802.11ax

Prior to 802.11ax, the individual MPDUs must all be of the same 802.11e QoS access category when A-MPDU frame aggregation is used. Voice MPDUs cannot be mixed with Best Effort or Video MPDUs within the same aggregated frame. 80211ax introduces Multi-Traffic Identifier Aggregated MAC Protocol Data Unit (Multi-TID AMPDU), which allows the aggregation of frames from multiple traffic identifiers (TIDs), from the same or different QoS access categories. The ability to mix MPDUs of different QoS traffic classes allows 802.11ax radios to aggregate more efficiently, reducing overhead and thus increasing throughput and therefore overall network efficiency.

The AP still has to compete against legacy 802.11 stations for a transmission opportunity (TXOP). Once the AP has a TXOP, the AP is then in control of up to nine 802.11ax client stations for either downlink or uplink transmissions on a 20 MHz channel. The number of RUs used can vary on a per TXOP basis. For the uplink (UL-OFDMA), the AP signals to the clients a synchronized schedule and RU allotment using a trigger frame.

The 802.11ax draft proposes an operating mode indication (OMI) procedure for this purpose. A client station can signal to the AP, the maximum number of spatial streams and the maximum channel bandwidth that the client can support for either uplink or downlink transmission. As shown in Figure 2, an 802.1ax client uses the OM Control subfield in 802.11 data and management frames to indicate a change of either transmission or receiver mode of operation. For example, a client can switch between single-user or multi-user UL-OFDMA operations. A client can suspend and resume responses to the trigger frames sent by an AP during the UL-OFDMA process.

subcarrier spacing = 78.125kHz 20M/78.125=256 subcarriers

有效的就是242 subcarrier.

subdividing a 20 MHz channel, an 802.11ax access point can designate 26, 52, 106, and 242 subcarrier resource units (RUs), which equates roughly to 2 MHz, 4 MHz, 8 MHz, and 20 MHz channels.

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