Hello everyone,
Today I will share with you Packet Extension in 802.11ax.
802.11ax uses 78.125 kHz subcarriers. The number of subcarriers is four times higher than that of 802.11ac 312.5 kHz. 802.11ax needs to improve the receiving processing speed by at least four times. Therefore, the 802.11ax protocol stipulates that the transmitter needs to add a PE at the end of the packet so that the receiver has sufficient time to receive the packet, reducing the complexity of the receiver manufacturing.
According to the protocol, PEs are discrete-time segments, including 0 μs, 4 μs, 8 μs, 12 μs, and 16 μs. The PE sends data blocks at the average transmit power. The padding content in the PE may be arbitrary. This means that the receiver does not parse the content in the PE. The content is used only to occupy the air interface time. This prevents the receiver from processing data in a timely manner due to a high speed of data transmission.
The following figure shows the position of the PE in the PPDU (using the HE SU PPDU as an example).
For an AP, the AP calculates the packet receiving capability of the AP when it functions as a receiver and forms a PE padding requirement for the STA. This field is advertised in the IEs of Beacon, Probe resp, and Assoc resp messages to inform the UE of the PE duration to be filled on the AP side. When an AP functions as a transmitter, the AP obtains the STA's PE capability from the STA association packet and delivers the PE capability to the MAC layer. When sending packets, the AP fills the PE field for different STAs based on the STA capability set.
As shown in the following flowchart, the PEs between the AP and the STA does not need to be the same, and the AP and the STA can perform negotiation according to their respective packet receiving capabilities. The corresponding data transmit end only needs to fill in the data according to the PE length required by the peer end before sending the data. The device needs to calculate and fill the length of the PE when sending packets and correctly advertise the PE capability in the corresponding management packets.
PEs can be used in fixed PEs or negotiated PEs.
Fixed PE: The PE required by the data receive end is fixed and does not change with the modulation mode, number of spatial streams, and RU allocation mode. The data transmit end only needs to transmit data according to the PE capability required by the receiving end. The fixed PE value ranges are 0 μs, 8 μs, and 16 μs. Because a fixed PE is only a strictest requirement for the PE specified by the data receive end, the PE padding length cannot be changed based on the modulation scheme, the quantity of spatial flows, and the RU allocation manner, the control on the PE is not precise enough, and overall performance is affected to a certain extent.
Negotiated PE: The data receive end sends its parameters for each modulation mode, each spatial flow, and each RU allocation mode to the data sender. The data sender calculates the PE based on the parameters and uses the PE during packet transmission. Compared with fixed PEs, the negotiated PEs can be 0 μs, 4 μs, 8 μs, 12 μs, or 16 μs. PEs of different lengths can be filled in different packet transmission scenarios, thereby achieving more precise control and improving air interface efficiency.
That is all I want to share with you! Thank you!
