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Cooperative Transmissions in Wireless Sensor Networks with Imperfect Synchronization

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}@binghamton.edu, hyusa@hyig.com

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Cooperative Transmissions in Wireless Sensor Networks with Imperfect Synchronization

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  1. 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}@binghamton.edu, hyusa@hyig.com http://ucesp.ws.binghamton.edu/~xli

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

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

  4. 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

  5. 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

  6. Cooperative transmissions inLEACH • 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

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

  8. 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

  9. 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

  10. Proposed STBC transmission scheme: • J transmitters transmit a data packet in P frames • Transmissions may be conjugated and time-reversed Special: J=2 nodes per cluster General: J nodes per cluster

  11. 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 J=2 to 5)

  12. Energy efficiency analysis and simulations • Transmission energy efficiency: • Energy saving ratio with respect to single-transmission

  13. Simulations: no loss of diversity while tolerating asynchronous transmissions

  14. 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 J=2,3,4,5, we have d=39,57,69,87 meters • Cooperative transmission is useful in sensor networks

  15. Sensor network simulations: 30% longer lifetime for J=2 than traditional LEACH

  16. 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

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