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MU-MIMO
TECHNICAL
CHALLENGESWLPC Phoenix 2016
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AGENDA
• MU-MIMO Benefits
• MU-MIMO Technology
• MU-MIMO Demonstration & Analysis
• Challenges & Conclusion
• Appendix – Wireshark MU-MIMO capture in more details
MU-MIMO is a new technology that improves spectrum utilization promising higher
practical throughputs and better user Quality of Experience. However there are
strong challenges in achieving effective implementation. This session will detail
these through demonstration and analysis of current commercial MU-MIMO APs
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11AC BEAMFORMING
• 11ac introduces Explicit method – Beamforming
• First time - client provides channel information
• Beamforming gain may allow higher MCS, especially mid-range
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BENEFITS
• MU-MIMO gives better spectrum utilization – higher throughputs
• Better client QoE
Lower Latency & Jitter
• Not straight forward as the diagram!
1x1
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MU-MIMO TECHNOLOGY
Complex to beamform
signal
11ac defined MU-MIMO
more effective clearer
environments
MIMO with multipath
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MU-MIMO TECHNOLOGY
1. MU-MIMO client
2. Sounding process
3. Grouping
4. MU DS Transmissions
5. BAs/BARs
Major differences
between chipsets!
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MU-MIMO QUESTIONSCategory Decision
Sounding
When should the AP sound?
Which clients does the AP sound?
How many clients does the AP sound?
How often should it sound those clients?
Sound the client for MU or SU beamforming?
Grouping
Who does the AP group?
How many clients are grouped?
How many groups?
TransmissionsWhen/how often does the AP do MU-MIMO Transmissions?
Use SU or MU beamforming?
On Mobility/TxFailure
Should the AP reduce the MCS for that client?
Should the AP transmit the data using SU?
How many failed transmissions/How far should the AP reduce the MCS before no longer performing MU-MIMO for that client?
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MU-MIMO GROUPING
Feedback Angles, SNR, DeltaSNR
Non-Orthogonal Clients
Close/Far Clients
Client MIMO level
? 2x2, 1x1, 1x1 client
? 4x 2x2
Client Access Category & QoS
Best Effort, Video..
How many groups? How many clients?
Same client in multiple groups
100 clients – 62 MU Groups – 248 pos.
Nature of Client Traffic
High/Low, TCP/UDP
Updates to client group assignment?
Static/Dynamic…
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VALUE
How do you know that MU-MIMO is happening?
Value in Scalable, Configurable, Repeatable, Independent
test environment, that provides Operation Visibility
under variety Traffic Models with QoE metrics
& can be Automated
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MU-MIMO TEST CONFIGURATION
Control Laptop
Wi-Fi & Ethernet Blade
Ethernet
In-cable RF
Linksys EA8500
Should be in a Chamber
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HOW IN-CABLE?
Handles all 4
simultaneous
users from
independent
“locations”
MU-MIMO Group
Any of 500 simulated
clients at any instance
MAC CPU
PHY FPGA
Signal
Chain
Know exactly the Vk for each client
Can see all information as in-cable connection
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MU-MIMO SCALING
• >10 clients see
drop off MU-MIMO
gains
• Reverts to SU
Clients only
0
100
200
300
400
500
600
700
800
900
1000
2 3 4 7 10 15
MU-Clients vs. UDP Forwarding Rate
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WIRESHARK
VHT NDP Announcement wlan.fc.type_subtype == 21
Action Frame(CBR) wlan.fc.type_subtype == 14
BF Report Poll wlan.fc.type_subtype == 20
Group ID Management wlan.vht.action == 1
MU-MIMO Data ixveriwave.VHT_GRPID < 63 &&
ixveriwave.VHT_GRPID > 0 && udp
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MU-MIMO PERFORMANCE (LATEST)
Highest 3 client performance, peaking 980Mbps
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CHALLENGES
1. AP Intelligence
• AP must make more decisions – non-deterministic/prioritization
• Unique solutions
2. Increased computational requirements
• Stressful on both Clients & APs
• Clients take Reference & respond within 16us SIFS – beyond a lot of clients today
• If AP looks at Orthogonality of each combination before transmission…
3. Channel Degradation
• Time since sounding – channel changes & degrades
• Especially sensitive in MU-MIMO
4. Sounding overhead
• As clients scale, how often & many do you sound?
• 10% -> Every 15ms with 1.5ms Sounding
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MU-MIMO CONCLUSION
• MU-MIMO promises!
• Success of MU-MIMO – in Intelligence
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MU-MIMO CAPTURE DETAILS IXIA – EA8500
…
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NDPA
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CBR
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CBR POLL
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MU-MIMO DATA
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MU-MIMO DATA BA
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MU-MIMO DATA BAR