# A Novel QoS-Aware ARQ Scheme for Multi-User Transmissions in IEEE802.11 ax WLANs
## 3 Methods  
### The Architecture of the BP-ARQ Scheme  
In this section, we design a novel BP-ARQ scheme for MU transmissions in IEEE802.11 ax WLANs. The BP-ARQ scheme is extended from the BAW-ARQ because the BAW-ARQ is superior to the ASR-ARQ for MU transmissions according to the comparison in Section 4. Similar to the BAW-ARQ, the proposed scheme prepares the packets that satisfy the limitations of the duration, size, and aggregation window to be aggregated in the A-MPDU for each user in the MU transmission. These packets are called the prepared packets in the following analysis. The prepared packets in each A-MPDU have different sequence numbers and are arranged in order of sequence number. The sequence number of the l-th packet is defined as \(SN_{l}\), where \(1\leq l\leq L\), and L donates the number of prepared packets in the A-MPDU. The proposed scheme lengthens the A-MPDU of a user by attaching replicates of packets selected from the prepared ones instead of padding bits to align with the scheduled duration of the MU transmission when constructing the A-MPDU. The replicate of a packet is called the backup. In the proposed scheme, the number of backups for each packet can vary, so that the backups can fill as much of the remaining duration as possible. The IEEE802.11 ax devices that construct A-MPDUs according to the procedure of the BP-ARQ scheme are treated as the enhanced devices in this article. The BP-ARQ scheme is considered protocol-friendly because the enhanced devices can communicate with the traditional IEEE802.11 ax ones normally. In the BP-ARQ scheme, the major modification of the protocol is to include backups in the A-MPDU, and other designs follow the rules of the MU transmission and A-MPDU aggregation. The modification can be implemented only in the originator of the A-MPDU, and the BP-ARQ scheme does not need to modify the procedure of receiving an A-MPDU in the recipient. The recipient can handle the A-MPDU as usual. Each sub-frame of the A-MPDU is processed independently. The order of received packets can be rearranged by the reordering buffer. The ability to detect and filter out a repeated frame is also mandatory in the IEEE802.11 ax protocol because the successfully received packets are also retransmitted when the BlockACK/ACK frame is lost. Therefore, the traditional devices are equipped with the ability to receive and handle backups normally. When the original sub-frame fails, the backup sub-frames in the same A-MPDU can act as if the packet is retransmitted. Consequently, the replicated packet is lost only when errors caused by the channel noise occur in both the original sub-frame and its backups simultaneously. The equivalent packet error rate (PER) of the l-th packet that is replicated for \(r_{l}\) times is shown as follows:  
\[P_{e,l}=e^{1+r_{l}}, \tag{1}\]  
where \(e\) represents the sub-frame error rate and is derived by  
\[e=1-(1-ber)^{(len_{MACheader}+len_{payload})}, \tag{2}\]  
where \(len_{MACheader}\) is the length of the MPDU header, and \(len_{payload}\) is the length of the average payload. The \(ber\) is the bit error rate (BER), which reflects the characteristics of the physical channel occupied by the user. Thus, the proposed scheme enhances the successfully delivered possibility of the replicated packets under the error-prone channels. Moreover, the higher successfully transmitted probability of the replicated packets also alleviates the blocking problem of the reordering buffer because the possibility that the replicated packets block the sliding procedure is also reduced. Figure 5 depicts an example of the BP-ARQ scheme where the remaining duration is enough for four backups in the first transmission. The BP-ARQ scheme utilizes the remaining duration of the user to transmit backups. Even if the BP-ARQ scheme is not implemented, the originator transmits the padding bits in the remaining duration. Therefore, the transmission of backups does not consume extra energy.