802.11AC UPDATE...• MU-MIMO clients may interference with each other › A new “inter-something” interference: inter-user interference • Clients must be separated “enough”

© 2012 Aerohive Networks CONFIDENTIAL

Matthew Gast

Networkshop 43, April 2015

802.11AC UPDATE

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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

© 2013 Aerohive Networks CONFIDENTIAL 4


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


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?


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%


…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!)


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


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


Focusing energy

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• Omnidirectional transmission sprays energy everywhere

• Application of a steering matrix will send energy preferentially in one direction


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


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)


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

Documents

802.11AC UPDATE...• MU-MIMO clients may interference with each other › A new “inter-something” interference: inter-user interference • Clients must be separated “enough”