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Investigation into the 802.11n Doppler Model

Investigation into the 802.11n Doppler Model. Date: 2009-05-11. Authors:. Introduction. Significant degradation to 802.11n transmit beamforming gain due to the 802.11n Doppler model occurs within 20ms delay [1]

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Investigation into the 802.11n Doppler Model

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  1. Investigation into the 802.11n Doppler Model Date: 2009-05-11 Authors: Eldad Perahia, Intel Corporation

  2. Introduction • Significant degradation to 802.11n transmit beamforming gain due to the 802.11n Doppler model occurs within 20ms delay [1] • Delay occurs between collection of CSI, computation of transmitter weights, and actual packet transmission • Feedback delay • Channel access delay • Since the TGac PAR includes multi-station throughput, techniques like SDMA or multi-user MIMO may be proposed • These techniques will face even longer delays between collection of CSI, computation of transmitter weights, and actual packet transmission • Simulation of these techniques will be required if they are necessary to meet the PAR • Measurements made by NTT in [3] demonstrated no degradation to Eigen-mode transmission after 100ms delay • Proper understanding and modeling of the change in the channel is critical Eldad Perahia, Intel Corporation

  3. Brief Overview of 802.11n Doppler Model [2] • Bell shaped Doppler spectrum is used in the 802.11n channel model • The doppler model includes a parameter for environmental speed, set to 1.2 km/h • The coherence time at 5 GHz is ~60 ms • All channel taps are filtered by the Doppler spectrum • Channel Model F has an extra Doppler component on the third tap Eldad Perahia, Intel Corporation

  4. Simulation Parameters: 4 TX antennas at transmitting device, 4 RX antennas at receiving device Basic SDM w/ MMSE receiver Transmit beamforming w/ MMSE receiver Channel Model D SNR = 30 dB No TxBF gain after 20ms Impact of Delay on TxBF Capacity with 802.11n Doppler Model Eldad Perahia, Intel Corporation

  5. New Measurements • Measurements were captured in an office environment between a device acting as an AP and a device acting as a stationary client (e.g. laptop on a desk) to determine the how much the channel changes with time • Measurements with different types of motion in the environment were captured • someone waving their hands in front of the device at both ends of the link (Double Motion) • someone waving their hands in front of the device at one end of the link (Single Motion) • many people known to be walking around (People Motion) • typical motion in office environment (Light Motion) Eldad Perahia, Intel Corporation

  6. Circled numbers indicate device locations Each device can act as an AP or STA Measurements made between all devices Each device transmits and receives with three antennas Actual three stream 802.11n packets transmitted Channel state information measured from LTFs TxBF capacity computed from measured LTFs and SNR 13 4 9 12 11 3 1 2 10 5 Measurement Detail Eldad Perahia, Intel Corporation

  7. Measurement Device Eldad Perahia, Intel Corporation

  8. Impact of Delay on TxBF Capacity with “Double Motion” Eldad Perahia, Intel Corporation

  9. Impact of Delay on TxBF Capacity with “Single Motion” Eldad Perahia, Intel Corporation

  10. Impact of Delay on TxBF Capacity with “People Motion” Eldad Perahia, Intel Corporation

  11. Impact of Delay on TxBF Capacity with “Light Motion” Eldad Perahia, Intel Corporation

  12. People waving hands at both ends of the link causes the most motion, but still much less degradation to TxBF gain than 802.11n doppler model Typical motion (LM, PM, SM) causes small amount of degradation and links can still retain majority of TxBF gain after 200 ms Variation of TxBF increases with people walking around Summary of Measurements Eldad Perahia, Intel Corporation

  13. Conclusion • Doppler component of 802.11n channel model results in significant degradation to TxBF after 20 ms • Recent measured results in [3] show no degradation to Eigen-mode transmission after 100ms delay • New office environment measurements show minimal degradation to TxBF gain after 20 ms even when motion is exaggerated (e.g. hands waving in front of AP and STA) • Measurements show that majority of TxBF gain retained after 200 ms • Coherence time of the channel has a big impact on gain and architecture • With short coherence time frequent overhead may eliminate gain • Short coherence time may result in over emphasis of MAC architecture on immediate and frequent feedback • More investigation of the applicability of the 802.11n doppler model to 802.11ac is necessary • 11n doppler model is applied to every tap more like a device slowly moving rather than stationary devices like a laptop upon a desk or set top box • Need more measurements to determine the impact in a typical environment • Perhaps apply Doppler to channel impulse response in a different way Eldad Perahia, Intel Corporation

  14. References • Perahia, E. and Stacey, R., Next Generation Wireless LANs: Throughput, Robustness, and Reliability in 802.11n, Cambridge University Press, 2008 • Erceg, V., Schumacher, L., Kyritsi, P., et al., TGn Channel Models, IEEE 802.11-03/940r4, May 10, 2004 • Honma, N., Nishimori, K., Kudo, R., Takatori, Y., Effect of SDMA in 802.11ac, IEEE 802.11-09/303r1, March 12, 2009 Eldad Perahia, Intel Corporation

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