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Short Block Ack. Date: 2012-07-16. Authors:. Introduction. 11ah STAs may have limited capabilities L ow TX power, low transmission rates, limited buffers capacity Sleep most of the time and wake up to TX/RX limited bursts of MPDUs

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short block ack
Short Block Ack

Date: 2012-07-16

Authors:

introduction
Introduction
  • 11ah STAs may have limited capabilities
    • Low TX power, low transmission rates, limited buffers capacity
    • Sleep most of the time and wake up to TX/RX limited bursts of MPDUs
  • Power imbalance between heterogeneous TX/RX STAs implies issues with MCS selection in response frames
  • Optimized energy saving requires optimized frame formats
  • Short frames were introduced, which improve MAC efficiency and simplify control response MCS selection [1, 2]
    • Short CTS and short Ackdefined as a PHY preamble only
  • An important control response frame is missing among them: the Block Ack
block acknowledgement
Block Acknowledgement
  • BA is useful in 11ah
    • Aggregation is possible/likely at 1 and >=2MHz rates: BA enables it
      • (~8 MPDUs@4Mbps and ~16 MPDUs@8Mbps, 1KB MPDU, TXOP 20ms)
    • BA allows to reduce the number of transmissions
      • E.g. a sensor sending a single BA to acknowledge multiple PPDUs, instead of sending multiple ACKs
  • However, the benefits may be significantly reduced when used in 802.11ah
    • Increased TX times for sending BA frames
      • Uncompressed Block Ack (TX time of~8.68ms@1MHzRep2)
      • Compressed Block Ack (TX time of ~2.28ms@1MHzRep2)
  • The proposed short Block reduces the TX time and energy consumption
    • Block ACK information stored in SIG field of PHY Preamble
      • Same approach as for short Ack frames [1]
    • TX time of 560us @ 1MHz, 240us @ >=2Mhz
block ack mcs selection rules
Block Ack MCS selection rules
  • Block Ack is a control response frame
    • Must obey rate selection rules imposed to control response frames
      • Must be sent with the highest MCS in the Basic Rate Set which is lower or equal to the MCS of the soliciting frame
    • Likely power imbalance between AP and STA causes problems when this rules applies
      • BA frames may not be received by the AP due to high MCS selected by above rule and low transmit power
  • Using short control response frames eliminates the need for rate selection rules
    • NDP frames consist only of the PHY preamble
    • ACK[1], CTS[2] already accepted
  • Short BA completes the set of short control response frames, eliminating the issue with MCS selection rules
short block ack1
Short Block ACK
  • Reload bits in SIG Field in the PHY Header to store Block Ack info
    • Short Block Ack indicated using one reserved value of MCS field (e.g., 11)
    • 22 reloadable bits for 1 MHz PHY Header format
    • 34 reloadable bits for 2 MHz PHY Header format
  • Use available bits in the SIG field for
    • Block ACK ID
    • Starting Sequence Control
    • Block Ack Bitmap

Basic

BA frame

Short

BA frame

STF

LTF1

SIG

Starting Sequence Control

Block Ack Bitmap

SIG

Block Ack ID

MCS, CRC, Tail

short block ack details
Short Block Ack Details
  • No modifications required to the Block Ack protocol
    • Buffer size is negotiated during Block Ack session initiation
      • Buffer size specified by the Responder in the ADDBA Response is binding
  • Short Block Ack to be used for immediate Block Ack
    • Delayed block Ack is generally not used anyways
  • Short Block ACK frame includes
    • Block Ack ID (TBD)
    • Starting Sequence Control (12 bits)
      • Include Starting Sequence Number only
    • Block Bitmap (8/16 bits)
      • 8 bits for 1 MHz and 16 bits for 2MHz PHY Preamble
    • Other fields (TBD)
conclusions
Conclusions
  • We propose a new short frame for Block Ack, using NDP format
    • Use a reserved MCS value to identify this frame type
    • Includes 8 or 16-bit bitmap (for 1 or 2 MHz)
    • No change to current BA protocol, only frame format change
  • Benefits
    • Resolve issue with MCS selection in case of UL/DL link imbalance
    • Reduce BA time from ~2.8ms to 560us at 1MHz rep2
  • Further developments
    • Same frame format could be used also for fragments acknowledgment (TBD)
references
References

[1] 11-12-0324-02-00ah-short-ack

[2] 11-12-0643-00-00ah-short-cts

straw poll 1
Straw Poll 1
  • Do you support to include in the specification framework the definition of a new short Block Ack frame

with

    • Block ACK ID (length TBD)
    • Starting Sequence Control (12 bits)
    • Block Bitmap with length (TBD)
    • Other fields (TBD)

STF

LTF1

SIG