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MIMO Time Reversal Communications. Hee-Chun Song, W.S. Hodgkiss, and W.A. Kuperman Marine Physical Laboratory/Scripps Institution Oceanography University of California, San Diego 9500 Gilman Drive, La Jolla September 14, WUWNet 2007, Montreal, Canada. Outline.

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mimo time reversal communications

MIMO Time Reversal Communications

Hee-Chun Song, W.S. Hodgkiss, and W.A. Kuperman

Marine Physical Laboratory/Scripps Institution Oceanography

University of California, San Diego

9500 Gilman Drive, La Jolla

September 14, WUWNet 2007, Montreal, Canada

outline
Outline
  • Underwater Acoustic (UWA) Channel
  • Time reversal mirror (TRM)
  • Application to communications
  • Multi-user communications: Experiments
    • Downlink (active)
    • Uplink (passive)
  • Summary
slide4

Challenges in UWA Channel

  • Delay Spread (ISI)
  • Doppler Spread

Stojanovic et al., “Phase-coherent digital communications for underwater

acoustic channels,” IEEE J. Oceanic Eng., 1994.

Commercial modem: Incoherent FSK (Frequency shift keying)

Research community: Multi-channel DFE with 2nd order PLL

background trm
BACKGROUND: TRM
  • CLAY AND PARVELESCU (underwater acoustics) 1965
  • ZEL’DOVICH (non-linear optics): Phase Conjugation 1972
  • Retrograde antenna (microwave) 1964
  • BUCKER (underwater acoustics - mfp) 1976
  • TAPPERT et al. (underwater acoustics - “retrograde …”)
  • FINK et al. (ultrasonics): Laboratory experiment1989
  • JACKSON AND DOWLING (underwater acoustics) 1991,1992
  • Marine Physical Laboratory and SACLANTCEN (implementation of TRM in ocean) 1996 - present

http://www-mpl.ucsd.edu/people/wak

3 5 khz sra 99 and 00
3.5 kHz SRA (’99 and ’00)

L = 78 m

N = 29

3.5 kHz tranceiver

application to acomms
Application to Acomms

Probe Source

Pulse

Normal

Time-Reversal

Time-Reversal

Communications

Self-equalization

ACTIVE (downlink)

tr self equalization process
TR: Self-Equalization Process
  • Channel complexity (number of multi-paths)
  • Number of array elements
  • Spatial distribution (spacing and aperture)

TR + Equalization

  • Remove the residual ISI
  • Compensate for time variations in the channel
performance comparison theory
Performance Comparison: Theory

Channel Model

TR+EQ

TR+EQ

TR

OutputSNR

Stojanovic, JASA (2005)

Song et al., JOE (2006)

Song et al., JASA (2007)

Input SNR

faf04 active time reversal dfe
FAF04: Active Time Reversal + DFE

TRM+DFE

TRM Only

SNR0=13.7 dB

SNR0=26.3 dB

Song et al., IEEE JOE (2006)

slide13

SIMO

PASSIVE

ACTIVE

MISO

Song et al., JASA (2006)

slide14

Multi-channel DFE

w/ implicit matched filter

Time Reversal

Passive time reversal

Followed by a single

Channel DFE

Yang, JOE (2005)

Correlation-based DFE

multi user miso active time reversal

Multi-User MISO(Active Time Reversal)

Downlink

Song et al., IEEE JOE 31, 2006

faf03 miso 3 user acomm qpsk
FAF03: MISO 3-User Acomm (QPSK)

Range = 8.6 km

29.7 dB

27.7 dB

25.5 dB

multi access simo passive time reversal

Multi-Access SIMO(Passive Time Reversal)

Uplink

Song et. al., JOE (2007)

faf05 multi access simo r 4 km
FAF05: Multi-Access SIMO, R=4 km

Co-channel interference

LFM

Receiver

faf05 single user simo r 4 km
FAF05: Single User SIMO, R=4 km

Single User

ISI: 45 symbols

96-m

cir comparison @ r 4 and 20 km
CIR Comparison @ R=4 and 20 km

R=4 km, ISI: 45 symbols

R=20 km, ISI: 10 symbols

Use bottom 20 elements for processing

summary
Summary
  • Time reversal mirrors, either active or passive, exploit spatial diversity to achieve temporal and spatial focusing in complex environments (e.g., acoustic waveguide)
  • Temporal focusing mitigates ISI while spatial focusing enables a straightforward extension to multi-user MIMO communications.
  • Active (downlink) and passive (uplink) time reversal is equivalent mathematically with the communications link being in the opposite direction.
  • Time reversal approach provides optimal performance in theory when combined with adaptive channel equalization.
  • At-sea experimental results in shallow water have demonstrated the effectiveness of time reversal communications.
performance prediction faf05 qpsk
Performance Prediction: FAF05, QPSK

Song et al., JASA (2007)

M=2

Fractionally-sampled DFE

Theory: Stojanovic, JASA (2005)

faf06 f 15 khz w 7 5 khz
FAF06: f =15 kHz, W=7.5 kHz

ISI:100 symbols

ISI:120 symbols

16 kbits/s at 2 km range

15 kbits/s at 5 km range

slide32

Array Resolution: Free Space

L

R

Rayleigh diffraction limit

slide33

Array Resolution: Waveguide

1

V(q)

qc

qe

B

S

B

S

B

S

Le

Lc

B

qe

S

B

S

qc

B

S

slide34

0

-5

-10

Amplitude

-15

-20

-25

-30

-10

-5

0

5

10

Distance (mm)

Spatial focusing of the time reversed wave

Mobile hydrophone

Time reversed signals

Fink et al

single channel communication w a moving source
Single Channel Communication w/ a Moving Source

T = 1-ms, 3 kHz, N=9002 symbol

LFM, 100-ms, 2-4 kHz

time reversal vs equalizer
Time Reversal vs Equalizer
  • MLSE
  • MMSE
    • LE
    • DFE

Song et al., IEEE JOE 31, 2006

Song et al., JASA 120, 2006

Yang, JOE 29, 2004