Underwater Acoustic OFDM: Past, Present, and Futurewuwnet.acm.org/2011/slides/papers-talks/Shengli...

Underwater Acoustic OFDM: Past, Present, andFuture

Shengli Zhou

Dept. of Electrical and Computer EngineeringUniversity of Connecticut

http://uwsn.engr.uconn.edu

WUWNET’11

Dec. 2, 2011

Shengli Zhou (University of Connecticut) Plenary Talk Dec. 2, 2011 1 / 26

Underwater Communications

Cable

Acoustic communications (ACOMM)

Electromagnetic communications (Wireless Fibre Systems)

Optical communications (Blue-Green Laser)


ACOMM Techniques

Frequency shift keying (FSK)I e.g., Teledyne Benthos, WHOI Micro-modem

Direct sequence spread spectrum (DSSS)I e.g., LinkQuest, DSPCOMM, Tritech

Single carrier phase-shift-keying (PSK) transmissionsI e.g., WHOI Micro-modem, Benthos (additional processing card)

Multicarrier modulation (in the form of OFDM)


OFDM: A Prevalent Choice for Broadband WirelessSystems

DSL Modem

WiFi (IEEE 802.11)

WiMax (IEEE 802.16)

3GPP-LTE

4G and beyond


Number of Publications on OFDM

1994 1996 1998 2000 2002 2004 2006 2008 2010 20120

2

4

6

8

10

12

14

Year

Num

ber

of P

ublic

atio

ns

OCEANS Conference PapersIEEE/JASA Journal Papers

1994 - 2005: sporadic effort and little progress

2006 - 2011: sustained effort and great progress


Outline

OFDM basics: Pros and Cons

Algorithm development

Prototype development


Underwater Acoustic Channel CharacteristicsThe sound propagates too slow!

I Long multipathI Fast variation

The SPACE’08 experiment, Martha’s Vineyard, depth 15 m

2 4 6 8 10 120

20

40

60

80

100

delay [ms]

ampl

itude

Fast-varying multipath channel with a large delay spread


Block Transmission over LTI ChannelsConsider a linear time invariant (LTI) multipath channel

h(t) =

Np∑

p=1

Apδ(τ − τp)

Time domain waveform distortion; intersymbol interference (ISI)arises; complex channel equalizer needed

y(t) = s(t) ∗ h(t)

Frequency domainY (f) = H(f)S(f)

If s(t) is carefully constructed with no ISI in frequency domain

S(f)|f=fm= s[m]

Then no ISI at the receiver side

Y (f)|f=fm= H(fm)s[m]


Basics of Orthogonal Frequency Division MultiplexingFrequency domain; fm − fk = (m − k) 1

T

S(f) =∑

k

s[k]sinc(

(f − fk)T)

, S(f)|f=fm= s[m]

−5 −4 −3 −2 −1 0 1 2 3 4 5

−0.2

0

0.2

0.4

0.6

0.8

1

1.2

Frequency

Fre

qu

en

cy F

esp

on

se

s[k] s[k+1]s[k−1]

Time domain waveform; g(t): rectangular pulse shaper

s(t) =∑

k

s[k]ej2πfktg(t), fk = fc +k

T


Pros and Cons of OFDM

Pros:I Convert a dispersive channel to a set of parallel simple channels

{

zm = H(fm)s[m] + nm

}K/2−1

m=−K/2

I Receiver complexity does not depend on the channel delay spread!


Pros and Cons of OFDM

Pros:I Convert a dispersive channel to a set of parallel simple channels

{

zm = H(fm)s[m] + nm

}K/2−1

m=−K/2

I Receiver complexity does not depend on the channel delay spread!

Cons:I Poor performance on faded subchannels.I Sensitive to the Doppler effect

F Doppler shifts destroy the subcarrier orthogonality, and hence leadsto intercarrier interference (ICI)

I Large peak-to-average power ratio (PAPR)


How to Drastically Enhance Performance in FadingChannels?

0 5 10 15 20 2510

−6

10−5

10−4

10−3

10−2

10−1

100

SNR(dB)

aver

age

BE

R

fadingAWGN

(a) BER vs SNR

−5 0 5 10 15 20 250

1

2

3

4

5

6

7

8

9

SNR(dB)

Cap

acity

fadingAWGN

(b) Capacity vs SNR

Fading channel drastically affects the uncoded performance

Fading channel has the potential for reliable data transmission

Solution: coded OFDM with strong codes, e.g., Turbo, LDPCcodes that are capacity-achieving


How to Deal With the Doppler Effect?

ICI is inevitable!

