BBN: Throughput Scaling in Dense Enterprise WLANs with B lind B eamforming and N ulling
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Transcript of BBN: Throughput Scaling in Dense Enterprise WLANs with B lind B eamforming and N ulling
BBN: Throughput Scaling in Dense Enterprise
WLANs with Blind Beamforming and Nulling
Wenjie Zhou (Co-Primary Author), Tarun Bansal (Co-Primary Author), Prasun Sinha and Kannan Srinivasan
The Ohio State University
Changes in Uplink Traffic
2
Cloud Computing
Online Gaming
Sensor Data Upload
Code Offloading VoIP,
Video Chat
Traditionally, WLAN traffic: • downlink heavy• less attention to uplink traffic
Recently, uplink traffic increased rapidly : • mobile applications
Can we scale the uplink throughput with the number of clients?
Network MIMO
Huge bandwidth consumption
C2C1 C3
Exchange raw samplesAP1 AP2 AP3
[1] Rahul, H., Kumar, S., and Katabi, D. MegaMIMO: Scaling Wireless Capacity with User Demand. In Proc. of ACM SIGCOMM 2012.
MegaMIMO1
Does not apply to uplink :Clients do not share a backbone network
[1] Cadambe, V. R., and Jafar, S. A. Interference Alignment and the Degrees of Freedom for the K User Interference Channel. IEEE Transactions on Information Theory (2008).
Interference Alignment1
• 4 packets, 3 slots• Enough time slots, everyone gets half the cake • Exponential slots of transmissions, not suitable for mobile clients• Heavy coordination between clients
C2
C1
C3
AP1
AP2
AP3
Existing interference alignment and beamforming techniques are not suitable to mobile uplink traffic.
How can we bring the benefits of beamforming to uplink traffic?
AP Density in Enterprise WLANs
8
50 70 90 110 130 150 170 1900
0.25
0.5
0.75
1
Number of Access Points (APs)
CDF
(140,0.5)
BBN leverages the high density of access points
Single Collision Domain
C1 C2 C3
x1 x2 x3
AP1 AP2
AP3 AP4
Switch
Omniscient TDMA
Time Slot: 1Time Slot: 2Time Slot: 3
Three Packets received in Three Slots Only one AP is in use 9
10
h(1)12x1 + h(1)
22x2 + h(1)32x3h(1)
11x1 + h(1)21x2 + h(1)
31x3
Blind Beamforming and NullingSingle Collision Domain
Time Slot: 1
C1 C2 C3
x1 x2 x3
AP1 AP2
AP3 AP4
Switchh(1)
13x1 + h(1)23x2 + h(1)
33x3 h(1)14x1 + h(1)
24x2 + h(1)34x3
h(1)13 h(1)
23h(1)
33
11
Receives:a11x1 + s1h(1)
21x2 + s1h(1)31x3
Receives:a12x1 + a22x2 + a32x3
Transmits:
v4 (h(1)14x1 + h(1)
24x2 + h(1)34x3)
Transmits:
(h(1)13x1 + h(1)
23x2 + h(1)33x3)
Time Slot: 2
Blind Beamforming and NullingSingle Collision Domain
AP1 AP2
AP3 AP4
Switch
v3
12
AP1 AP2
AP3 AP4
Switch
Slot 2: a11x1 + s1h(1)21x2 + s1h(1)
31x3 Slot 2: a12x1 + a22x2 + a32x3
Slot 1: h(1)11x1 + h(1)
21x2 + h(1)31x3 Slot 1: h(1)
12x1 + h(1)22x2 + h(1)
32x3
Three Packets received in Two Slots
Blind Beamforming and NullingSingle Collision Domain
(s1h(1)11-a11)x1
Slot 2: a11x1 + s1h(1)21x2 + s1h(1)
31x3
Number of APs Required
• In a network with APs, APs in BBN can
receive N uplink packets in two slots
• 3 clients, 4 APs
• 4 clients, 7 APs
• 10 clients, 46 APs
13
2
22 NN
Throughput Improvement
• Previous Example Topology– APs in BBN receive three packets in two slots: an
improvement of 50%
• General Topology– Uplink throughput in BBN scales with the number of
clients (N/2 packets per slot). – Half of the cake as in Interference Alignment
• Always two slots• No coordination between clients
14
BBN Highlights
• Leverages the high density of access points • All computation and design complexity shifted to
APs • APs only need to exchange decoded packets over
the backbone instead of raw samples
15
Further Optimizations to Improve SNR
• Which subset of APs act as transmitters and which subset as receivers?
• Which AP decodes which packet?
C1 C2 C3
AP1AP2
AP3
AP4
Switch
16
BBN Approach: xi is decoded at the APj where it is expected to have highest SNR
Transmitters
Receiversx1 x2, x3
Challenge 1/4: Synchronization of APs
• To perform accurate beamforming, APs need to be tightly synchronized with each other
• Solution: – SourceSync (Rahul et al., SIGCOMM 2010):
synchronizes APs within a single collision domain – Vidyut (Yenamandra et al., SIGCOMM 2014):
uses power line to synchronize APs in the same building
17
Challenge 2/4 : MultiCollision Domain
• Not all APs may be able to hear each other directly
• Solution: Make smaller groups where all APs in a single group can hear each other.
