Scheduled spatial reuse with collaborative beamforming
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Scheduled Spatial Reuse with Collaborative Beamforming. Date: 2010-05-16. Authors:. Abstract. Spatial reuse is a key aim for 802.11ad [1] potentially a major increase in aggregate data throughput within a PBSS mutual interference mitigated by directional antennas

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Scheduled Spatial Reuse with Collaborative Beamforming

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Scheduled spatial reuse with collaborative beamforming

Scheduled Spatial Reuse with Collaborative Beamforming

Date: 2010-05-16

Authors:

Thomas Derham, Orange Labs


Abstract

Abstract

  • Spatial reuse is a key aim for 802.11ad [1]

    • potentially a major increase in aggregate data throughput within a PBSS

    • mutual interference mitigated by directional antennas

  • Beamforming capability of phased-array antennas can be usedto our advantage to optimally mitigate interference

    • Antenna Weight Vectors (AWVs) can be determined that are optimizedtaking into account mutual interference due to spatial reuse

      • based on simple extension of Beam Refinement Procedure (BRP) [2]

    • resulting SINR of each link can be accurately predicted and used by PCP asthe basis for co-scheduling (in addition to Directional Channel Quality reports)

  • Substantial increase in throughput; low complexity and impact

Thomas Derham, Orange Labs


A typical 802 11ad usage scenario

A typical 802.11ad usage scenario

  • in dense networks, multiple links must share the same channel

    • but one uncompressed HD video link can exhaust all bandwidth in one channel

      • video links tend to be continuously active over extended periods

      • spatial reuse necessary to support all links (e.g. video + data) [3]

repeater

Media server

FTTH

BSS

Set-top box

Home Gateway

Thomas Derham, Orange Labs


Scheduled spatial reuse

Scheduled spatial reuse

  • beacon interval (BI)

beacon

service periods (SPs)

contention-based period (CBP)

CSMA/CA

TDMA

  • high-QoS data transmissions are generally scheduled in SPs

    • since CSMA/CA is inefficient with directional antennas [4]

    • PCP (PBSS Control Point) schedules SPs and transmits the beacon [5]

  • spatial reuse: PCP may co-schedule overlapping SPs (e.g. 2x spatial reuse)

STA1 => STA2

STA3 => STA4

STA3

STA1

STA2

STA4

Thomas Derham, Orange Labs


Example of co scheduling process

Example of co-scheduling process

  • STA3 requests SPs for link STA3=>STA4 by sending “Extended mmWave TSPEC” to PCP

  • If no free time in BI, PCP sends “Spatial Reuse BRP Request” to the STAs ofa subset of links (STA1=>STA2, STA3=>STA4) that it is considering to co-schedule

  • STAs perform BRPs with other STAs in the subset to determine optimal AWVs

  • Receiver STAs (STA2, STA4) send “Spatial Reuse SINR Report” to PCP

  • PCP determines scheduling (all/part of subset), broadcasts “Extended Schedule”

STA1 => STA2

< x

STA1 => STA2

Extended mmWave TSPEC

(Src STA = 3), Dest STA = 4, Length = x

Spatial Reuse BRP Request

Co-scheduled links: STA1=>STA2, STA3=>STA4

STA3 => STA4

Spatial Reuse SINR Report

SINR1,2, SINR3,4

STA3

STA1

STA2

PCP

STA4

Extended Schedule

(1) Src STA = 1, Dest STA = 2, Start = 0, Length=x’

(2) Src STA = 3, Dest STA = 4, Start = 0, Length=x

Thomas Derham, Orange Labs


Pcp determines a subset of links that it is considering to co schedule

PCP determines a subset of links that it is considering to co-schedule

  • based on two metrics that approximately indicate spatial separation

    • metrics already known by PCP, so no additional overhead

    • large receiver spatial separation => greater chance links can be co-scheduled

range separation is channel strength forPCP<=>Rx STA of ith link

angular separation is beamforming vector used by PCP for PCP<=>Rx STA of ith link

PCP

Thomas Derham, Orange Labs


Each tx sta performs brp with all rx stas in the subset to determine optimal tx awv

Each Tx STA performs BRP with all Rx STAs in the subset to determine optimal Tx AWV

  • Tx STA initiates transmit beam refinement with each Rx STA in turn

    • i.e. Tx STA1 performs transmit BRP for both its own link (STA1=>STA2)and the “cross-link” it may interfere with (STA1=>STA4)

  • Rx STA shall fix its beam to the best known AWV for its own link

    • all links have already done conventional beam training

    • i.e. Rx STA4 fixes its beam to AWV chosen in previous SLS/BRP with STA3

  • Tx STA cycles through an orthogonal codebook matrix of AWVs

    • CSI “Channel Measurement”/“Tap Delay” fed back for each AWV in codebook

link

cross-link

STA3

STA1

Rx beam fixed to best AWV for STA3=>STA4 link

STA2

TRN-T transmitted M times(number of Tx elements)

