doc.: ieee 802.11-09/0538r0 submission may 2009 eldad perahia, intel corporationslide 1...
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May 2009
Eldad Perahia, Intel Corporation
Slide 1
doc.: IEEE 802.11-09/0538r0
Submission
Investigation into the 802.11n Doppler Model
Date: 2009-05-11
Name Company Address Phone email Eldad Perahia Intel
Corporation
2111 NE 25th Ave Hillsboro, OR 97124
503-712-8081 [email protected]
Tom Kenney Intel Corporation
Robert Stacey Intel Corporation
Anmol Sheth Intel Corporation
Dan Halperin Intel Corporation
Authors:
May 2009
Eldad Perahia, Intel Corporation
Slide 2
doc.: IEEE 802.11-09/0538r0
Submission
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
May 2009
Eldad Perahia, Intel Corporation
Slide 3
doc.: IEEE 802.11-09/0538r0
Submission
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
May 2009
Eldad Perahia, Intel Corporation
Slide 4
doc.: IEEE 802.11-09/0538r0
Submission
Impact of Delay on TxBF Capacity with 802.11n Doppler Model
• 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 20ms0 10 20 30 40 50 60 70 80 90 100
18
20
22
24
26
28
30
Cap
acity
(bi
ts/s
ymbo
l/sub
carr
ier)
Delay (msec)
AP Tx=4; STA Rx=4; channel model D; SNR=30
TxBF
Basic SDM
May 2009
Eldad Perahia, Intel Corporation
Slide 5
doc.: IEEE 802.11-09/0538r0
Submission
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)
May 2009
Eldad Perahia, Intel Corporation
Slide 6
doc.: IEEE 802.11-09/0538r0
Submission
Measurement Detail
• 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
12
10
4
9
3
11
12
5
13
May 2009
Eldad Perahia, Intel Corporation
Slide 7
doc.: IEEE 802.11-09/0538r0
Submission
Measurement Device
May 2009
Eldad Perahia, Intel Corporation
Slide 8
doc.: IEEE 802.11-09/0538r0
Submission
Impact of Delay on TxBF Capacity with “Double Motion”
0 20 40 60 80 100 120 140 160 180 20015
16
17
18
19
20
21
22
23C
apac
ity (
bits
/sym
bol/s
ubca
rrie
r)
Delay (msec)
AP 2; STA 4; SNR=30; Double Motion
TxBF
Basic SDM
May 2009
Eldad Perahia, Intel Corporation
Slide 9
doc.: IEEE 802.11-09/0538r0
Submission
Impact of Delay on TxBF Capacity with “Single Motion”
0 20 40 60 80 100 120 140 160 180 20014
15
16
17
18
19
20
21C
apac
ity (
bits
/sym
bol/s
ubca
rrie
r)
Delay (msec)
AP 5; STA 4; SNR=27; Single Motion
TxBF
Basic SDM
May 2009
Eldad Perahia, Intel Corporation
Slide 10
doc.: IEEE 802.11-09/0538r0
Submission
Impact of Delay on TxBF Capacity with “People Motion”
0 20 40 60 80 100 120 140 160 180 20024
25
26
27
28
29
30
31C
apac
ity (
bits
/sym
bol/s
ubca
rrie
r)
Delay (msec)
AP 5; STA 10; SNR=37; People Motion
TxBF
Basic SDM
May 2009
Eldad Perahia, Intel Corporation
Slide 11
doc.: IEEE 802.11-09/0538r0
Submission
Impact of Delay on TxBF Capacity with “Light Motion”
0 20 40 60 80 100 120 140 160 180 2007.5
8
8.5
9
9.5
10
10.5
11
11.5
12
Cap
acity
(bi
ts/s
ymbo
l/sub
carr
ier)
Delay (msec)
AP 11; STA 9; SNR=20; Light Motion
TxBF
Basic SDM
May 2009
Eldad Perahia, Intel Corporation
Slide 12
doc.: IEEE 802.11-09/0538r0
Submission
Summary of Measurements
• 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
20ms 50ms 100ms 200msDM avg 18.6 38 49 58
std err 1.3 3.2 3.6 4.1SM avg 9.2 20.3 28.9 34.2
std err 1.5 3.4 4.0 4.4PM avg 20.2 27.1 33.3 38.7
std err 6.0 6.4 7.2 7.7LM avg 8.3 12.6 17.2 22.0
std err 2.4 3.1 4.3 5.1
% degradation of TxBF gainMotion type
May 2009
Eldad Perahia, Intel Corporation
Slide 13
doc.: IEEE 802.11-09/0538r0
Submission
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
May 2009
Eldad Perahia, Intel Corporation
Slide 14
doc.: IEEE 802.11-09/0538r0
Submission
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