18
19
Distributed System
Group Head
Group Head
• Within a group, all APs can hear each other• When one group is communicating, neighboring groups
remain silent
Challenge 3/4 : Inconsistency in the AP density
• Number of APs may be less than
• Solution: Appropriate MAC layer algorithm that restricts the number of participating clients
2
22 NN
20
21
Uplink
Poll Approve A, B and C
Keep Silent – Allow neighboring groups to transmit
Downlink Uplink
....... ....... .......
Time
Notification Period
Time Slot 1 Time Slot 2
Uplink
MAC Timeline
Compute pre-coding vectors in the background
C1 C2 C3
x1 x2 x3
AP1 AP2
AP4 AP5
Switch
Challenge 4/4 : Robustness• Nulling is not always perfect.
x1, x2 , x3x1
Decoding Error
Can’t Subtract x1
22
C1 C2 C3
x1 x2 x3
AP1 AP2
AP3 AP4
Switch
Challenge 4/4 : Robustness• What if we have extra APs
AP5
AP6AP7
x1, x2 , x3x1 x1
23
Experiments
24
C1 C2
x1
x2, x3AP1 AP2
AP3 AP4
Switch
Intended Signal = x1
Interference from x2, x3
x2
C2
x3
USRP N210
25
Throughput
BBN provides 1.48x throughput compared to TDMA
1.48X
Trace-Driven Simulation• Over multiple collision domains (divided into groups)
• Field Size: 500m X 500m
• Number of clients: 1000
• Vary the number of APs
• Residual interference distribution from experiment
• Other algorithms simulated– Omniscient TDMA– IEEE 802.11 26
27
• 2000 APs• 4.6X throughput
gain• ~76 APs near each
client
Throughput
BBN
Fairness
28
BBN achieves higher fairness• Beamforming increased SINR of clients that are far
away
BBN
29
Summary and Future Work• BBN leverages the high density of APs to scale the uplink
throughput for single antenna systems– Throughput scales linearly with the number of clients– All computational and design complexity shifted to APs
• Future Work– Coexist with legacy network
– Data rate selection
Thank you
Backup Slides
30
Long Term Results
OctoClock-G
Frequency Accuracy w/ out : 25 ppb Frequency Accuracy with GPS Lock : <1 ppb PPS Accuracy with GPS Lock : 50 ns
Vidyut : approximately 225 ns
Multiple Antenna AP
• Assume each AP has K antennas• For N clients, APs required• For M APs, clients
K
KNN
2
22
MK2
Estimate SNR of C1 at AP2
SNR of C1 at AP2 is low
C1
AP1 AP2
AP3
Switch
AP4
34
No path with high
SNR
Estimate SNR of C1 at AP1
SNR of C1 at AP1 is high
C1
AP1 AP2
AP3 AP4
Switch
35
One path with high
SNR
• C1 should be decoded by AP1
• AP1 should act as a receiver in slot 2
3
2
1
00
00
00
)( Clientsby dTransmitte Data
x
x
x
X
37363534
27262524
17161514 )( APs Receiving toClients from Channel
hhhh
hhhh
hhhh
H
Blind Nulling in BBN
36
4
3
2
1
000
000
000
000
)( Vectors Precoding
v
v
v
v
V
73'
72'
71'
63'
62'
61'
53'
52'
51'
43'
42'
41'
)( APs Among Channel
hhh
hhh
hhh
hhh
H '
AP1 :
C1 : AC1 Packet 1
C2 : AC2 Packet 2
C3 : AC3 Packet 3
IACS IACS IACS
Approve
MAC Layer: Phase 1
37
AP1 :
AP2:
AC3AP3 :
AP4 : v4* Samples4
BIFSAC1
AC2
SIFS SIFS SIFS
AP5 : v5* Samples5
AP6: v6* Samples6
AP7 : v7* Samples7
MAC Layer: Phase 2
38
Experiments Setup
• Performed using USRP N210 Radio
• Testbed of 4 APs and 3 clients
• Modulation Scheme: OFDM with BPSK
• Channel: Central Frequency 400 Mhz, Bandwith set to 500 KHz
39
Existing Schemes
• Interference Alignment– Existing IA schemes perform alignment over exponential number of time
slots [Cadambe et al., IEEE Transactions on Information Theory 2007]
• MU-MIMO (Multi User MIMO)– Requires transmitters to exchange each other’s data before transmission
• MU-MIMO (Multi User MIMO) in EWLAN– All APs together act as a single AP with multiple antennas– Requires APs to exchange samples over the backbone which is cost-
prohibitive [Gollakota et al., SIGCOMM 2009; Gowda et al., INFOCOM 2013]
40
Existing Schemes
• Interference Alignment– Existing IA schemes require each transmitter to transmit
exponential amount of data [Cadambe et al., IEEE Transactions on Information Theory 2007]
• MU-MIMO– All APs together act as a single AP with multiple antennas– Requires APs to exchange samples over the backbone
which is cost-prohibitive [Gollakota et al., SIGCOMM 2009]
41
Related Work (contd.)
