Beamforming on mobile devices: A first study

Transcription

1 Beamforming on mobile devices: A first study Hang Yu, Lin Zhong, Ashutosh Sabharwal, David Kao

2 Two invariants for wireless Spectrum is scarce Hardware is cheap and getting cheaper 2

3

4 3.2 cm Passive directional antennas 3.2 cm Ardalan Amiri Sani, Lin Zhong, and Ashutosh Sabharwal, "Directional antenna diversity for mobile devices: characterizations and solutions," in Proc. ACM MobiCom, September

5 Findings: ~3 db gain Multifold throughput increase at network edge ~50% TX power reduction at network center 5

6 Can we go beyond 3 db? 6

7 Beamforming? Studied in the past for use on cellular base station, access points, vehicles, and even wireless sensor nodes, e.g., MobiSteer (MobiSys 07), R2D2 (MobiSys 09), DIRC (SIGCOMM 09) 7

8 Beamforming primer 8

9 Beamforming primer Fixed transmission power

10 Beamforming primer Fixed transmission power

11 Beamforming primer Fixed transmission power

12 Beamforming primer Fixed transmission power

13 Is beamforming practical? Beamforming Mobile devices Antenna array Small form factor Narrow beam Rotate and move Power hungry Battery powered 13

14 Peak beamforming gain (db) Form factor? antennas 3 antennas 2 antennas Antenna spacing (wavelength) λ : cm at 2 GHz

15 Form factor! 18 cm 6 cm 24 cm 12 cm λ (4.5-6 cm at 2 GHz) 15

16 Rotation? Client Node Infrastructure Node 16

17 Beamforming gain (db) Rotation? Indoor 6 Max Static 90d/s 180d/s 3 0 N=2 N=4 CSI estimation every 100 ms

18 Beamforming gain (db) Rotation! Indoor 6 Max Static 90d/s 180d/s 3 0 N=2 N=4 CSI estimation every 10 ms

19 Power? (uplink only) P Circuit P PA =P TX / η Baseband Signal DAC Filter Mixer Filter PA 1 Frequency Synthesizer N P Shared Baseband Signal DAC Filter Mixer Filter PA N P = P shared + N P Circuit + P TX / η 19

20 Tradeoff No. 1 P=P shared + 1 P Circuit + P TX / η Fixed receiver SNR

21 Tradeoff No. 1 P=P shared + 2 P Circuit + P TX / η Fixed receiver SNR

22 Tradeoff No. 1 P=P shared + 3 P Circuit + P TX / η Fixed receiver SNR

23 Tradeoff No. 1 P=P shared + 4 P Circuit + P TX / η Fixed receiver SNR

24 Tradeoff No. 1 Optimal number of antennas for efficiency N opt = a P O /P Circuit b P O

25 Transmitter Power Consumption (mw) Hardware is cheap & getting cheaper P = P shared + N P Circuit + P TX / η SISO 2x2 MIMO Sources: Year IEEE Int. Solid-State Circuits Conferences (ISSCC) and IEEE Journal of Solid-State Circuits (JSSC)

26 Power! Beamforming with state-of-the-art multi-rf chain realization is already more efficient! Tradeoff No. 1 is increasingly profitable!

27 Beyond a single link 27

28 What the carrier wants: Use all your antennas! 28

29 What you want: N opt = a P O /P Circuit b P O 29

30 Tradeoff No. 2 Network capacity vs. client efficiency 30

31 How can clients figure out its N without talking to each other? 31

32 BeamAdapt Distributed algorithm to minimize TX power under uplink capacity constraints No explicit inter-client cooperation Iterative Guaranteed to converge Converge in a few iterations in practice Converge to a good solution in practice Can be built on top of uplink power control in cellular networks 32

33 WARPLab-based prototype Infrastructure Node 1 Infrastructure Node 2 Ethernet Router Uplink (Wireless) Uplink (Wireless) Client Node 1 Client Node 2 Laptop with MATLAB 33

34 Beamforming size SINR (db) Received SNR stable Client Node Time (s) Time (s) Link SNR constraint: 5 db 34

35 Power close to optimal Power consumption (mw)2000 5dB BeamAdapt Genie-aided I/S I/M O/S O/M I: Indoor O: Outdoor S: Stationary M: Mobile / Rotational Link SNR constraint: 5 db

36 4km 4km UMTS; Client movement: 0-70 mph; Client rotation: /s

37 Client Power Consumption (mw) Power reduced Beamforming/Omni BeamAdapt N=1 N=2 N=4 N=8 CBR traffic

38 Network Throughput (b/s) Network throughput maintained x 10 5 Beamforming/Omni BeamAdapt N=1 N=2 N=4 N=8 CBR traffic

39 Conclusions Beamforming is feasible for mobile devices Lower-power uplink for mobile devices Distributed optimization feasible

40 Looking forward Benefits of beamforming orthogonal to other spectrum efficiency technologies such as network MIMO Network capacity implications

41 Treating interference as noise Strong interference regime: Far from optimal from information theoretic perspective

42 Treating interference as noise Weak interference regime: Existing architecture yields close to optimal capacity

43