Post on 11-Mar-2020
© 2012 Aerohive Networks CONFIDENTIAL
Matthew Gast
Networkshop 43, April 2015
802.11AC UPDATE
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© 2013 Aerohive Networks CONFIDENTIAL
At the end of 2013 there were more mobile devices than people on earth - SAP
“ “
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Our error: 2 is too small!
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Tablets Media Transport
Gaming Automation & security Wearables
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So, now what?
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Hang 10! Catch the 802.11ac waves
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• “Wave 1” and “Wave 2” are not terms for the standard › 802.11ac was completed in 2013 › The standard is complete done and has been for 18 months
Wave 1
• Widen channels to 80 MHz
• Add 256-QAM
• Keep 3 streams
Wave 2
• Widen channels to 160 MHz?
• Add MU-MIMO
• Start adding a fourth stream
© 2013 Aerohive Networks CONFIDENTIAL
Major features of 802.11ac
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802.11ac protocol feature
Description First-wave gain over 802.11n
Second-wave gain over 802.11n
80 & 160 MHz channels
802.11n supports only 40 MHz channels; wider channels support higher data rates
~2x (80 MHz)
~4x (160 MHz)
Up to 8 spatial streams
802.11n is largely 3 SS; currently planned 802.11ac chips only support up to 4 SS
~1x (3 SS) ~1.33x (4 SS)
256-QAM 64-QAM is the maximum in 802.11n
~1.33x ~1.33x
Multi-user MIMO Beamforming was not widely supported in 802.11n
n/a ~2x?
Total ~2.5x ~15x?
© 2013 Aerohive Networks CONFIDENTIAL
MU-MIMO, the theory: You paid for multiple streams…
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Single-stream devices 3-stream devices
Capacity: 33%
MU-MIMO efficiency gain!
Capacity: 100%
Capacity: 3 x 33% = 100%
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…SO HOW DO WE ACCOMPLISH THAT?
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Beamforming in 802.11ac
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• Objective of beamforming is to steer the energy towards particular receivers › By increasing SNR, we can increase
data rates
• Beamforming may be explicit or implicit › Implicit measures by inference › Explicit is an actual channel measurement
All figures from 802.11ac: A Survival Guide (and used with permission!)
© 2013 Aerohive Networks CONFIDENTIAL
Multi-user MIMO details
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• Totally new technology to Wi-Fi › Huge R&D effort over many years – and still continuing
• No extra speed: you still have N streams › Some chips limit number of simultaneous MU receivers
• High level: divide up all the streams you paid for › Multi-user transmissions go to multiple receivers at the same
time (up to 4 in the spec) › Spatial streams can be divided up between the four receivers
• Less obvious implications › Explicit channel measurement needs to happen more often –
maybe 10x as frequently (10 ms instead of 100ms?) › MU-MIMO trades peak speed to a single client for total system
throughput › Effectiveness depends greatly on client mix
© 2013 Aerohive Networks CONFIDENTIAL
Matrix Math!
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• Doing beamforming requires knowing what the path is like between each {transmitter, receiver} pair
• Practically speaking, the only way to do this is matrix math, where each matrix entry describes on of the many paths
• Several matrices are used › H is the channel matrix that describes the
path between transmitter & receiver › Q is the steering matrix that alters the
distribution of energy along a path › V is the feedback matrix, sent as part
of the measurement process to derive Q
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Focusing energy
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• Omnidirectional transmission sprays energy everywhere
• Application of a steering matrix will send energy preferentially in one direction
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Null steering
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• MU-MIMO clients may interference with each other › A new “inter-something” interference: inter-user interference
• Clients must be separated “enough” to avoid interfering with each other
• Ideal situation: the steering matrix creates a “null” for all other users
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Assembling explicit feedback
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• 802.11ac uses one type of beamforming: Null Data Packets (NDPs), instead of having several options
• The NDP is a well-known format of pilot carriers that enables calculation of the feedback matrix
• Multi-user MIMO requires a polling system to fetch all feedback matrices (V)
© 2013 Aerohive Networks CONFIDENTIAL
Acknowledgement and Queueing in MU-MIMO
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• Acknowledgements use well-known (from 802.11n) Block ACK procedures
• Queuing is interesting because high-priority frames may “pull forward” lower priority frames
Thank YOU