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  1. Decentralized Jointly Sparse Optimization byReweighted Lq Minimization Qing Ling Department of Automation University of Science and Technology of China Joint work with Zaiwen Wen (SJTU) and Wotao Yin (RICE) 2012/09/05

  2. A brief introduction to my research interest optimization and control in networked multi-agent systems autonomous agents - collect data - process data - communicate problem: how to efficiently accomplish in-network optimization and control tasks through collaboration of agents?

  3. Large-scale wireless sensor networks: decentralized signal processing, node localization, sensor selection … blind anchor how to localize blinds with anchors? how to fuse big sensory data? e.g. structural health monitoring difficulty in data transmission → decentralized optimization without any fusion center how to assign sensors to targets?

  4. Computer/server networks with big data: collaborative data mining new challenges in the big data era - big data is stored in distributed computers/servers - data transmission is prohibited due to bandwidth/privacy/… - computers/servers collaborate to do data mining distributed/decentralized optimization

  5. Wireless sensor and actuator networks: with application in large-scale greenhouse control • wireless sensing • temperature • humidity • … • wireless actuating • circulating fan • wet curtain • … • disadvantages of traditional centralized control • communication burden in collecting distributed sensory data • lack of robustness due to packet-loss, time-delay, … decentralized control system design

  6. Recent works wireless sensor networks - decentralized signal processing with application in SHM - decentralized node localization using SDP and SOCP - decentralized sensor node selection for target tracking collaborative data mining - decentralized approaches to jointly sparse signal recovery - decentralized approaches to matrix completion • wireless sensor and actuator networks • modeling, hardware design, controller design, prototype • theoretical issues • convergence and convergence rate analysis

  7. Decentralized Jointly Sparse Optimization byReweighted Lq Minimization Qing Ling Department of Automation University of Science and Technology of China Joint work with Zaiwen Wen (SJTU) and Wotao Yin (RICE) 2012/09/05

  8. Outline • Background • decentralized jointly sparse optimization with applications • Roadmap • nonconvex versus convex, difficulty in decentralized computing • Algorithm development • successive linearization, inexact average consensus • Simulation and conclusion

  9. Background (I): jointly sparse optimization • Structured signals • A sparse signal: only few elements are nonzero • Jointly sparse signals: sparse, with the same nonzero supports zeros nonzeros • Jointly sparse optimization: to recover X from linear measurements measurement matrix measurement noise

  10. Background (II): decentralized jointly sparse optimization • Decentralized computing in a network • Distributed data in distributed agents & no fusion center • Consideration of privacy, difficulty in data collection, etc • Decentralized jointly sparse optimization Goal: agent i has y(i) and A(i), to recover x(i) through collaboration

  11. Background (III): applications • Cooperative spectrum sensing [1][2] • Cognitive radios sense jointly sparse spectra {x(i)} • Measure from time domain [1] or frequency selective filter [2] • Decentralized recovery from {y(i)=A(i)x(i)} • Decentralized event detection [3] • Sensors {i} sense few targets represented by jointly sparse {x(i)} • Decentralized recovery from {y(i)=A(i)x(i)} • Collaborative data mining, distributed human action recognition, etc [1] F. Zeng, C. Li, and Z. Tian, “Distributed compressive spectrum sensing in cooperative multi-hop wideband cognitive networks,” IEEE Journal of Selected Topics in Signal Processing, vol. 5, pp. 37–48, 2011 [2] J. Meng, W. Yin, H. Li, E. Houssain, and Z. Han, “Collaborative spectrum sensing from sparse observations for cognitive radio networks,” IEEE Journal on Selected Areas on Communications, vol. 29, pp. 327–337, 2011 [3] N. Nguyen, N. Nasrabadi, and T. Tran, “Robust multi-sensor classification via joint sparse representation,” submitted to Journal of Advance in Information Fusion

  12. Roadmap (I): nonconvex versus convex • Convex model: group lasso or L21 norm minimization regularization parameter • Nonconvex versus convex • Convex: with global convergence guarantee • Nonconvex: often with better recovery performance • Look back on nonconvex models to recover a single sparse signal • Reweighted L1/L2 norm minimization [4][5] • Reweighted algorithms for jointly sparse optimization? [4] E. Candes, M. Wakin, and S. Boyd, “Enhancing sparsity by reweighted L1 minimization,” Journal of Fourier Analysis and Applications, vol. 14, pp. 877–905, 2008 [5] R. Chartrand and W. Yin, “Iteratively reweighted algorithms for compressive sensing,” In: Proceedings of ICASSP, 2008

  13. Roadmap (II): difficulty in decentralized computing • A popular decentralized computing technique: consensus objective function in agent i common optimization variable local copy in agent i neighboring copies are equal • Obviously, two problems are equivalent for a connected network • Efficient algorithms (ADM, SGD, etc) for if it is convex [6] • Nothing for consensus in jointly sparse optimization! • Signals are different; common supports bring nonconvexity [6] D. Bertsekas and J. Tsitsiklis, Parallel and Distributed Computation: Numerical Methods, Second Edition, Athena Scientific, 1997

  14. Roadmap (III): solution overview • Nonconvex model + convex decentralized computing subproblem • Nonconvex model -> successive linearization -> reweighted Lq • Natural decentralized computing, one nontrivial subproblem • Inexactly solving the subproblem still leads to good recovery

  15. Algorithm (I): successive linearization smoothing parameter • Nonconvex model (q=1 or 2) regularization parameter • “Successive linearization” to the joint sparsity term at t Actually a majorization minimization approach

  16. Algorithm (II): reweighted algorithm • Centralized reweighted Lq minimization algorithm • Updating weight vector weight vector u=[u1; u2; uN] • Updating signals • From a decentralized implementation perspective … • Natural decentralized computing in x-update • One subproblem needs decentralized solution in u-update

  17. Algorithm (III): average consensus • Check u-update: average consensus problem • Rewrite to more familiar forms

  18. Algorithm (IV): inexact average consensus • Solve the average consensus problem with ADM (time t, slot s/S) • Updating weight vectors (local copies) • Updating Lagrange multipliers (c is a positive constant) • Exact average consensus versus inexact average consensus • Exact average consensus: exact implementation of reweighted Lq • Introducing inner loops: cost of coordination & communication • Inexact average consensus: one iteration in the inner loop

  19. Algorithm (V): decentralized reweighted Lq • Algorithm outline • Updating weight vectors (local copies) • Updating Lagrange multipliers (c is a positive constant) • Updating signals

  20. Simulation (I): simulation settings • Network settings • L=50 agents, randomly deployed in 100×100 area • Communication range=30, bidirectionally connected • Measurement settings • Signal dimension N=20, signal sparsity K=2 • Measurement dimension M=10 • Random measurement matrices and random measurement noise • Parameter settings

  21. Simulation (II): recovery performance

  22. Simulation (III): convergence rate

  23. Conclusion • Decentralized jointly sparse optimization problem • Jointly sparse signal recovery in a distributed network • Reweighted Lq minimization algorithms • Feature #1: nonconvex model <- successive linearization • Feature #2: decentralized computing <- inexact average consensus • Good news and bad news • Local convergence of the centralized algorithms • Excellent performance of the decentralized algorithms • No theoretical performance guarantee (recovery and convergence) • Outlook: many open questions in decentralized optimization

  24. Thanks for your attention!