STA4

Thomas Derham, Orange Labs


Each tx sta performs brp with all rx stas in the subset to determine optimal tx awv 2

Each Tx STA performs BRP with all Rx STAs in the subset to determine optimal Tx AWV (2)

  • Tx STA estimates effective MISO channel

    • e.g. MIMO channel model for subcarrier i:

    • transmit-side spatial covariance matrix of MISO channel given by:

  • Tx STA calculates optimal AWV using max-SLNR criterion

    • Signal to Leakage & Noise Ratio: “leakage” is interference caused to other Rx

MIMO channel

fixed Rx AWV

codebook of Tx AWVs

channel estimates for Tx AWVs

(eig{X} is dominant eigenvector of X)

“cross-link” to qth Rx STA

own link

Thomas Derham, Orange Labs


Each rx sta performs brp with all tx stas in the subset to determine optimal rx awv

Each Rx STA performs BRP with all Tx STAs in the subset to determine optimal Rx AWV

  • Rx STA initiates receive beam refinement with each Tx STA in turn

    • i.e. Rx STA2 performs receive BRP for both its own link (STA1=>STA2)and the “cross-link” that may cause it interference (STA3=>STA2)

  • Tx STA shall fix its beam to the best known AWV for its own link

    • determined in previous stage

    • i.e. Tx STA3 fixes its beam to AWV chosen for use with STA4

  • Rx STA cycles through an orthogonal codebook matrix of AWVs

cross-link

link

TRN-R transmitted N times(number of Rx elements)

STA3

STA1

Tx beam fixed to best AWV for STA3=>STA4 link

STA2

STA4

Thomas Derham, Orange Labs


Each rx sta performs brp with all tx stas in the subset to determine optimal rx awv 2

Each Rx STA performs BRP with all Tx STAs in the subset to determine optimal Rx AWV (2)

  • Rx STA estimates effective SIMO channel

    • e.g. MIMO channel model for subcarrier i:

    • receive-side spatial covariance matrix of SIMO channel given by:

  • Rx STA calculates optimal AWV using max-SINR criterion

    • Signal to Interference & Noise Ratio: “interference” is caused by other Tx

MIMO channel

codebook of Rx AWVs

fixed Tx AWV

channel estimates for Rx AWVs

(eig{X} finds dominant eigenvector of X)

“cross-links” from pth Tx STA

own link

Thomas Derham, Orange Labs


Each rx sta sends sinr report to pcp and pcp determines scheduling

Each Rx STA sends SINR report to PCPand PCP determines scheduling

  • Rx STA calculates SINR assuming subset is co-scheduled

  • PCP schedules links based on these SINRs

    • links are co-scheduled (overlapping SPs) if all SINR are above a threshold

      • threshold may be chosen according to QoS requirement for that link

    • if one or more SINRs are too low, two choices:

      • (a) remove dominant interfering pair (lowest SLNR) and retry, or

      • (b) perform iteration of Tx-side and Rx-side BRP and AWV calculation

        • since optimal Rx AWV are conditional on Tx AWV, and vice versa

own link

“cross-links” from pth Tx STA

Thomas Derham, Orange Labs


System level simulation setup

System-level simulation setup

  • conference room model

    • inter-cluster parameters between all pairs of STAs from ray-tracing [7]

      • correctly models interference between all STAs

    • TGad channel model code used for intra-cluster parameters [8]

Link-n

Thomas Derham, Orange Labs


Sta positions as per evaluation methodology

STA positions as per Evaluation Methodology

  • 3 STA-STA pairs on table [9]: STA2=>1 (LoS), STA3=>5 (NLoS), STA7=>8 (LoS)

    • STA-AP links ignored; STA5=>STA3 ignored since direct leakage with STA3=>5 not modeled

  • beamforming: (a) conventional training, (b) collaborative beamforming

SINR threshold: 4 dB

complementary CDF of aggregate throughput

complementary CDF of number of co-scheduled links

aggregate throughput increased by 40% @Pr=0.5

no. of co-scheduledlinks increased in approx. 80% ofchannel instances

Thomas Derham, Orange Labs


Sta positions randomized on table

STA positions randomized on table

  • 10 pairs of STAs on 2.5 x 1 m table (random orientation) ==> dense network

    • note: not all pairs are simultaneously active (due to scheduler SINR rule)

