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STA Antenna Indication in VHT Capabilities

STA Antenna Indication in VHT Capabilities. Date: 2011-11-09. Slide 1. Executive Summary. Situation Via VHT Capabilities element, an AP can find out the minimum number of RX and TX antennas of a STA

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STA Antenna Indication in VHT Capabilities

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  1. STA Antenna Indication in VHT Capabilities Date: 2011-11-09 Slide 1 Brian Hart, Cisco Systems

  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

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