Need signal processing algorithms to address ICI explicitly

Signal processing tailored to

underwater channels

OFDM demodulation

One example: Progressive receiver [JSTSP’2011]

There are other alternative approaches


Progressive Receiver

Adapt the receiver to channel conditions automatically without any apriori information

Achieves both low complexity and robust performance over time-varyingUWA channels

H0 H1

H2 H3

z

..

.

..

...

.

..

..

..

..

...

.

..

...

.

..

...

.

..

...

.

..

.

= + ns


Receiver Structure

Nosuccess or

D = Dmax

Yes

Output

decisions

Nonbinary

LDPC decoding

ICI equalization

Noise variance

estimation

Channel estimation

Increase D;

provide soft

information

Pre-processing;

set D = 0

z = HDs + n 1. The system model keepsbeing updated

Increase the span of ICIin equalization model

Increase the maximumpossible Doppler spreadin channel estimation

2. Soft information from thechannel decoder is utilized

3. No extra pilot-overheadneeded


Block Success Percentage: SPACE08

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8

D = 3

D = 2

D = 1

D = 0

Success percentage vs. number of phones

S1 (60 m)

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

1 2 3 4 5 6 7 8

D = 3

D = 2

D = 1

D = 0

Success percentage vs. number of phones

S5 (1000 m)

Averaged over Julian dates 295-302

With 4 phones: 90% (D = 0), 95% (D = 1), and up to 98.8% (D = 3)


How to Alleviate the PAPR Impact?

6 8 10 12 14 1610

−3

10−2

10−1

100

Thresh [dB]

Pr(

PA

PR

>T

hres

h)

QPSK8−QAM16−QAM

Multicarrier without

PAPR control

Multicarrier with PAPR

control

Single−Carrier

QPSK: The gap between OFDM and single-carrier is about 6dB

QAMs: The gap between OFDM and single-carrier is about 4dBDesign considerations on power amplifier and transducer:

I Peak or average power constrained?


Summary

OFDM is an elegant scheme

It has a clear advantage for short-range long dispersive multipathchannels.

It is an appealing technique for shallow-water high-data-rateacoustic applications


WUWNet’07 Demo, Sept. 2007

Link A to B

Link B to A

Single-input single-output (SISO) OFDM in-air demonstration

The data rate is 3.1 kb/s, with QPSK modulation, rate 1/2 LDPCcoding, and bandwidth of 5.5 kHz


WUWNet’08 Demo, Sept. 2008

Multi-input multi-output (2 × 2) OFDM in-air demonstration

The data rate is 6.2 kb/s, with QPSK modulation, rate 1/2 LDPCcoding, and bandwidth of 5.5 kHz


WUWNet’09 and ’10 Demo, Nov. 2009 & Nov. 2010

Aqua-fModem Prototype: With keyboard input and LCD display

Floating-point TMS320C6713 DSP board; running @ 225 MHz

Fixed-point TMS320C6416 DSP board; running @ 1 GHz


WUWNET’11 Demo: A Network of Modems

One-hop network, RTS/CTS based MAC protocol


Current / Future Issues (1)Multi-input multi-output (MIMO)

I Co-located: Increase the data rate via spatial modulation

x1

x2

x Nt

. . .

y1

y2

y Nr

. . .

h11

h12

hNtNr

I Distributed MIMO, asynchronous MIMO

User 1 User 2

Re

civ

ers

User 3

v1

v3 = 0

v2


Current / Future Issues (2)

Interference (Sonar, impulse interference, multiuser interference)

0 0.5 1 1.5 2 2.5

x 106

−600

−400

−200

0

200

400

600Timedomain_for_637_phone_10

I Interference mitigation / avoidance / alignment / management


Current / Future Issues (3)

Networking issuesI MAC

I Routing

I Reliable data transfer

I Applications

How to efficiently interact with higher layers?

Joint optimization (cross-layer design)?


Thank you!


Environmental Impact: S1

295 296 297 298 299 300 301 302 3030

1

2

3

4

Julian Date in 2008

Wav

e he

ight

(m

)

295 296 297 298 299 300 301 302 3030

5

10

15

20

Julian Date in 2008

Win

d sp

eed

(m/s

)

295 296 297 298 299 300 301 302 303

0

1

2

3

4

5

6

7

8

9

All success, D

max = 0

All success, Dmax

= 1

All success, Dmax

= 2

All success, Dmax

= 3

With errors, Dmax

= 3

S1 (60 m)

“All success”: all 20 blocks in a file correctly decoded

ICI-ignorant receiver (D = 0) works well during calm days

ICI-aware receiver (D > 0) is needed when the channel conditionsbecome worse


Underwater Acoustic OFDM: Past, Present, and Futurewuwnet.acm.org/2011/slides/papers-talks/Shengli...

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