Power Consumption by Wireless Communication

1. Power Consumption by Wireless Communication Lin Zhong ELEC518, Spring 2011

2. Power consumption (SMT5600)

3. Power consumption (T-Mobile) Bluetooth Cellular Wi-Fi

4. Power consumption (Contd.) • Theoretical limits • Receiving energy per bit > N * 10-0.159 • N: Noise spectral power level • Wideband communication PRX PTX∝ PRX*da Distance: d Propagation constant: a (1.81-5.22)

5. Power consumption (Contd.) • What increases power consumption • Government regulation (FCC) • Available spectrum band (Higher band, higher power) • Limited bandwidth • Limited transmission power • Noise and reliability • Higher capacity • Multiple access (CDMA, TDMA etc.) • Security • Addressability (TCP/IP) • More……

6. Wireless system architecture Network protocol stack Hardware implementation Host computer Application Transport Network Data link Baseband Physical Network interface RF front ends

7. Power consumption (Contd.) Low-noise amplifier LNA Intermediate Frequency (IF) signal processing Antenna interface Baseband processor Local Oscillator (LO) IF/Baseband Conversion PA MAC Layer & above Physical Layer Power amplifier >60% non-display power consumed in RF RF technologies improve much slower than IC

8. Power consumption (Contd.) Source: Li et al, 2004

9. Low-noise amplifier (LNA) • Bandwidth (same as the signal) • Gain (~20dB) • Linearity (IP3) • Noise figure (1dB) • Power consumption

10. Circuit power optimization • Major power consumers Low-noise amplifier High duty cycle Huge dynamic range 105 LNA Intermediate Frequency (IF) signal processing Local Oscillator (LO) Almost always on Antenna interface Baseband processor IF/Baseband Conversion PA MAC Layer & above Physical Layer Power amplifier High power consumption

11. Circuit power optimization (Contd.) • Reduce supply voltage • Negatively impact amplifier linearity • Higher integration • CMOS RF • SoC and SiP integration • Power-saving modes

12. Circuit power optimization (Contd.) • Power-saving modes • Complete power off • (Circuit wake-up latency + network association latency) on the order of seconds • Different power-saving modes • Less power saving but short wake-up latency

13. Power-saving modes Radio Deep Sleep Wake-up latency on the order of micro seconds Low-noise amplifier LNA Intermediate Frequency (IF) signal processing Antenna interface Baseband processor Local Oscillator (LO) IF/Baseband Conversion PA MAC Layer & above Physical Layer Power amplifier

14. Power-saving modes (Contd.) Sleep Mode Wake-up latency on the order of milliseconds Low-rate clock with saved network association information Low-noise amplifier LNA Intermediate Frequency (IF) signal processing Antenna interface Baseband processor Local Oscillator (LO) IF/Baseband Conversion PA MAC Layer & above Physical Layer Power amplifier

15. Network power optimization • Use power-saving modes • Example: 802.11 wireless LAN (WiFi) • Infrastructure mode: Access points and mobile nodes • Example: Cellular networks

16. 802.11 infrastructure mode • Mobile node sniffs based on a “Listen Interval” • Listen Interval is multiple of the “beacon period” • Beacon period: typically 100ms • During a Listen Interval • Access point • buffers data for mobile node • sends out a traffic indication map (TIM), announcing buffered data, every beacon period • Mobile node stays in power-saving mode • After a Listen Interval • Mobile node checks TIM to see whether it gets buffered data • If so, send “PS-Poll” asking for data

17. Buffering/sniffing in 802.11 Gast, 802.11 Wireless Network: The Definitive Guide 802.15.1/Bluetooth uses similar power-saving protocols: Hold and Sniff modes

18. Cellular networks • Discontinuous transmission (DTX) • Discontinuous reception (DRX)

19. Wireless energy cost • Connection • Establishment • Maintenance • Transfer data • Transmit vs. receive

20. Energy per bit transfer Oppermann et al., IEEE Comm. Mag. 2004

21. Wasteful wireless communication Time Micro power management Spectrum Efficiency-driven cognitive radio Space Directional communication

22. Space waste • Omni transmission huge power by power amplifier (PA)

23. Time waste • Network Bandwidth Under-Utilization • Modest data rate required by applications • IE ~ 1Mbps, MSN video call ~ 3Mbps • Bandwidth limit of wired link • 6Mbps DSL at home 23

24. Spectrum waste

25. Observed from an 802.11g user Energy per bit Distribution of observed 802.11g throughput

26. Temporal waste 90% of time & 80% of energy spent in idle listening Four 802.11g laptop users, one week

27. Fundamental problem with CSMA • CSMA: Carrier Sense Multiple Access • Clients compete for air time • Incoming packets are unpredictable

28. Fundamental problem with CSMA

29. Micro power management (µPM) • Sleep during idle listening • Wake up in time to catch retransmission • Monitor the traffic not to abuse it • ~30% power reduction • No observed quality degradation J. Liu and L. Zhong, "Micro power management of active 802.11 interfaces," in Proc. MobiSys’08.

30. Directional waste Ongoing project with Ashutosh Sabharwal

31. Directional waste

32. Two ways to realize directionality • Passive directional antennas • Low cost • fixed beam patterns • Digital beamforming • Flexible beam patterns • High cost Desclos, Mahe, Reed, 2001 Phased-array antenna system from Fidelity Comtech

33. Challenge I: Rotation!!! Solution: Don’t get rid of the omni directional antennas Use multiple directional antennas But can we select the right antenna in time?

34. Challenge II: Multipath fading

35. Challenge III • Can we do it without changing the infrastructure?

36. Characterizing smartphone rotation • How much do they rotate? • How fast do they rotate? • 11 HTC G1 users, each one week • Log accelerometer and compass readings • 100Hz when wireless in use

37. Device orientation described by three Euler angles • θ and φ based on tri-axis accelerometer • ψ based on tri-axis compass and θ and φ

38. Rotation is not that much • <120° per second

39. Directionality indoor 5 dBi 8 dBi

40. 5dBi antenna 8dBi antenna

41. Measurement setup • RSSI measured at both ends Data packets ACK packets

42. Directional channel still reciprocal

43. Directional beats omni close to half of the time Field collected rotation traces replayed

44. RSS is predictable (to about 100ms)

45. Multi-directional antenna design (MiDAS) • One RF chain, one omni antenna, multiple directional antennas • Directional ant. only used for data transmit and ACK Reception • Standard compliance • Tradeoff between risk and benefit

46. Packet-based antenna selection • Assess an antenna by receiving a packet with it • Leveraging channel reciprocity • Continuously assess the selected antenna • Find out the best antenna by assessing them one by one • Potential risk of missing packets • Stay with omni antenna when RSS changes rapidly • No change in 802.11 network infrastructure

47. Symbol-based antenna selection • Assess all antennas through a series of PHY symbols • Similar to MIMO antenna selection • Needs help from PHY layer Antenna training packet Regular packet SEL ACK

48. Trace based evaluation • Rotation traces replayed on the motor • RSSI traces collected for all antennas • Algorithms evaluated on traces offline

49. An early prototype Finalist of MobiCom’08 Best Student Demo

50. The busier the traffic, the better