RobinHood: Sharing the Happiness in a Wireless Jungle Tarun Bansal, Wenjie Zhou, Kannan Srinivasan and Prasun Sinha Department of Computer Science and Engineering Ohio State University, Columbus, Ohio

Enterprise Wireless LAN (EWLAN) 2 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 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

C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 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

C1C1 C2C2 C3C3 x1x1 x2x2 x3x3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 Example Topology (Single Collision Domain) with RobinHood Time Slot: 1 h 14 x 1 + h 24 x 2 + h 34 x 3 h 15 x 1 + h 25 x 2 + h 35 x 3... Time Slot: 2 v 4 * (h 14 x 1 + h 24 x 2 + h 34 x 3 ) v 5 * (h 15 x 1 + h 25 x 2 + h 35 x 3 ) v 6 * (...) v 7 * (...) a 11 x 1 a 12 x 1 + a 22 x 2 a 13 x 1 + a 23 x 2 + a 33 x 3 8

AP 1 AP 2 AP 3 Switch Example Topology (Single Collision Domain) with RobinHood Time Slot: 2 a 11 x 1 a 12 x 1 + a 22 x 2 a 13 x 1 + a 23 x 2 + a 33 x 3 - a 12 x 1 = a 22 x 2 - a 13 x 1 - a 23 x 2 = a 33 x 3 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. 10

Further Optimizations to improve SNR Which subset of APs act as transmitters and which subset as receivers? Which AP decodes which packet? C1C1 C2C2 C3C3 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 11 RobinHood Approach: x i is decoded at the AP where it is expected to have highest SNR Transmitters Receivers x1x1 x2x2 x3x3

Example: Estimate SNR of C 1 at AP 1 SNR of C 1 at AP 1 is high C1C1 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 12 One path available with high SNR

Example: Estimate SNR of C 1 at AP 3 SNR of C 1 at AP 3 is low C1C1 AP 1 AP 2 AP 3 AP 4 AP 5 Switch AP 6 AP 7 13 No path available with high SNR C 1 should be decoded by AP 1 AP 1 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

Simulation Results: Throughput 15

Challenges Synchronization MultiCollision domain Inconsistency in the number of APs Robustness Reducing the overhead of channel estimation

Summary RobinHood leverages the high density of APs to scale the uplink wireless throughput for single antenna mobile clients. 17