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Opportunistic Large Arrays

Opportunistic Large Arrays. By Anna Scaglione and Yao-Win Hong. Introduction . The reach back problem Issues and limitations Cooperative transmission of Ad Hoc nodes to remote receiver otherwise not reachable – Opportunistic Large Arrays

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Opportunistic Large Arrays

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  1. Opportunistic Large Arrays By Anna Scaglione and Yao-Win Hong IEEE ISWC 2002

  2. Introduction • The reach back problem • Issues and limitations • Cooperative transmission of Ad Hoc nodes to remote receiver otherwise not reachable –Opportunistic Large Arrays • OLAas the avalanche of cheers by spectators in a stadium • OLA distributed modem, adaptive receiver and characteristic features • Conclusions and future work IEEE ISWC 2002

  3. Related Work • Increasing Capacity with Mobility • M. Grossglauser, D. Tse, “Mobility Increases the Capacity of Ad-Hoc Wireless Networks,” IEEE Proc. INFOCOM 2001 • Richard H. Frenkiel, B. R. Badrinath, Joan Borras, and Roy D. Yates, “The Infostations Challenge: Balancing Cost and Ubiquity in Delivering Wireless Data,” IEEE Personal Comm., April 2000 • Cooperative Diversity • J.N. Laneman, G.W. Wornell, D.N.C. Tse,“An Efficient Protocol for Realizing Cooperative Diversity in Wireless Networks”, ISIT 2001 IEEE ISWC 2002

  4. Reach Back Problem • Problem: • Design a scheme that allows two-way transmission from a set of asynchronous transmitters, with low battery life and low peak power, to a remote destination with which they cannot individually exchange data reliably • Cooperative Transmission Technique • Is cooperation possible through the network connectivity? • How can we enforce scalability and adaptability? IEEE ISWC 2002

  5. Bottleneck: Inter-node Communications • Signaling is necessary to form a Phased array or do Space-Time Coding • Throughput per node vanishes in the limit as the number of nodes diverges [Gupta-Kumar] • Obstacle in coordinating large scale networks to perform any form of synchronized activity • Is this an insurmountable obstacle? • In the uplink network connectivity should not be used to support the bandwidth consuming inter-node signaling IEEE ISWC 2002

  6. New approach • How can we impress coordination if the nodes responses are asynchronous? • Idea: Generate avalanches of signals • Opportunistic Large Array (OLA) • Formed by firing signals in response to signals fired by special nodes (leaders) • Issues: • Diversity under the OLA configuration • Signal Estimation and Receiver Training IEEE ISWC 2002

  7. How it works: • Leader triggers transmission. • Avalanche effect from Ad Hoc nodes (like the OLA in a stadium) IEEE ISWC 2002

  8. Does it work for large networks? • Basis: experimental evidence of neural communications • The communication of any neuron pair is almost entirely lost • The avalanche of pulses fired by the neurons provide coordination, redundancy for reliability, stability, robustness and all sorts of desirable properties • Bio-Inspired Communication Networks IEEE ISWC 2002

  9. System Model • Let leader trigger with a pulse pm(t): where An is the complex fading coefficient, n(t) is AWGN with variance N0, and τn is the delay of node n. • Spread Spectrum System • BW=1/Tp>>1/Ts=Rate • Multipath Fading Problem • Delay Spread >> Tp • Frequency Selective Fading IEEE ISWC 2002

  10. OLA Structure • Let where c is the speed of light. • Assume smaller than radius of transmission. • Let Nk = # of nodes in ring Dk. • If N sufficiently large, sm(kTp) is Gaussian with variance IEEE ISWC 2002

  11. OLA Structure • Parameters that affect the transmitter alphabet • Distribution of the nodes in the network • Nodes modulation technique • Leader selection • Pulse-width of symbol waveform • Size of network • Spatial distribution of nodes provide diversity in the most non-artificial way IEEE ISWC 2002

  12. OLA Modulations I • Linear Modulations • Let sm be the complex symbol for an M-ary constellation (QAM, ASK, PSK), p(t) = pulse shape at each node g(t) = effective aggregated pulse shape. Signal Processing perspective: OLA is like a multi-path channel with positive gain! • Sample at every Tp, • ML receiver: assuming (sme+n) is Gaussian, IEEE ISWC 2002

  13. OLA Modulations II • Orthogonal Modulations. • Frequency Shift Keying • When Tp is large enough, s.t. all pulses overlap then no need for training with incoherent detection • Pulse Position Modulation – UWB applications • OLA using UWB generates signals that are still nearly orthogonal. IEEE ISWC 2002

  14. OLA Modulations III • Leader Position Modulation • Choose leaders to be at positions sufficiently apart. • The waveform generated by the Mth leader or group of leaders represent the Mth symbol waveform. where An is reordered, and τn is reassigned. • Advantage: • Nodes need only to transmit one type of pulse. • Need not decode symbols, just react to the received power variation. IEEE ISWC 2002

  15. OLA Modulations IV • Leader Position Modulation IEEE ISWC 2002

  16. Adaptive Receiver with Training I • Let where r(i)=[r(iTs), r(iTs+Tp), …, r(iTs+kmaxTp)]T, sm is the sampled symbol waveform, and n(i) the sampled noise. • M.L. Estimation of the OLA signatures • Mean Square Estimation Error contributes to the received noise • Use pseudo-noise sequences during training for Low Probability of Detection IEEE ISWC 2002

  17. Adaptive Receiver with Training II • Adaptive LMMSE Estimator • To minimize , • Decision directed mode • as in memory and learning each decision updates the OLA set of waveforms • Problem: • Training for newly setup nodes • Solution Blind Estimation IEEE ISWC 2002

  18. Adaptive Receiver with Blind Estimate • Constant Modulus Algorithm [Godard ‘80] • To minimize , • Subspace Method • Assume si a white process • Estimated with • Find noise subspace eigenvectors ud: • The equivalent channel is the null space of noise subspace. IEEE ISWC 2002

  19. Intra-OLA Communication • Flooding network with the data from one source • Using exact same receiver structure for both nodes and remote receiver • Each parameter different for every node i. • Communication between OLA “coalitions” • Compressing the joint information of each node while distributing data through the network IEEE ISWC 2002

  20. Performance Analysis I • Error performance between joint transmission and individual transmission • Effect of estimation error IEEE ISWC 2002

  21. Performance Analysis I • Robustness toward Moving nodes or Unfriendly nodes IEEE ISWC 2002

  22. OLA Downlink - Distributed Receiver • Many faulty receivers = One good receiver • The nodes share with close by nodes their detections, compress and detect jointly the data as they travel through the network (distributed detection for ad-hoc networks) IEEE ISWC 2002

  23. Conclusion • OLA introduces the concept of cooperative communication systems • Advantages in terms of diversity and robustness • Easily constructed on top of existing systems • Suitable for applications such as security, maintenance, control signal etc. IEEE ISWC 2002

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