1. STA Antenna Indication in VHT Capabilities
Month Year
Month Year
doc.: IEEE yy/xxxxr0
doc.: IEEE yy/xxxxr0
STA Antenna Indication in VHT Capabilities
Date:
Slide 1
Brian Hart, Cisco Systems
Page 1
John Doe, Some Company
John Doe, Some Company

2. Executive Summary Situation
Via VHT Capabilities element, an AP can find out the minimum number of RX and TX antennas of a STA
A client might silently have additional antennas and use them for proprietary PHY processing
Problem
AP cannot make optimal decisions with silent antennas
E.g. a client switches antennas after sounding and AP sees unexpectedly lower BFing performance, and could lead AP to sound a channel more often than needed
Solution
We propose to redefine two reserved bits in “VHT Capabilities Info field” to provide indication of the possibility of antenna pattern change.

3. Situation: A STA advertizes min capabilities only
RX/TX MCS Map in VHT Supported MCS Set field in VHT Capabilities element lists the number of spatial streams and supported MCSs per SS
So AP can infer the minimum number of RX and TX antennas
Yet, there are many devices with more antennas than they indicate in their RX/TX MCS Map
The additional antennas are usually used for various PHY processing to enhance PHY layer robustness by antenna diversity techniques
802.11ac forces a STA to remain silent about its additional antennas

4. Problem: AP lacks info to make better decisions
There are devices with more RX antennas that perform antenna switching
Antenna switching causes the already sounded channel to be mostly useless for subsequent transmissions without AP knowing it, particularly for MU transmission
Frequent antenna switching could appear as channel change due to mobility, leading AP to interpret the associated client as a mobile STA and to sound the channel more frequently than it’s needed
Some STAs might switch RX antennas often, even per frame
A STA after antenna switching usually stays on the selected antenna(s), causing its TX antenna pattern to change without AP knowing it
This causes problem for the AP that performs advanced signal processing

5. Solution: Allow STA to clarify its antenna capabilities
1 bit: “RX Antenna Pattern Consistency” in VHT Capabilities element
Set to 0 if RX antenna pattern can change during association
Set to 1 if RX antenna pattern remains fixed during association
1 bit: “TX Antenna Pattern Consistency” in VHT Capabilities element
Set to 0 if TX antenna pattern can change during association
Set to 1 if TX antenna pattern remains fixed during association
B B15
B B18
B19
B20
B21
B22
B B25
B B27
B28
B29
B29-B31
Compressed Steering Number of BF Antennas Supported
Number of Sounding Dimensions
MU BFer Capable
MU BFee Capable
VHT TXOP PS
+HTC-VHT Capable
Max A-MPDU Length Exponent
VHT Link Adaptation Capable
RX Antenna
Pattern Consistency
TX Antenna
Reserved
Bits: 3 3 1 2