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HEW Beamforming Enhancements

HEW Beamforming Enhancements. Authors:. Date: 2013-07-3. Background. “Area throughput/Average Throughput per STA” is a new metric in HEW for dense deployment scenarios 1 Performance enhancement for cell edge has also been highlighted as a key issue in dense deployment 2

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HEW Beamforming Enhancements

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  1. HEW Beamforming Enhancements Authors: Date: 2013-07-3 James Wang (MediaTek)

  2. Background “Area throughput/Average Throughput per STA” is a new metric in HEW for dense deployment scenarios1 Performance enhancement for cell edge has also been highlighted as a key issue in dense deployment2 Current CSMA baseline channel access protocol is based on the omni-directional transmission There are some ambiguities related to beamformed transmission under different scenarios This contribution presents some possible performance beamforming enhancements for HEW James Wang (MediaTek) Slide 2

  3. Cell-Edge Performance Improved cell edge performance with TX or RX beamforming, more graceful degradation in dense deployment James Wang (Mediatek )

  4. Beamforming for Cell Edge Performance Enhancement • Beamforming is an effective way to enhance cell edge performance • Higher phy rate can be achieved (versus non-beamformed) • Reduced interference to OBSS (beamformed transmission) • Reduced interference from OBSS interference (beamformed reception) • Reduced delay spread • When more energy is delivered to the targeted receiver through TX beamforming, the less interference toward others. RX beamforming mitigates the interference level. Higher number of antennas results in less interference. • Potential to increase spatial reuse (higher area throughput) James Wang (MediaTek)

  5. Scenario : Station at the cell edge AP1 STA3 STA1 BSS1 AP2 STA2 • STA1 at the cell edge of BSS1 typically suffers from • Difficulty with channel access (CCA busy) due to OBSS interference • Higher packet collision rate from OBSS hidden nodes • STA1 becomes interference to OBSS • Goal: How to use beamforming to improve STA1 situation while reducing interference to OBSS (STA2 and AP2). Specifically, we want to explore the possibility of spatial reuse to improve network capacity James Wang (MediaTek)

  6. Spatially Reciprocal Transmission AP1 STA3 STA1 BSS1 AP2 STA2 • Define spatial reciprocity as a device capable of transmitting and receiving with same antenna pattern, via implicit or explicit calibration or other means. • Spatially reciprocal devices provides the following benefits : • A spatially reciprocal device can use its channel knowledge derived the received signal use it in its transmission • It reduces the overhead of the over-the-air (explicit or implicit) calibration. • Note that some vendors are shipping pre-calibrated devices already. James Wang (MediaTek)

  7. Condition 1 - Spatially Orthogonal Condition AP1 STA3 STA1 BSS1 SO condition AP2 STA2 • Supposed OBSS STA2 and AP2 are communicating and STA1 wishes to transmit to AP1 at the same time. • To avoid STA1 interfering with OBSS STAs (STA2 and AP2), STA1 should satisfy spatially orthogonal (SO) condition before it transmits (allowing it to reset NAV) • SO condition: STA1 does not receive AP2 and STA2 signal under beamformed condition during an observation duration * Note if STA1’s CCA is idle, the STA1 is allowed to transmit. James Wang (MediaTek)

  8. Condition 1- Spatially Orthogonal Condition AP1 STA3 STA1 BSS1 SO condition AP2 STA2 • STA1 decides the beamforming weight for SO detection based on its knowledge of channel to AP1 and the OBSS interference (STA2&AP2) situations. STA1’s is only interested in communicate to AP1. • STA1 can establish SO condition via • beamforming toward AP1 to suppress interference, or • using active nulling toward interference source (STA2/AP2) James Wang (MediaTek)

  9. Condition 2 – Use RTS/CTS to start a SO Frame Exchange AP1 CTS STA3 RTS STA1 BSS1 AP2 STA2 • STA1 should start the frame exchange with RTS/CTS with AP1 to avoid AP1 from interfering with AP2 or STA2 • Note that the intended recipient of RTS (AP1) will not transmit CTS if its NAV≠0*. AP1 would not cause interference to OBSS STAs (AP2 and STA2). * REVmb: 9.3.2.6 CTS procedure: If the NAV at the STA receiving the RTS indicates the medium is not idle, that STA shall not respond to the RTS frame. James Wang (MediaTek)

