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## Opportunistic Large Arrays

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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

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

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

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

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

How it works:

- Leader triggers transmission.
- Avalanche effect from Ad Hoc nodes (like the OLA in a stadium)

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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

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

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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

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

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

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

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.

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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

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

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

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

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Performance Analysis I

- Error performance between joint transmission and individual transmission
- Effect of estimation error

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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)

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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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