1. 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. 2 Major Contributions Distributed STBC-encoded transmissions that tolerates imperfect synchronization Sensor network energy efficiency considering 1.Cooperative diversity 2.Cooperation overhead 3.Synchronization cost

3. 3 Contents 1.Introduction 2.Cooperative transmissions in LEACH: overhead analysis 3.Distributed cooperative transmission: synchronization problem 4.New STBC with imperfect synchronization 5.Energy efficiency analysis and simulations 6.Conclusions

4. 4 1.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. 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. 6 2.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

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

8. 8 3.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. 9 4.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. 10 Special: J=2 nodes per cluster General: J nodes per cluster Proposed STBC transmission scheme: J transmitters transmit a data packet in P frames Transmissions may be conjugated and time-reversed

11. 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. 12 5.Energy efficiency analysis and simulations Transmission energy efficiency: –Energy saving ratio with respect to single- transmission

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

14. 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. 15 Sensor network simulations: 30% longer lifetime for J=2 than traditional LEACH

16. 16 6.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