Acoustic Technologies for Underwater Communication Networks
Transcript of Acoustic Technologies for Underwater Communication Networks
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Acoustic Technologies for Underwater
Communication NetworksBayan Sharif
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Sub-Sea Acoustic Communications
High data rate video/sonar: 16 Kbps @ 3 km
Low data rate command/control: 100 bps @10 km
Sensor data
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Typical Applications
Command link
Very high integrity, low data rate: Control of valves, lights, pumps
Enable/disable functionsremotely
AUV navigation
Reverse link
High integrity and data-rate:
Video and sonar images
Monitoring of sensors: depth,
temperature, pressure Instrument data
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Sensor nodes deployed
from helicopter or any
small vessel
Surface nodes provide acoustic
communication/tracking for
subsea nodes, GPS and
radio/satellite gateway to
central workstation
Wireless Sensor Network Potential Application Scenario
Movement with
Currents
Active buoyancy
Sensor Payload: Chemical or biological
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Main Technologies
Acoustic Communications
Point-to-point
Mobility
Low Power
Other Electromagnetic
Optical
EM Acoustic
Acousto Optical
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Why ~Kbps and not ~Mbps?
~Mbps can be achieved using EM
propagation, however, seawater ishighly conductive and signal
attenuation is therefore very high.
Feasible distances can only beachieved by acoustic propagation,
but only at low frequenciestherefore limiting achievable
bandwidth, hence ~Kbps.
~10 dB/m
~.001 dB/m
Joseph Hansen, Nav al Postgraduate School
Channel Range (km) Bandwidth (kHz)
Very Short < 0.1 >100
Short 0.1-1 20-50
Medium 1-10 10
Long 10-100 2-5
Very Long >100
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Challenges to Acoustic Receivers
1803-1853
Mulltipath Noise Doppler
Low speed ofacoustic waves
(1500m/s) cause
Doppler effects up
to 1% for moving
underwater targets.
Sampling
clock
Transmit
waveform
Receive
waveform
Acoustic Speed
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Other System-related Limitations
Battery life
Transducer frequency response roll-off Fouling/corrosion
Half-duplex transmission
Sound speed depends on depth,
temperature, salinity, etc.
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Deployment Cost
$5000
$10000
$15000
$20000
Deployment Method
DailyCost
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More about the Underwater Channel- Multipath, Doppler and Noise
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Multipath: Shallow Water Channel (1 km)
Short-Term VariationLong-Term Variation
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Multipath Spread and Coherence Bandwidth
14 ms, 250 Hzrms coh
rms
B
=
FFT,
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Coherence time and Doppler Bandwidth
FFT
Hz4.0dBs5.21
=d
cohB
T
,
4 ms 0.4 Hz 0.0016 1rms dB =
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7
3 54
2
8
1
3 m
3 m
3 m
3 m
Side view
1 m
Junctionbox
Transducer Array
Ambient Noise
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Adaptive Receivers
Doppler Compensation
Acoustic Receiver Structures
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Adaptive Receiver with Phase Recovery
Chirp(Doppler tolerant)
BPSK-modulatedPN Sequence
QPSK-modulatedInformation Bits
Training Sequence DataChannelEstimation
ReceiveElement
Down-
conversionUnit
TimingUpdate
)(nPhaseUpdate
)( nje
AdaptiveAlgorithm
)(ny)( n
fw
MMSEFilter
DataDecisionDevice
)(nd
)(nd
)(ne
)( nb
w
FeedbackFilter
TrainingSequence
FeedforwardFilter
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2D-Rake Receiver Architecture (Temporal Combiner)
From otherreceive
elements
l
)( 1 nlw
lK
Data
)(nd
)(nd
)(ne
TrainingSequence
DecisionDevice
AdaptiveAlgorithm
CarrierPhaseUpdate
)( nje
MMSE
Flter
MMSE
Flter
MMSE
Flter
MMSE
Flter
)( 2 nlw )( nlKw)( 1 nlKw
)(1 nyl )(2 nyl )(1 nylK )(nylK
)(nyls )(nz
)(1 nyl
1l
)(nrl )( 1ll nr )( lKl nr
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2-D Rake Multi-channel Receiver
