1. Proposal for enabling overlay FEC in GCR Block Ack
March 2011
doc.: IEEE /0293r1
March 2011
Proposal for enabling overlay FEC in GCR Block Ack
Date:
Authors:
Jochen Miroll, Saarland University
Jochen Miroll

2. Motivation To enable optional Coded Block Ack for Groupcast
March 2011
doc.: IEEE /0293r1
March 2011
Motivation
To enable optional Coded Block Ack for Groupcast
To reduce overhead of GCR polled Block Ack
Jochen Miroll, Saarland University
Jochen Miroll

3. Proposal Extend polled Block Ack by polled Coded Ack
March 2011
doc.: IEEE /0293r1
March 2011
Proposal
Extend polled Block Ack by polled Coded Ack
After completing a GCR Data transmission, an AP or Mesh STA will transmit Unicast BlockAckReq frames to (a subset of) GCR STAs participating in the GCR stream.
In Polled Coded Ack mode, the BlockAckReq contains an integer number k to be evaluated by the BlockAckReq addressed STA.
Each STA receiving BlockAckReq frame will respond with Ack if the Coding Condition is met.
The Coding Condition is met, if k frames of the preceding Block have been received.
Each STA receiving BlockAckReq frame will remain silent if the Coding Condition is not met.
Jochen Miroll, Saarland University
Jochen Miroll

4. March 2011
doc.: IEEE /0293r1
March 2011
Issues
How to handle moving the buffered frames from MAC to upper layers in GCR stream receiving STA?
Use same mechanism as defined for Block Ack and GCR Block Ack
How to determine the Coding Condition?
Coding Condition (as well as Block Ack block length e.g.) has to be determined by the AP or Mesh STA. Out of scope for TGaa.
For overlay FEC, there is typically only systematic data and parity. Could be a good use for the DEI bit. Out of scope for TGaa.
Multicast-Broadcast sequence numbers
AP or Mesh STA needs to add the number of interim MC/BC frames to k
Jochen Miroll, Saarland University
Jochen Miroll

5. Advantages This enables overlay FEC support at the MAC layer.
March 2011
doc.: IEEE /0293r1
March 2011
Advantages
This enables overlay FEC support at the MAC layer.
Overlay FEC is considered good practice for Multicast.
The scheme consumes less medium time compared to GCR Block Ack
Because Coded Block Ack BAR-ACK sequence is known from Delayed Block Ack, there will be no legacy client compatibility issue
Jochen Miroll, Saarland University
Jochen Miroll

6. Typical exchange with GCR polled Block Ack
March 2011
doc.: IEEE /0293r1
March 2011
Typical exchange with GCR polled Block Ack
Data
Data
Data
BAR
BAR AP … BA
GCR Group Member 1 BA
GCR Group Member 2
Not included as part of polled Ack
GCR Group Member 3
Slide taken from Suri, Hart, /1179r0
Jochen Miroll, Saarland University
Jochen Miroll

7. Typical exchange with polled Coded Ack
March 2011
doc.: IEEE /0293r1
March 2011
Typical exchange with polled Coded Ack
Data 1
Data 2
any
mgmt
Data k
BAR
BAR
BAR
Data k+1
(parity) AP …
ACK
GCR Group Member 1
Did not receive k+1
GCR Group Member 2
ACK
GCR Group Member 3
Not included as part of polled Ack
GCR Group Member 4
Slide adapted from Suri, Hart, /1179r0
Jochen Miroll, Saarland University
Jochen Miroll

8. March 2011
doc.: IEEE /0293r1
March 2011
Straw Poll
Should this scheme be adopted, extending the polled GCR Block Ack mechanism?
Yes - 1
No - 7
Abstain - 4
Jochen Miroll, Saarland University
Jochen Miroll

9. March 2011
Overlay FEC example
Usage of erasure codes (Vandermond-Code, Reed-Solomon)
Usage of „Virtual Interleaving“ (cf. ETSI)
Result is a Block-of-packets FEC code that has:
As input: a number of Bytes
As output: a number of equal length packets (e.g Bytes)
The property: out of n packets, any k packets are sufficient to decode n k
Redundancy Data Table
Application Data Table
Jochen Miroll, Saarland University

10. March 2011
Case 1: Packet loss
This case addresses the problem that within a block of upto 64 frames (GCR Block Ack), loss may be uncorrelated among the receivers
Jochen Miroll, Saarland University

11. Case 1: Packet loss, GCR … March 2011 D D D D D D D D D D D D D D D D
doc.: IEEE /0293r1
March 2011
Case 1: Packet loss, GCR
BAR start seq#
Same seq# as before, need retransm. of all frames D D D D
BAR
BAR
BAR
BAR D D D D
BAR
BAR AP D D D BA D D D BA
GCR1 D BA D D BA D D D
GCR2 … D D D BA D D D
GCR3 D D D BA D D D
GCR4
Jochen Miroll, Saarland University
Jochen Miroll

12. Case 1: Packet loss, coded GCR
March 2011
doc.: IEEE /0293r1
March 2011
Case 1: Packet loss, coded GCR
Did not get k
Everyone has k upto now
BAR start seq#
Did you get k?
Did not get k D D D D
BAR P
BAR P
BAR
BAR
BAR
BAR AP D D D P BA
GCR1 D BA D D P P
GCR2 D D D P BA
GCR3 D D D P P BA
GCR4
Jochen Miroll, Saarland University
Jochen Miroll

13. Case 2: Frame seq# mix w/ single counter
March 2011
Case 2: Frame seq# mix w/ single counter
This case addresses the problem that within a block of upto 64 frames (GCR Block Ack), an arbitrary number of frames using the same single sequence number counter (Management and Broadcast/Multicast frames) may be scheduled intermediately
Jochen Miroll, Saarland University

14. Case 2: Frame seq# mix w/ single counter
March 2011
doc.: IEEE /0293r1
March 2011
Case 2: Frame seq# mix w/ single counter
GCR as defined in P802.11aa D3.01, possible sequence (example)
BAR start seq#
BAR for one Data frame
D,1
MC,2
MGMT,3
MC,4 …
*,64
BAR …
D,65 AP
Proposed coded GCR, possible sequence (same example)
BAR for all Data frames, AP asks each receiver individually for „how many“ plus number of intermediate (red or blue)
BAR start seq#
D,1
MC,2
MGMT,3
MC,4 …
D,65
D,66
BAR AP
The AP knows how many MGMT or MC frames it has sent to each receiver, so it may ask each for a different number k+x. Assumes those x frames are indeed received.
Jochen Miroll, Saarland University
Jochen Miroll

15. March 2011
Rate Adaptation
Rate (MCS) adaptation based on immediate receiver feedback is possible in …
Unicast
GCR Block Ack
Coded GCR Block Ack
Yet, adding an overlay FEC onto a Groupcast transmission has to rely on receiver reports
=> Rate adaptation is out of scope for TGaa, but remains in the same way possible as in the above 1., 2. and 3.
Jochen Miroll, Saarland University

16. March 2011
Motion
Move to instruct the TGaa editor to incorporate normative text changes in 11/0294r1 into the P802.11aa Draft 3.01
Moved:
Seconded:
Result (Yes/No/Abstain):
Jochen Miroll, Saarland University