  • beamforming types: (a) conventional training, (b) collaborative beamforming

  • SINR threshold: 4 dB

    complementary CDF of aggregate throughput

    complementary CDF of number of co-scheduled links

    aggregate throughput increased by 70% @Pr=0.5

    Thomas Derham, Orange Labs


    Sta positions randomized on table1

    STA positions randomized on table

    • 10 pairs of STAs on 2.5 x 1 m table (random orientation) ==> dense network

      • note: not all pairs are simultaneously active (due to scheduler SINR rule)

    • beamforming types: (a) conventional training, (b) collaborative beamforming

    SINR threshold: 9.5 dB

    complementary CDF of aggregate throughput

    complementary CDF of number of co-scheduled links

    aggregate throughput increased by 60% @Pr=0.5

    Thomas Derham, Orange Labs


    Supporting scheduled spatial reuse with collaborative beamforming in 802 11ad

    Supporting scheduled spatial reuse with collaborative beamforming in 802.11ad

    • provide basic framework to allow implementation

      • (1) a field in BRP request which tells the responder STA to fix its AWV to the best known beam for communicating with a specified STA

      • (2) a supporting STA should maintain a table of best known AWVs for communicating with each STA when specified other links are co-scheduled

        • e.g. transmitter AWVs receiver AWVs

    Thomas Derham, Orange Labs


    Supporting scheduled spatial reuse with collaborative beamforming in 802 11ad 2

    Supporting scheduled spatial reuse with collaborative beamforming in 802.11ad (2)

    • to allow control by PCP

      • (1) support “Spatial Reuse BRP Request” for both Tx and Rx sides

        • from PCP to Tx/Rx STAs

      • (2) support “Spatial Reuse SINR Report”

        • from Rx STAs to PCP

    • complementary to existing beamforming (SLS/BRP) and measurement reports

      • predicted SINR reports are more accurate than Directional Channel Quality

        • not affected by bursty traffic

        • based on the responder AWV that will actually be used

    Thomas Derham, Orange Labs


    Overhead and complexity

    Overhead and complexity

    • small additional overhead to perform cross-link BRPs, but...

      • can allow PCP implementation (or STA) to manage overhead tradeoff

      • spatial reuse always involves some additional measurements, so is best targeted to low mobility channels (many realistic 11ad scenarios)

      • all Rx STAs could simultaneously “listen” to TRN if framework supported it

    • computational complexity reduced due to efficient algorithms

      • calculate AWVs

        • division by Hermitian matrix, e.g. Cholesky factorization

        • find dominant eigenvector, e.g. power iteration method

      • calculate SINRs for scheduling

        • matrix multiplication

    Thomas Derham, Orange Labs


    Conclusion

    Conclusion

    • A method of scheduled spatial reuse with collaborative beamforming is proposed

      • significantly increases aggregate throughput

      • significantly increases the number of concurrent links that are supported

      • scheduling guarantees the QoS of each link

    • Low complexity, overhead and impact

      • only provide framework to enable implementations

      • shown that low complexity implementations are possible

      • complementary to existing beamforming and measurement mechanisms

    Thomas Derham, Orange Labs


    References

    References

    • [1] C. Cordeiro et al, “Spatial Reuse and Interference Mitigation in 60 GHz”, 802.11-09/0782r0

    • [2] C. Cordeiro et al, “PHY/MAC Complete Proposal Specification”, 802.11-10/0433r0

    • [3] M. Park et al, “QoS Considerations for 60 GHz Wireless Networks, Globecom 2009

    • [4] S. Nandagopalan et al, “MAC Channel Access in 60 GHz”, 802.11-09/0572r0

    • [5] C. Cordiero et al, “Implications of Usage Models on TGad Network Architecture”, 802.11-09/0391r0

    • [6] M. Lim et al, “Spatial Multiplexing in the Multi-user MIMO Downlink Based on Signal-to-Leakage Ratios”, Globecom 2007

    • [7] M. Park et al, “TGad Interference Modeling for MAC Simulations”, 802.11-10/0067r0

    • [8] A. Maltsev et al, “Channel Models for 60 GHz WLAN Systems”, 802.11-09/0334r7

    • [9] E. Perahia et al, “Evaluation Methodology”, 802.11-09/0296r16

    Thomas Derham, Orange Labs


    Appendix an additional usage case two stas in each device

    Appendixan additional usage case - two STAs in each device

    • effectively MxN spatial multiplexing with max. rank 2

      • “SU-MIMO” for additional throughput withstrong multipath channel

        • e.g. “bottlenecks” to/from repeater

      • “MU-MIMO” to allow simultaneouscommunication between central point andtwo different destinations

        • e.g. multi-stream video from media server

      • the same collaborative beamforming techniqueoptimizes AWVs to minimize interference

    Thomas Derham, Orange Labs


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