Date: 2010-05-18

1. Date: 2010-05-18 [TXOP Enhancement for DL MU-MIMO Support] Authors: Slide 1

2. Abstract A Multi-User Transmit Opportunity (MU-TXOP) mechanism is proposed to extend the existing 802.11 TXOP concept to support DL MU-MIMO.

3. The Limitation of 802.11 TXOP The current EDCA TXOP mechanism (referred to as SU-TXOP) is per AC based. A SU-TXOP obtained by a STA can only be used to transmit frames belonging to the AC that won the TXOP. A STA may initiate multiple frame exchange sequences to transmit MMPDUs and/or MSDUs during a SU-TXOP, but only within the same AC. Internal contention among EDCAFs of different ACs allows only one EDCAF to win the internal competition for the TXOP. These rules would not allow multiple frames belonging to different ACs to be transmitted simultaneously.

4. Definitions The following terms are defined to simplify the discussion Primary AC: the AC that wins the TXOP for channel access after both external and internal competition. There could be only one primary AC at any time. Secondary AC: an AC that does not win a TXOP but wants to share the TXOP obtained by the primary AC for simultaneous transmissions. There could be multiple secondary ACs at any time. Primary Destinations: destinations targeted by the frames belonging to the primary AC. There could be one or more primary destinations at any time. Secondary Destinations: destinations targeted by the frames belonging to secondary ACs. There could be one or more secondary destinations at any time.

5. Main Ideas Each EDCAF of the AP’s MAC layer competes for TXOP as in current spec. Once an EDCAF wins a TXOP it becomes the owner of the TXOP its corresponding AC becomes Primary AC, and Its corresponding destination(s) become Primary Destination(s). The Primary AC can share the TXOP with Secondary ACs that have data in their buffers (the TXOP becomes MU-TXOP). Group the Secondary Destinations together with the Primary Destination(s) for simultaneous transmission (e.g. using GroupID). When sharing, a secondary AC may transmit even though that AC has not won contention and the backoff counter for that AC is not zero.

6. More on MU-TXOP In this presentation, EDCA MU-TXOP for the downlink is discussed. A MU-TXOP can only be assigned to a MU-MIMO capable AP STA. An EDCA MU-TXOP has three modes the initiation of the EDCA MU-TXOP occurs when the EDCA rules permit access to the medium this is kept the same as in the current spec the sharing of the EDCA MU-TXOP occurs after an EDCAF is granted the TXOP this will be a new mode of TXOP the multiple frame transmission within an EDCA MU-TXOP occurs when an EDCAF retains the right to access the medium following the completion of multiple frame exchange sequences, each for one receiving STAs. this is enhanced to allow multiple, simultaneous frame exchange sequences.

7. Mode #1: Initiation If an EDCAF’s backoff timer counts down to zero when its slot boundary is reached, the EDCAF is allowed to access the medium. This AC becomes the Primary AC if it also wins the internal competition. An EDCA TXOP is granted to this EDCAF. Note the rules the AP uses to contend for the channel access among all other STAs remain the same as current ones. Each AC of the AP uses a set of EDCA parameters of its own, e.g. AIFS[AC]. E.g. the AP should not always use parameters of its highest priority AC to contend for channel access. This maintains fairness among AP and other non-AP STAs.

8. Mode #2: Sharing (1/2) If the primary AC decides to share the TXOP with secondary ACs, the TXOP becomes MU-TXOP. Sharing is not always possible or appropriate Frames for multiple STAs must exist DL MU-MIMO transmission must be possible counter example: two or more ACs have frames but they are heading to the same STA. DL MU-MIMO transmission must be appropriate counter example: two or more destinations are too close to each other space wise Sharing is done by grouping primary destination(s) with secondary destination(s) Grouping policy is an implementation issue. GroupID or other methods can be used to identify the selected destinations.

9. Mode #2: Sharing (2/2) Frames may be transmitted from a secondary AC queue if that queue is sharing the TXOP, even if the backoff counter for that secondary AC is not at zero. Internal competition for TXOP is resolved by allowing secondary ACs transmit in the same TXOP of the primary AC. When parallel transmissions is not possible, e.g. two or more ACs want to transmit to the same destination, the lower priority AC needs to backoff as in the current spec.

10. Illustration: Sharing the TXOP Figure. 1

11. Mode #3: Frame Exchange During an EDCA MU-TXOP won by an EDCAF, the AP may initiate multiple frame exchange sequences for frames belonging to the same or different ACs. Propose to use same GroupID during the entire TXOP. Using the same GroupID benefits power saving. The length of the TXOP is determined by the primary AC Once the primary AC finishes its transmission, the MU-TXOP is ended, even if secondary ACs still have frames to send. The MU-TXOP can be ended by the AP sending a CF-End frame if the remaining time is enough for the transmission. This is for the fairness of other STAs. All STAs contend for the TXOP again after the MU-TXOP ends.

12. Illustration: Frame Exchanges Figure. 2

13. Summary MU-TXOP breaks the current limitation of TXOP and enables simultaneous transmissions of multiple traffic streams at the MAC layer. Expand current two EDCA TXOP modes to three A “share” mode is added after the initiation of the TXOP. ACs are divided into two classes, primary AC and secondary AC for each obtained MU-TXOP. MU-TXOP is obtained using EDCA parameters of the primary AC but may be shared by multiple traffic streams, which may or may not belong to the same AC. Same GroupID is used during the entire MU-TXOP Internal collision can be partially resolved by TXOP sharing. Secondary ACs’ channel access rules need to be changed. Secondary ACs can transmit their frames as long as their transmission times are not longer than the transmission time of the primary AC.