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Adaptive Frequency-Domain equalization for Underwater Acoustic Communications

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### Adaptive Frequency-Domain equalization for Underwater Acoustic Communications

### Backup

Abdelhakim Youcef

Supervised by

Christophe Laot and Karine Amis

LabSticc seminary, Brest, February 9th , 2012

Introduction (1/2)UWA channel

Multipath propagation (reflection at the surface and the bottom)

Doppler effect due to the movement of the platforms

Differential Doppler effect due to the movement on the sea

Compression/dilatation of the symbol duration

Why acoustic propagation?

When the frequency increases:

The transmission range decreases (signal is attenuated)

The Doppler effect increases

Radio and optical waves are strongly attenuated

Speed of the sound

page 1

Abdelhakim Youcef

- Arrival of the cable from port
- Signal input

50m

15m

30m

1.5km

10m

Introduction (2/2)- Underwater acoustic (UWA) communication:
- Strong frequency selectivity (ISI)
- Time-variation
- Limited bandwidth (acoustic waves & transdictor )

page 2

Abdelhakim Youcef

Outline

- Underwater acoustic (UWA) communication:
- Digital receiver for UWA communication
- Frequency-domain equalization (FDE)
- Cyclic-prefix adaptive FDE (CP-AFDE)
- Overlap-and-save adaptive FDE (OS-AFDE)
- Simulation results (CP-AFDE vs. OS-AFDE)
- Joint OS-AFDE and phase synchronization
- Multiple input receiver
- Experimental results
- Conclusions and perspectives

page 3

Abdelhakim Youcef

UWA communication system

Transmitter

fc: 35kHz

Bit rate: 10 kbps

- Source:
- Image
- Speech
- Data

Channel Coding

Frame

QPSK

Modulation

Underwater Acoustic Channel

4 hydrophones

Receiver

Frequency

Domain

equalizer

Down conversion

Phase

synchronizer

Timing

recovery

Channel

Decoding

Adaptive processing + PLL

Abdelhakim Youcef

Some applications on UWA communications

- The off-shore oil industry
- Aquaculture and fishing industry
- Pollution control
- Climate recording
- Ocean monitoring for prediction of natural disturbances
- Detection of objects on the ocean floor
- Scientific data collection
- Security and military applications

page 5

Abdelhakim Youcef

Frequency-domain Equalization (1/3)Principle

- Performance: equivalent to the time-domain equalization
- The equalization is performed block by block
- Fast Fourier Transform (FFT) ~ circular convolution

Serial

To

Parallel

Conversion

F

F

T

I

F

F

T

Parallel

To

Serial

Conversion

.

.

.

.

.

.

.

.

.

Abdelhakim Youcef

Frequency-domain Equalization (2/3)Cyclic prefix based FDE (circular model)

N

Block of N symbols

Copy of the

last

symbols

N

N

CP

CP

S/P

P/S

FFT

IFFT

Transmitter

Receiver

page 8

Abdelhakim Youcef

N symbols

Copy of the

last symbols

Block of N symbols

Frequency-domain Equalization (2/3)Cyclic prefix based FDE (circular model)- Advantages and properties:
- CP length equal to the maximum channel delay spread in terms of symbol duration
- Circular convolution in the channel
- Removes the inter block interference
- Inconvenient:
- A loss in the spectral efficiency
- Additional treatment at the transmitter (CP insertion)

(dB)

Abdelhakim Youcef

Frequency-domain Equalization (3/3)Overlap-and-save based FDE (linear model)

Sequence 1: incoming data blocks

Circular Convolution

between

the sequences 1 and 2

in the time-domain

Initiate zeros

N

N

N

N

N

The last N samples

correspond to a linear

convolution result

The first samples

correspond to a circular

convolution result

Sequence 2: Equalizer vector

Nzeros

Each equalizer input vector

contains N samples from the

current block and the last

Samples from the previous one

page 10

Abdelhakim Youcef

Frequency-domain Equalization (3/3)Overlap-and-save (linear model)

- Overlapping and sectioning methods (e.g. overlap and save)
- The transmission of CP intervals is not necessary
- Allows to perform linear convolution using FFT
- The block/FFT size is selected at the receiver
- Overlapping of 50% (block size equal to equalizer size)

Input data 2N

Equalizer vector

N

N

N

N samples

Nzeros

N

Equalizer

Output

N

. . .

page 11

Abdelhakim Youcef

Simulation results (1/2)OS-AFDE vs. CP-AFDE

(a) Porat channel model

(b) Proakis B channel model

Bit error rate (Ber) vs. Eb/N0 calculated over 320 data blocks

N = 64, = 16, number of blocks : 400, training sequence :80 data blocks

Abdelhakim Youcef

Joint OS-AFDE and phase synchronizationMultiple input receiver

- Adaptive processing is used to track the time-varying channel
- Multiple input receiver

Timing recovery

+

Sample rate

conversion

Low pass

Filter

frequency-domain

equalizer

Oversampling

Timing recovery

+

Sample rate

conversion

Low pass

Filter

frequency-domain

equalizer

Oversampling

Adaptive

processing

page 14

Abdelhakim Youcef

The proposed multiple input equalizerJoint optimization of the OS-AFDE and phase synchronization

Concatenate

two blocks

Gradient Constraint

FFT

T

Delete last

block

GC

Delete last

block

Conjugate

IFFT

Concatenate

two blocks

T

GC

Conjugate

page 15

Abdelhakim Youcef

- Arrival of the cable from port
- Signal input

50m

15m

30m

- Experiment A:
- Sonar images
- v = 1.4 m/s

1.5km

10m

Experimental results (1/2)- fc = 35 kHz
- R =10 kbits/s
- N = 32
- Training period: 1 s
- Pe: 180 dB ref μ Pa at 1m
- Experiment B:
- The transmitter is submerged and fixed at a buoy
- Text sentences
- v = 0.5 m/s
- D= 500 m

Abdelhakim Youcef

Experimental results (2/2)OS-AFDE vs. LMS-TDE

- OS-AFDE: block by blockequalization in the frequency-domain
- LMS-TDE: symbol by symbol equalization in the time-domain
- After channel decoding, the bit error rate is equal to zero

Experiment B

D=500 m

Experiment A

D=1.5 Km

Abdelhakim Youcef

Conclusion & perspectives

- Frequency-domain equalization: alternative to time-domain equalization
- Computational complexity gain
- Simple equalizer parameters setting
- OS-AFDE vs. CP-AFDE: spectral efficiency and flexibility
- Joint adaptive compensation of residual frequency offsets
- Multiple input receiver
- Influence of the block/FFT size on the performance of the OS-AFDE
- Hybrid frequency-time domain decision Feedback equalization
- SC-FDMA multiple access

page 19

Abdelhakim Youcef

Abdelhakim Youcef

The proposed multiple input equalizerJoint optimization of the OS-AFDE and phase synchronization

Concatenate

two blocks

Gradient Constraint

FFT

T

Delete last

block

GC

Delete last

block

Conjugate

IFFT

Concatenate

two blocks

T

GC

Conjugate

page 22

Abdelhakim Youcef

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