1. DAC: Distributed Asynchronous Cooperation for Wireless Relay Networks 1 Xinyu Zhang, Kang G. Shin University of Michigan

2. Outline 2 Introduction DesignConclusion DAC (routing) PHY layerMAC layer CSMA/CR Implementation & evaluation GNURadio /USRP analysis simulation

3. Motivation: sync problem in cooperative relaying Non-orthogonal cooperative relaying Multiple transmitters sending the same packet (V-MISO) 3 A B C S Major obstacle towards practical use: Sync among distributed relays In theory, realized via STBC or beamforming

4. DAC: asynchronous non-orthogonal relaying (b) DAC, asynchronous non- orthogonal relaying 4 (a) Synchronous non- orthogonal relaying protocol A B C S A B C S Preserve transmit diversity Circumvent the sync problem DAC: Via a new MAC/PHY: CSMA/CR (CSMA with collision resolution)

5. The DAC network stack 5 PHYMAC Resolve collisions via signal processing Encourage resolvable collisions via intelligent sensing and scheduling CSMA/CR Cooperative relaying Asynchronous, non-orthogonal relaying protocol DAC

6. CSMA/CR: PHY layer Resolve the collided packet by iterative decoding S --- the received symbol. A’ --- estimated based on A C = S – A’ 6P1 P1 A A' B B' C C' S=A' + C D E D'E' Y' Z' Y Z Key problem: how to reconstruct A’ based on A? A B C

7. Challenges and solutions: Channel estimation (phase and amplitude): correlation Frequency offset estimation: Costas loop Symbol and sample level timing offset: MM circuit Identify exact start time of packets: sample level correlation Transmitter distortion: reverse engineering tx filter 7 A A' B B' C C' S=A' + C D E D'E' Y' Z' Y Z

8. Implementation on GNURadio and verification on an SDR network: A D B A, B transmit the same packets to D 8 PER performance of forward-direction collision resolution

9. CSMA/CR: MAC layer Key rule: If the channel is busy, and the packet on the air is the one to transmit, then start the transmission. Sensing and scheduling: --- encourage resolvable collision 9 Otherwise, degenerate to CSMA/CA P1 P1 P1 P1 A B C

10. DAC: CSMA/CR-based cooperative relaying Objective: Improve throughput performance of cooperative relaying using collision resolution Basic idea: 7 5 6 S 8 9 D 3 4 1 2 10 DAC (Distributed Asynchronous Cooperation) Add secondary relays to primary relays 11 How to select relays? Establish a primary path

11. DAC: relay selection Select secondary relays: Optimal relay selection: primary relay secondary relay Select resulting in minimum delay from to A model-driven approach, based on average link quality 11

12. DAC: Diversity-multiplexing tradeoff Q: Does DAC improve total network throughput? Interference range 12 Throughput ++ Secondary relay provides diversity gain for the primary path, but may reduce the multiplexing opportunity of other flows. Throughput −−

13. Network model: Homogeneous erasure network with reception probability Grid network: Arbitrary network topology: Wireless LAN: throughput of DAC throughput of DAC throughput of the single-path routing protocol throughput of the single-path routing protocol 13 Analytical results: Sufficient conditions for : A: DAC improves the throughput of lossy networks (e.g. Roofnet)

14. Implement DAC in ns-2 Benchmark protocol: ETX routing * D. Couto, D. Aguayo, J. Bicket and R. Morris, A High-throughput Path Metric for Multi-hop Wireless Routing, In Proc. of ACM MobiCom, 2003 DAC: Simulation experiments 14 Routing metric: expected transmission count

15. Single-unicast scenario: DAC throughput gain ranges from 1.1 to 2.9, avg 1.7 Throughput gain is higher for low-throughput paths 15

16. Multiple-unicast scenario: DAC results in higher network throughput DAC shows a higher level of fairness 16

17. Multiple-unicast scenario, non-lossy networks: DAC may have lower network throughput DAC still maintains a higher level of fairness 17

18. Related work: Iterative cancellation: 18 * S. Gollakotam, D. Katabi. ZigZag Decoding: Combating Hidden Terminals in Wireless Networks, in Proc. of ACM SIGCOMM, 2008. Cooperative relaying: * R. Mudumbai, et al. On the Feasibility of Distributed Beamforming in Wireless Networks, in IEEE Trans. On Wireless Communications. Vol. 6, No. 5, May 2007. * J. Zhang, J. Jia, Q. Zhang and E. M. K. Lo, Implementation and Evaluation of Cooperative Communication Schemes in Software-Defined Radio Testbed. In Proc. of IEEE INFOCOM, 2010

19. Conclusion 19 Circumvent sync problem in cooperative relaying via PHY layer signal processing DAC (Distributed Asynchronous Cooperation): Diversity-multiplexing tradeoff Collision tolerant scheduling & relay selection DAC: asynchronous cooperative relaying, based on a SDR PHY

20. Thank you!