RobinHood: Sharing the Happiness in a Wireless Jungle

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

2. Enterprise Wireless LAN (EWLAN) AP AP AP AP AP AP

3. Uplink Traffic • 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 Code Offloading VoIP, Video Chat Sensor Data Upload

4. Existing Schemes • Interference Alignment • Existing IA schemes perform alignment over exponential number of time slots [Cadambeet 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 [Gollakotaet al., SIGCOMM 2009; Gowdaet al., INFOCOM 2013]

5. AP Density in Enterprise WLANs • Can we leverage the underutilized backbone and the high density of APs to scale the uplink throughput? CDF of number of APs observed (Measurements conducted at Ohio State University campus)

6. RobinHood Highlights • 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

7. Example Topology (Single Collision Domain) with Omniscient TDMA Time Slot: 2 Time Slot: 1 Time Slot: 3 AP1 AP2 AP3 Switch AP4 AP5 AP6 AP7 C1 C2 C3 x1 x2 x3 Three Packets received in Three Slots. Only one AP is in use.

8. Example Topology (Single Collision Domain) with RobinHood a11x1 a12x1 + a22x2 a13x1 + a23x2 + a33x3 Time Slot: 2 Time Slot: 1 AP1 AP2 AP3 h15x1 + h25x2 + h35x3 h14x1 + h24x2 + h34x3 Switch ... ... AP4 AP5 AP6 AP7 v4 * (h14x1 + h24x2 + h34x3 ) C1 C2 C3 v5 * (h15x1 + h25x2 + h35x3 ) v6 * (...) v7 * (...) x1 x2 x3

9. Example Topology (Single Collision Domain) with RobinHood a11x1 a12x1 + a22x2 a13x1 + a23x2 + a33x3 Time Slot: 2 Time Slot: Background AP1 AP2 AP3 Switch - a12x1 = a22x2 - a13x1 - a23x2 = a33x3 • Three Packets received in Two Slots

10. 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.

11. Further Optimizations to improve SNR x1 x2 AP3 AP2 x3 AP1 Receivers • Which subset of APs act as transmitters and which subset as receivers? • Which AP decodes which packet? AP5 AP7 AP6 Switch Transmitters AP4 C1 C2 C3 RobinHood Approach: xi is decoded at the AP where it is expected to have highest SNR

12. Example: Estimate SNR of C1 at AP1 SNR of C1 at AP1 is high AP1 AP2 AP3 One path available with high SNR Switch AP4 AP5 AP6 AP7 C1

13. Example: Estimate SNR of C1 at AP3 SNR of C1 at AP3 is low AP1 AP2 AP3 No path available with high SNR Switch AP4 AP5 AP6 AP7 • C1 should be decoded by AP1 • AP1 should act as receiver in slot 2 C1

14. 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

15. Simulation Results: Throughput

16. 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