1. Considerations on MU-MIMO Protection in 11ac
Month Year
doc.: IEEE yy/xxxxr0
Considerations on MU-MIMO Protection in 11ac
Date:
Authors:
Slide 1
ETRI

2. Introduction
MU-MIMO is considered to be supported in 11ac for improving system performance
11ac devices should protect its transmission from legacy devices operating in 5GHz
Protection mechanism for MU-MIMO data transmission should be considered
It is necessary to start to consider possible mechanism for NAV distribution for MU-MIMO protection

3. Downlink MU-MIMO functions
General Downlink MU-MIMO functions
Sounding
Group control schemes
NAV distribution  Scope of this contribution
Beamformed data transmissions
Response phase

4. NAV distribution for legacy protection
Month Year
doc.: IEEE yy/xxxxr0
NAV distribution for legacy protection
It is necessary to distribute NAV information to reserve the medium for a transmission of a nonbasic rate frame
Using RTS/CTS exchange
The exchange of RTS and CTS frames prior to the actual data frame is one means of distribution of this medium reservation information
Using CTS-to-Self for NAV distribution
A CTS frame with the RA field equal to its own MAC address (CTS-to-Self) and with a duration value that protects the pending transmission, plus possibly an ACK frame is transmitted at first
CTS-to-self is less robust against hidden nodes and collisions than RTS/CTS
John Doe, Some Company

5. Overview of NAV distribution in MU-MIMO
Month Year
doc.: IEEE yy/xxxxr0
Overview of NAV distribution in MU-MIMO
By medium access rules
NAV protection
NAV distribution
MU-MIMO data transmission AP
STAs
Response frames
xIFS time
xIFS time
NAV distributing frames contain a Duration field that defines the period of time that the medium is to be reserved to transmit the actual beamformed data frame and the returning response frames.

6. Possible approaches for NAV distribution using RTS/CTS in MU-MIMO
We are considering 3 different NAV distribution mechanisms for MU-MIMO transmissions
Multiple RTS/CTS exchanges
RTS and multiple CTS exchanges
CTS-to-self frame transmission
Slide 6

7. Multiple RTS/CTS exchanges for NAV distribution in MU-MIMO
By medium access rules
NAV protection AP
RTS
RTS
RTS
MU-MIMO data transmission
SIFS
Response frames
SIFS time
STA1
SIFS
CTS
STA2
xIFS time
CTS
xIFS time
STA3
xIFS time
CTS
xIFS time
NAV distribution
Pros.
Using existing RTS and CTS frame formats
Supporting robust protection
Cons.
Having overhead problem for NAV distribution by using multiple RTS/CTS exchanges
Slide 7

8. RTS and multiple CTS exchanges for NAV distribution in MU-MIMO
By medium access rules
NAV protection AP
RTS
MU-MIMO data transmission
Response frames
xIFS time
STA1
CTS
xIFS time
STA2
CTS
xIFS time
xIFS time
STA3
CTS
xIFS time
NAV distribution
NAV distributing
Pros.
Using existing CTS frame format
Supporting robust protection as same as the previous solution
Having less overhead problem for NAV distribution than the previous solution
Cons.
Using RTS frame with multiple destination information
Requiring accurate timing information to transmit each CTS frame without any transmitting address
Slide 8

9. CTS-to-self frame transmission for NAV distribution in MU-MIMO
By medium access rules
NAV protection AP
CTS
MU-MIMO data transmission
Response frames
STA1
STA2
xIFS time
STA3
xIFS time
NAV distribution
Pros.
Significantly reduces overhead compared with the previous solutions
Cons.
Supporting less robust against hidden nodes and collisions than the previous solutions
Slide 9

10. Considerations on sounding exchange with NAV distribution mechanisms
Example of sounding exchange with RTS/CTS
Pros.
Reduces the overhead for sounding exchange
Cons.
CTS carried in a staggered sounding PPDU is may not be broadly received
does not provide robust protection
Separating the sounding exchange from the NAV distribution seems to be a better approach
By medium access rules
TRQ AP
RTS
Beamformed data transmission
SIFS time
SIFS time
STA
CTS
Response frame
Sounding PPDU
SIFS time
Slide 10

11. Conclusion
Protection mechanisms for MU-MIMO transmission were considered
Possible approaches for NAV distribution using RTS/CTS in MU-MIMO
Multiple RTS/CTS exchanges
RTS and multiple CTS exchanges
CTS-to-self frame transmission
There are cons and pros for the above approaches
Need additional considerations for better solutions for NAV distribution
Need to separate the sounding exchange from NAV distribution
NAV distribution mechanisms carried in non-sounding PPDUs to protect the MU-MIMO data transmission and response frames seems to be a better approach
It is necessary to start discussing possible MU-MIMO protection mechanisms in TGac
Slide 11