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

Enterprise Wireless LAN (EWLAN)

2

AP AP AP

AP AP AP

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

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]

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?

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

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

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

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

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

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

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

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

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

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

Challenges

• Synchronization

• MultiCollision domain

• Inconsistency in the number of APs

• Robustness

• Reducing the overhead of channel estimation

17

Summary

• RobinHood leverages the high density of APs to scale the uplink wireless throughput for single antenna mobile clients.

Thank you