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RobinHood: Sharing the Happiness in a
Wireless Jungle
Tarun Bansal, Wenjie Zhou, Kannan Srinivasan and Prasun Sinha
Department of Computer Science and EngineeringOhio State University, Columbus, Ohio
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Enterprise Wireless LAN (EWLAN)
2
AP AP AP
AP AP AP
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Uplink Traffic
3
• Traditionally, uplink traffic has received less attention in the design of algorithms/solutions for WLANs
• Recently, uplink traffic has been increasing at a rapid pace due to increasing popularity of mobile applications such as:
Cloud Computing
Online Gaming
Sensor Data Upload
Code Offloading VoIP,
Video Chat
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Existing Schemes
4
• 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]
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AP Density in Enterprise WLANs
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
5
CDF of number of APs observed (Measurements conducted at Ohio State University campus)
Can we leverage the underutilized backbone and the high density of APs to
scale the uplink throughput?
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RobinHood Highlights
6
• Leverages the high density of access points
• Uplink throughput scales with the number of clients in the network
– Schedule length: Two Slots• First slot: Mobile clients transmit• Second slot: APs perform Blind Nulling
– APs only need to exchange decoded packets over the backbone
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C1 C2 C3
x1 x2 x3
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
Example Topology (Single Collision Domain) with Omniscient TDMA
Time Slot: 1Time Slot: 2Time Slot: 3
Three Packets received in Three Slots. Only one AP is in use. 7
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C1 C2 C3
x1 x2 x3
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
Example Topology (Single Collision Domain) with RobinHood
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
8
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AP1 AP2 AP3Sw
itch
Example Topology (Single Collision Domain) with RobinHood
Time Slot: 2a11x1 a12x1 + a22x2 a13x1 + a23x2 + a33x3
- a12x1
= a22x2- a13x1
- a23x2
= a33x3
9
Three Packets received in Two Slots
Time Slot:Background
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Number of APs Required for Blind Nulling
• In a network with APs, APs in RobinHood can receive
N uplink packets in two slots
• With M APs in a single collision domain, RobinHood provides uplink
throughput of compared to O(1) for omniscient TDMA.
• Uplink throughput in RobinHood scales with the number of
clients.
2
22 NN
10
MO
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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
AP5
Switch
AP6 AP7
11
RobinHood Approach: xi is decoded at the AP where it is expected to have highest SNR
Transmitters
Receiversx1 x2 x3
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Example: Estimate SNR of C1 at AP1
SNR of C1 at AP1 is high
C1
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
12
One path available with high
SNR
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Example: Estimate SNR of C1 at AP3
SNR of C1 at AP3 is low
C1
AP1 AP2 AP3
AP4 AP5
Switch
AP6 AP7
13
No path available with high
SNR • C1 should be decoded by AP1
• AP1 should act as receiver in slot 2
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Trace-Driven Simulation
• All clients and APs are in a single collision domain
• Vary the number of clients (N)
– Number of APs is always
• Assume no power adaptation
• Other algorithms simulated– Omniscient TDMA– IEEE 802.11
2
22 NN
14
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Simulation Results: Throughput
3 5 8 10 13 150
50
100
150
200
RobinHoodOmniscient TDMAIEEE 802.11
Number of Clients
Tota
l Th
roug
hput
(in
Mbp
s)
15
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Challenges
• Synchronization
• MultiCollision domain
• Inconsistency in the number of APs
• Robustness
• Reducing the overhead of channel estimation
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17
Summary
• RobinHood leverages the high density of APs to scale the uplink wireless throughput for single antenna mobile clients.
Thank you