1 Energy Efficiency of MIMO Transmissions in Wireless Sensor Networks with Diversity and Multiplexing Gains Wenyu Liu, Xiaohua (Edward) Li and Mo Chen Department of Electrical and Computer Engineering State University of New York at Binghamton {xli,

2 Energy efficiencies of some MIMO transmission schemes in wireless sensor networks are analyzed considering the trade-off between diversity and multiplexing gains Optimal energy efficiency requires both diversity and multiplexing gain Cooperative MIMO are potential for enhancing energy efficiency Abstract

3 Outline 1.Introduction 2.MIMO Transmission Schemes 3.Energy Efficiency of non-cooperative MIMO 4.Energy Efficiency of cooperative MIMO 5.Simulations 6.Conclusions

4 1.Introduction Cooperative transmissions in sensor networks: exploit the collaborative nature of sensors Cooperative STBC to improve energy efficiency: depending on transmission distance Overheads –Circuitry energy consumption increases –Cooperation overhead reduces energy efficiency Cooperative MIMO: Is it better for energy efficiency? –Even higher overheads –There is a fundamental trade-off between the diversity gain and the multiplexing gain.

5 2. MIMO Transmission Schemes a)non-cooperative MIMO b) one-side, half-cooperative MIMO Physical antenna array in Rx A cluster of sensors forming an virtual array at Tx Physical antenna array in Rx Physical antenna array in Tx

6 c) two-side, cooperative MIMO A cluster of sensors forming an virtual array at Tx A cluster of sensors forming an virtual array at Rx General Cooperative MIMO Description: Cooperative transmission: The primary head sensor first broadcasts to the secondary head sensors the data to be transmitted. Then at the next time slot, all the heads (the primary and secondary) perform cooperative transmission. Cooperative receiving: All the secondary heads forward their received signals to the primary head, where the MIMO signal detection is performed.

7 MIMO Signal Model M r x 1 received signal M r x M t channel matrix power adjuster M t x 1 transmitted Signal, zero mean, σ s M r x 1 AWGN, zero mean, σ v The received signal-to-noise ratio (SNR) at each antenna :

8 3. Energy Efficiency of Non-cooperative MIMO 3.1, Transmission Energy Efficiency Bit Error Rate Transmission Date Rate Diversity Gain – Improve energy efficiency Multiplexing Gain – Achieve higher rate → higher trans. power – Reduce time → enhance energy efficiency

9 Trade off between d r and r for some MIMO schemes

10 Transmission Energy Large scale path loss with exponent n Total data to be transmitted Transmission energy depends on both diversity gain d r and multiplexing gain r N/-logP e Cσ2vCσ2v

11 Circuitry Energy Circuit Energy Constant Overall transmission and circuitry energies

12 Non Cooperative MIMO energy efficiency (J tc /K tc ×10 9 ) M t =M r =2, P e =0.001, n = 2 and d =10 meters E t =100pJ / 249 and E c =50 nJ

13 4. Energy Efficiency of Cooperative MIMO With either cooperative or half-cooperative MIMOs, there is energy consumption in cooperative overhead. Primary heads chooses M t -1 secondary heads Step 1: (Overhead is small, and can be skipped) Step 2: Primary heads broadcasts to all the secondary heads its data (N bits) (Major overhead : broadcasting of the N data bits)

14 The data rate of broadcasting from the primary head to the secondary head Signal noise ratio to broadcasting Scale factor due to the fact that the symbol alphabet of broadcast may be different from cooperative transmission Step 3: Primary heads and the secondary heads perform cooperative transmission with energy consumption J tc Total Energy Consumption:

15 Step 4: The M r -1secondary heads quantize their received samples, and transmit them as new symbol sequences to the primary head, where MIMO receiving is performed to recover the original N bits Composite result of quantization and symbol mapping

16 Overall energy consumption of the cooperative MIMO transmission The cooperative or half –cooperative MIMO energy efficiency can be optimized as Overall energy consumption of the half- cooperative MIMO transmission

17 Cooperative MIMO energy efficiency (J a /K tc ×10 9 ) M t =M r =2, P e =0.001, n = 2 and d =100 meters, E t =100pJ / 249 and E c =50 nJ

18 Half-cooperative MIMO energy efficiency (J h /K tc ×10 9 ) M t =M r =2, P e =0.001, n = 2 and d =100 meters, E t =100pJ / 249 and E c =50 nJ

19 5. Simulation Compare the simulated transmission energy consumption with the theoretical values

20 6. Conclusion Derived energy consumption representations for MIMO and cooperative MIMO Cooperative overheads were considered in addition to transmission energy efficiency The MIMO tradeoff between diversity and multiplexing was exploited for transmission energy efficiency optimization MIMO and cooperative MIMO were shown beneficial to sensor network energy efficiency