• Interference Alignment– Existing IA schemes work over exponential number of time slots
[Cadambe et al., IEEE Transactions on Information Theory 2007]
– Or, work only for downlink [Suh et al., IEEE Transactions on Communications 2011]
– Or, require multiple antennas at clients [Gollakota et al., SIGCOMM 2009]
– Or, require APs to exchange samples over backbone [Annapureddy et al., IEEE Transactions on Information Theory 2012] 42
Related Work
• Backbone Usage–MegaMIMO (Rahul et al., SIGCOMM 2012):
Works only for downlink
– Symphony (Bansal et al., MobiCom 2013): Works only in multiple collision domain
43
Related Work (contd.)
• Wireless Relays– Use special relay nodes to assist high speed
communication between specific transmitters and receivers
– Existing algorithms do not make use of the backbone
– BBN leverages the backbone to improve throughput
– BBN can extend to multiple rounds to decode packets with low SNR
44
BBN Highlights
• Leverages the high density of access points • Uplink throughput scales with the number of
clients in the network• All computational and design complexity shifted
to APs• APs only need to exchange decoded packets over
the backbone
45
C1 C2 C3
x1 x2 x3
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
Example Topology: What we ideally want
46
x2x1 x3
Works! But can we make the requirements less strict?
Matching in BBN
47
C1
C2
C3
AP1
AP2
AP3
AP4
AP5
AP6
AP7
Edge Weight = Expected SINR of C2 at AP3
Find the Maximum Weight Matching
• Which AP decodes which packet.• Which AP transmits in the second slot.
Number of APs Required: Example Topology
• Two packets (x2 and x3) need to be nulled at AP1
• One packet (x3) needs to be nulled at AP2
• Three transmitting APs required• Guarantee non degenerate solution: Four APs required
48
x1 x2x3
AP Density in Enterprise WLANs
49
60 80 100 120 140 160 180 2000
0.25
0.5
0.75
1
CDF of Number of APs Observed
Number of Access Points (APs)
CDF
CDF of number of APs observed (Measurements conducted at Ohio State
University campus)
Can we leverage the high density of APs to scale the uplink throughput?
Enterprise Wireless LAN
50
AP AP AP
AP AP AP
Internet
BBN Overview
• Leverages the high density of access points• Uplink throughput scales with the number of
clients in the network
– Schedule length: Two Slots• First slot: Clients transmit• Second slot: APs perform blind nulling
– APs only need to exchange decoded packets over the backbone
51
52
Contents
• BBN Design• Experiments• Simulations• Challenges• Related Work• Conclusion
C1 C2 C3
x1 x2 x3
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
Example Topology (Single Collision Domain) with BBN
53
Time Slot: 1
C1 C2 C3
x1 x2 x3
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
Simultaneous Nulling Goal
54
Receive: x1, x2
Null: x3
Receive: x1
Null: x2, x3
Receive: x1,, x2, x3
C1 C2 C3
x1 x2 x3
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
Example Topology (Single Collision Domain) with BBN
55
Time Slot: 1
h14x1 + h24x2 + h34x3
h15x1 + h25x2 + h35x3
... ...
Time Slot: 2
v4 * (h14x1 + h24x2 + h34x3 ) v5 * (h15x1 +
h25x2 + h35x3 )
v6 * (...) v7 * (...)
a11x1 a12x1 + a22x2 a13x1 + a23x2 + a33x3
AP1 AP2 AP3Sw
itch
Example Topology (Single Collision Domain) with BBN
56
Time Slot: 2a11x1 a12x1 + a22x2 a13x1 + a23x2 + a33x3
- a12x1
= a22x2- a13x1
- a23x2
= a33x3
Three Packets received in Two Slots
Blind Nulling in BBN (Contd.)
• What we want: Blindly null x2 and x3 at AP1; and, blindly null x3 at AP2
• Assuming nulling and packet cancellation is perfect, Compute V such that
57
000
000
000'VHH
MAC Layer
58
Uplink
Poll Approve A, B and C
Keep Silent – Allow neighboring groups to transmit
Downlink Uplink
....... ....... .......
Time
Contention Period
Phase I Phase II Phase III
RSS Without Blind Beamforming and Nulling
59
-55
-45
-35
-25
-15
Signal Strength at AP1 Before Cancellation (in dB)
Interference Intended Signal
-8dB SINR
RSS After Blind Beamforming
60
-55
-45
-35
-25
-15
Signal Strength at AP1 After Cancellation (in dB)
Interference Intended Signal
13dB SINR
• Proper precoding increases SINR by 21 dB• Blind Beamforming and Nulling is practical
61
How to boost uplink throughput?
MIMO?
netwrok-MIMO?
Smartphones are small
High bandwidth consumption
Interference Alignment?
Smartphones are mobile