the MacDelimiter field(s) in an A-MPDU frame can make the receiver unable to detect the starting point(s) of subsequent MPDUs. In this paper the shortest MPDUs that we consider are of 168 bytes and the longest are of 1540 bytes. The MacDelimiter is 4 bytes and in the worst case about 2 − 3% of the MPDU's length, when considering MPDUs of 168 bytes. For MPDUs of 1540 bytes it is about 0.3%. Therefore, and as observed in real systems, the probability of not detecting the next MacDelimiter after a corrupted MPDU is very slight, and is therefore not mentioned in this paper.

Proposed transmission methods
In the A-MPDU and Two-Level aggregation schemes it is possible to transmit up to 64 MP-DUs, with different sequence numbers, in an A-MPDU/PSDU frame [3]. In the compressed BAck frame described in section 8.3.1.9.3 in [1] is a Block Ack Bitmap field containing 64 bits. Every one bit in this field acknowledges the reception of one MPDU in increasing order of sequence numbers, starting from a sequence number that is also included in the frame.
This is the reason for the limit of 64 MPDUs with different sequence numbers per A-MPDU frame.
Let K be the maximum number of MPDUs, with different sequence numbers, that are actually allowed in a PSDU frame. K ranges between 1 to 64. Notice however, that the standard does not prohibit the transmission of more than 64 MPDUs per A-MPDU frame, as long as there are at most 64 different sequence numbers, and the transmission time of the PSDU is not larger than 5.4ms. This limit is derived from Eq. 9-12 in Section 9.26.4 in [1].
We base our proposed methods on this observation.
We are given an infinite sequence of MPDUs to transmit from transmitter A to receiver B. All MPDUs are of the same length. Every MPDU has a probability 1 − p to move successfully from A to B . This probability 1 − p is the same for all MPDUs and all MPDUs' transmissions are independent.
We also have a From Figure 1 we define C 1 to be C 1 = AIF S + BackOf f + P HY preamble + SIF S + BAckT ime ( BAckTime contains the PHY Preamble preceding the BAck transmission).
Assuming OFDM PHY layer, T sym is 4µs and BitP erSymbol equals 4. L is the MSDU's size in bytes and the additional 22 bits in the denominator are due to the SERVICE (16 bits) and TAIL (6 bits) fields that are added to every transmission by the PHY layer conv.
protocol [3]. Finally, P succ denotes the probability that an MPDU arrives successfully at the receiver.
The rationale behind our proposed methods is to blindly retransmit several copies of MPDUs at the beginning of the T W , in order to increase the probability that the T W will slide forward and will contain new MPDUs for later transmissions. We examine the cases of retransmitting only several copies of the 1st, 2nd, 3rd and 4th MPDUs in the T W respectively. These cases are denoted later by Set1, Set2, Set3 and Set4 respectively. As an extreme measure, we also check the possibility of retransmitting several copies of each of the MPDUs are transmitted in a transmission attempt. This is done in Set5 further on.
We now describe the five methods for retransmission of the MPDUs and compare between the Throughputs that these schemes achieve. We use this example in the description of the schemes below.
• Base : The X min MPDUs are transmitted, a single copy of each MPDU. In our example the transmission contains one copy of MPDUs 1, 3, 9, 10 .
• Set1 -1MPDU2, 1MPDU3, 1MPDU4, 1MPDU5: One copy of each MPDU in X min is transmitted once, except to MP DU min . In 1MPDU2 this MPDU is transmitted twice in every transmission attempt. In 1MPDU3i this MPDU is always transmitted 3 times in every transmission attempt, and so on until 5 times.

Performance results
Our performance results are based on simulations. In all the simulations we set W , the size of the Transmission Window, to be 64, the maximum possible in the IEEE 802.11ac standard [1]. We checked the Throughput for all possible K, the number of MPDUs in every transmission, 1 ≤ K ≤ 64, and picked the maximum Throughput that is achieved by any of the Ks.  MPDUs, such as 1540 bytes, this increase is more significant and therefore, retransmitting the 4th MPDU of this size is not efficient.
In Figure 5 we show the same results for PHY rate 3466.8 Mbps. In this case the 'penalty' for retransmitting MPDUs is lower, however the same relative results are still observed.

Performance of Set1-Set4
The next set of results corresponds to the A-MPDU aggregation scheme. In Figure 6 we consider the first set of methods, Set1, in which only MP DU min is retransmitted. We assume  the improvements over the Base method are 12% and 5% respectively.
In Figure 7 we show the same results as in Figure 6, but for Set2 of methods. For PHY rates 866.7 and 433.3 Mbps there is a more significant improvement over the Base method compared to Set1, but for the larger rates of 3466.8 and 1299.9 Mbps the improvement is much more significant: 25% and 15% for PER=0.5 respectively. Again, the retransmission of size 2, i.e. 2MPDU2 is the best method.
In Figure 8 we show the results for Set3 of methods. The results are similar to those for Set1 and Set2. Notice that for PER=0.5 the improvements in Throughputs over the Base method are 30% and 17% in the PHY rates 3466.8 and 1299.9 Mbps respectively.
In Figures 9 and 10 we show the results for Set1 and Set4 respectively, when the MPDU size is 168 bytes. We omit the results for Set3 and Set4 as they fall between Set1 and Set4.
One can see that in all the PHY rates there is an improvement in the Throughput over the Base method because the MPDUs are relatively short. Therefore, the penalty due to retransmissions is small compared to MPDUs of 1540 bytes. In Set1, Figure 9, the maximum

Performance of Set5
In Figures 11 and 12

Overall Throughput improvement
In Figures 13-16 Figure 13(A) and PER=0.5, it is most efficient to transmit each MPDU 5 times. At the same point in Figure 14 it is best to use Clearly, as the PER decreases, the improvement in the Throughput decreases as well.
The channel becomes reliable, the need for retransmissions is smaller and the penalty of longer PSDUs' transmission times is therefore more significant. The reason that smaller MSDUs show greater improvement (in percentage ) in the Throughput is due to the overhead associated with every transmission. As the MSDU's length is smaller, the transmission time of an MSDU is smaller, and the size of the overhead (C 1 in Eq. 2) is more significant.
When the size of the overhead is more significant, the penalty of transmitting the same MSDU several times is relatively lower and therefore, the improvement in percentage is more significant. When later discussing the Two-Level aggregation method we provide an analytical explanation to the argument above.
Overall, from show that the improvement in the Throughput is achieved over a wide range of BER values.

Throughput improvement in Two-Level aggregation
In Figure 18 we consider the Two-Level aggregation, an MSDU's size of 1500 bytes and 2-7 MSDUs per A-MPDU frame in Figure 18(A)-(F) respectively. We again compare between the Throughput of the Base method to the maximum achieved by all the new methods.
One can see in Figure 18 that as the number of MSDUs in an A-MPDU frame increases, Lets assume now X MSDUs per A-MPDU frame, where 3 ≤ X ≤ 7 and X = 2 · α, α > 1. In this case, for the Base method B becomes αB and T becomes αT . Recall that we consider the same PER ! For S it turns out that B s becomes αB s , and T + T s becomes α(T + T s ). Now the percentage of the improvement is as shown in Eq. 4: One can easily verify that for α > 1 the percentage of improvement is lower, and it decreases as α increases. Intuitively, given the same overhead C 1 , multiplying the PSDU's transmission time at S by α is more significant than at the Base method. This is because in S the PSDU's transmission time is larger. This reduces the attractiveness of S as α increases.          All,5 All,3 All,3 All,3 All,5 All,3 All,3 All,2