Cooperative Transmissions in Wireless Sensor Networks with Imperfect Synchronization

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Cooperative Transmissions in Wireless Sensor
Networks with Imperfect Synchronization

• Xiaohua (Edward) Li, Mo Chen and Wenyu Liu
• Department of Electrical and Computer Engineering
• State University of New York at Binghamton
• xli, mchen0_at_binghamton.edu, hyusa_at_hyig.com
• http//ucesp.ws.binghamton.edu/xli

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Major Contributions

• Distributed STBC-encoded transmissions that
  tolerates imperfect synchronization
• Sensor network energy efficiency considering
• Cooperative diversity
• Cooperation overhead
• Synchronization cost

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Contents

• Introduction
• Cooperative transmissions in LEACH overhead
  analysis
• Distributed cooperative transmission
  synchronization problem
• New STBC with imperfect synchronization
• Energy efficiency analysis and simulations
• Conclusions

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Introduction

• Cooperative transmissions in sensor networks
  exploit the collaborative nature of sensors
• Cooperative STBC diversity induces energy
  efficiency
• Challenges
• Circuitry energy consumption increases
• Cooperation overhead reduces energy efficiency
• Imperfect synchronization makes STBC not directly
  applicable

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• Objectives study the impact of
• overhead of cooperation circuitry energy by
  considering jointly PHY-layer cooperative
  transmission and higher-layer LEACH protocol
• imperfect synchronization by developing new
  distributed STBC-encoded transmissions

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Cooperative transmissions in LEACH

• Protocol modification overhead analysis
• Phase I. Advertisement to determine primary head
• Phase II. Cluster setup
• one-byte more transmission
• Phase III. TDMA transmission schedule
• determine secondary heads
• one-byte more transmission
• Phase IV. Data transmission
• Primary head broadcasts to secondary heads
• Cooperative transmissions

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• Major overhead is in Data Transmission Phase
• Overhead is small

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Synchronization of distributed transmissions

• Secondary heads synchronize frequency timing to
  primary heads
• Carrier phase timing phase asynchronism makes
  channels dispersive ? ISI
• Different relative delays destroy STBC structure
• Non-dispersive channel model in flat-fading
  environment, and distances among cooperative
  sensors are small enough ? STBC directly
  applicable
• Dispersive channel with delays in frequency
  selective fading or large distance (for
  macro-diversity) ? STBC not directly applicable

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New STBC with imperfect synchronization

• Existing work on cooperative STBC idealized
  synchronization
• What if synchronization is imperfect?
• distance may be large for macro-diversity
• synchronization may be impossible in multi-hop
  networks

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• Proposed STBC transmission scheme
• J transmitters transmit a data packet in P
  frames
• Transmissions may be conjugated and time-reversed

Special J2 nodes per cluster
General J nodes per cluster
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• Receiving procedure
• From received signal
• linear (maximal ratio) combiner for STBC decoding
• linear equalizer for symbol estimation
• Properties
• Tolerate asynchronous delays dispersive
  channels
• Full diversity, with linear complexity
• Rate comparable to ordinary STBC (for J2 to 5)

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Energy efficiency analysis and simulations

• Transmission energy efficiency
• Energy saving ratio with respect to
  single-transmission

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• Simulations no loss of diversity while
  tolerating asynchronous transmissions

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• Overall energy efficiency
• Consider cooperation overhead, circuitry energy,
  and synchronization cost
• Use first-order energy consumption model
• If transmission distance d satisfy
• then cooperative transmission is advantageous.
• With typical parameters, for J2,3,4,5, we have
  d39,57,69,87 meters
• Cooperative transmission is useful in sensor
  networks

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• Sensor network simulations 30 longer lifetime
  for J2 than traditional LEACH

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Conclusions

• Propose a distributed STBC-encoded transmission
  scheme with tolerance to imperfect
  synchronization
• Study energy efficiency of cooperative
  transmissions considering cooperation overhead,
  circuitry energy, synchronization cost
• Demonstrate the advantage of cooperative
  transmission in sensor networks