  10. Spatial Reuse of Wireless Medium AP1 STA2 STA1 BSS1 AP3 STA3 TXOP Bemaformed Transmission and Reception Duration RTS WM Access (SO Detection) AP WM Access (SO Detection) CTS STA • A spatially reciprocal (SR) HEW device should be allowed to gain access to the channel when SO condition is satisfied, with beamforming weights determined by the HEW device based on its channel knowledge • Under this SO condition, the HEW device can start a SO frame exchange by employing the beamformed (preamble & payload) transmission and reception for the TXOP starting with a RTS/CTS • Note that under this condition , the spatial re-use of the wireless medium can be achieved. James Wang (MediaTek)

  11. Beamformed Transmission Issues Omni-preamble Time Beamformed Beamformed AP STA • PPDU-based Beamforming: Switching between a omni-preamble to a beamformed-transmission after omni-preamble is also hard for OBSS receiver to predict the channel conditions James Wang (MediaTek)

  12. TXOP-based Beamformed Frame Exchange TXOP Omni-Beam Duration Bemaformed Transmission and Reception Duration AP WM Access WM Access NAV STA NAV Reduced Interference to OBSS More Interference to OBSS • When an AP is engaged in a frame exchange with a selected STA, it should inform all STAs within the BSS defer properly. This requires some omni-directional transmission. • TXOP-based beamformed frame exchange: A proposed solution is to employ the omni-beam transmission at the beginning of an TXOP to set up protection duration (NAV) and then to switch to the beamformed (both preamble and payload) transmission and reception for the remainder of the TXOP duration • Desirable to have minimum omni-beam duration and then beamformed transmission thru the TXOP  minimum interference to OBSS James Wang (MediaTek)

  13. TXOP-based Beamformed and SO Frame Exchange AP1 STA2 BSS1 STA1 AP3 STA3 An OBSS STA can start a SO frame exchange (RTS/CTS) upon detecting the SO condition During a TXOP-based beamformed frame exchange (AP1 & STA2), an OBSS device (STA3 or AP3) should be allowed to start a frame exchange starting with a RTS/CTS if the SO condition is satisfied  spatial reuse cab be achieved AP1 can have more antennas for beamforming resulting in less interference TXOP-based beamformed FX enable simple STAs to spatially re-use the medium (only need to detect SO condition either with or without beamforming) James Wang(MediaTek)

  14. Combined SR beamformed and TXOP-based Beamformed FXs TXOP AP uses TXOP-based Beamformed FX Bemaformed Transmission and Reception Duration Omni-Beam Duration AP WM Access WM Access NAV STA NAV Bemaformed Transmission and Reception Duration STA uses SR Beamformed FX TXOP AP uses TXOP-based Beamformed FX Omni-Beam Duration Bemaformed Transmission and Reception Duration AP CTS WM Access WM Access NAV STA RTS NAV Beamformed Transmission and Reception Duration STA uses SR beamformed FX Note that the beamformed FX (by STA) and the TXOP-based beamformed FX (by AP) can be combined in an TXOP James Wang (MediaTek)

  15. Summary • Discuss potential beamforming enhancement ideas with the following benefits: • increase likelihood of channel access under dense deployment condition • reduce interference to OBSS • reduce collision during reception • increase likelihood of spatial re-use in dense deployment scenario, leading to higher network throughput • The proposed beamforming enhancement is suitable for HEW, because • there is no control of the WLNA deployment scenario due to unlicensed spectrum • no BSS-BSS coordination is required • distributed algorithm accommodates mobile APs/devices and changing channel conditions James Wang (MediaTek)

  16. References Ref 1: 11-13-0675 Usage models and requirements for IEEE 802.11 High Efficiency WLAN study group (HEW SG) –Liaison with WFA, Laurent Cariou, Orange Ref 2: 11-12-1123-00-0-WNG Carrier-oriented WiFi for cellular offload, Laurent Cariou, Orange James Wang (MediaTek)

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