Hello, everyone!
Today, I would like to continue on the novel features of IEEE 802.11ac amendment in WLAN.
Downlink Multiuser MIMO.
The fundamental difference between the IEEE 802.11ac and IEEE 802.11n amendments is that the latter now supports MU-MIMO transmissions in the downlink, allowing multiple simultaneous transmissions from the AP to distinct STAs. The AP can have up to eight antennas and send up to four spatial streams to two distinct users, or two spatial streams to four different users at the same time, according to the IEEE 802.11ac amendment.
An IEEE 802.11ac AP specifies the group of STAs to which a multi-user transmission is directed when it executes a multi-user transmission. The new IEEE 802.11ac PHY headers, which are broadcast omni-directionally to all STAs, carry this information.
The AP decides how STAs are grouped after receiving channel state information (CSI) feedback from all STAs. IEEE 802.11ac only considers an explicit channel sounding feedback technique called Explicit Compressed FeedBack to collect CSI information by the AP (ECFB).
The EDCA regulates channel access (Enhanced Distributed Channel Access). Because only one AC may be provided for each transmission attempt, the numerous access categories (AC) maintained by the AP should compete for the channel medium at each transmission attempt.
If the queue connected with the AC that won the internal contention does not include enough packets to enough various destinations to all of the available spatial streams, the remaining ones can be shared with the other ACs.
Packet Aggregation.
IEEE 802.11ac permits the transmission of several MPDUs aggregated in a single A-MPDU to boost the efficiency of each transmission by minimizing unnecessary overheads. Then, to acknowledge each MPDU individually, a Block ACK packet is utilized, which contains a bitmap to indicate that all contained MPDUs were correctly received.
As a result of the information in the Block ACK, the transmitter can selectively retransmit only the MPDUs that have failed rather than the entire A-MPDU.
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