User 1 User 2
Channel Impulse Responses
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2-D Rake Multi-channel Receiver
1x2-AC Rake:
SINRo = 10.46 dB1x1-AC Rake:
SINRo = 9.31 dB
Performance for User 2
1x4-AC Rake:
SINRo = 13.43 dB1x3-AC Rake:
SINRo = 12.51 dB
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Multichannel Receiver (Spatial-Temporal Combiner)
ReceiveArray
Elements
AdaptiveAlgorithm
AdaptiveCorrelators
1
2
K
)( 1 ncw
)( 2 ncw
( )K ncw
)(1 ny
( )Ky n
)(2 ny
Data
)(nd
)(nd
)(ne
TrainingSequence
)( nb
w
FeedbackFilter
)(nz
DecisionDevice
CarrierPhaseUpdate
)( nje
)(nys
1( )x n
( )Kx n
2( )x n
0.00 0.05 0.10 0.15 0.20 0.25
time (ms)
0.00
0.02
0.04
0.06
0.08
0.10
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0.14
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0.18
0.20
0.22
0.00 0.05 0.10 0.15 0.20 0.25
time (ms)
0.00
0.02
0.04
0.06
0.08
0.10
0.12
0.14
0.16
0.18
0.20
0.22
0.00 0.05 0.10 0.15 0.20 0.25
time (ms)
0.00
0.02
0.04
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The Sub Sea Acoustic Channel
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Array Configuration
1 2
3 4
5 6
The Sub-Sea Acoustic Channel
(2km)
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The Sub Sea Acoustic Channel
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Array Configuration
1 2
3 4
5 6
The Sub-Sea Acoustic Channel
(2km)
Significant temporal fluctuations
during single packets implies
requirement for adaptive
techniques
Relatively little separation
between elements (a few
wavelengths) results in low spatial
correlation between fluctuations
in channel response
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2 D Rake Multi channel Receiver
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2-D Rake Multi-channel ReceiverPerformance for User 2
1x4-AC Rake:
S INRo
= 13.43 dB
1x3-AC Rake:
S INR o = 12.51 dB1x2-AC Rake:
S INR o = 10.46 dB1x1-AC Rake:
S INR o = 9.31 dB
Combining Gain
6x4-AC Rake:
S I NR o = 17.91 dB6x3-AC Rake:
S INR o = 16.69 dB6x2-AC Rake:
S INR o = 15.37 dB6x1-AC Rake:
S INR o = 13.64 dB
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Adaptive Receivers Doppler Compensation
Acoustic Receiver Structures
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Conventional Adaptive Receiver Structure
with both Phase and Timing Recovery
)( nw
MMSEFilter
ReceiveElement
Down-conversion
Unit
TimingUpdate
)(nPhaseUpdate
)( nje
)(ny
DataDecisionDevice
TrainingSequence
)(nd
)(nd
)(ne
AdaptiveAlgorithm
)(nz
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Doppler-Shift in Wideband Communications
Doppler shift () modelled as a time scaling:
Equivalent (for a discrete-time sampled system) to scaling of the samplingperiod (interpolation or decimation):
Then, inverse time-scaling will remove carrier/symbol shift:
Which corresponds to a scaling of the sampling frequency:
])1([][ ss TnsnTr +=
+= ss T
nrnTs
1][
ss ff )1(' +=
)()( )1( tstr +=
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Doppler Compensation Methods
Block-based:
Closed-loop:
Interpolator
(1+)input
signalReceiver
/demodulator
Block Doppler
estimator
Interpolator
(1+)inputsignal
Receiver
/demodulator
Cost
function
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Adaptive Doppler Compensation
Interpolation Factor
where kp is a proportional constant.
npnn kII +=+ .1*.arg nnn dy=
Down-converter
(I-Q)
Linearinterpolator(s I.s)
RxSignal Forward fil ter
h0(N taps)
SFeedback
filterg(M taps)
x0
y
Training
Sequence
d
UpdateAlgorithm
Single element structure
Element 1
ElementK
For significant Doppler shift variations arising from vehicle acceleration, a more
robust alternative would be adaptive closed loop Doppler compensation.
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Block vs. Adaptive Doppler Compensation
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Multichannel Receiver with Doppler Compensation
ReceiveArray
Elements
AdaptiveAlgorithm
Adaptive
Correlators
1
2
K
)( 1 ncw
)( 2 ncw
( )K ncw
)(1 ny
( )Ky n
)(2 ny
Data
)(nd
)(nd
)(ne
Training
Sequence
)( nb
w
FeedbackFilter
)(nz
DecisionDevice
CarrierPhaseUpdate
)( nje )(nys
1( )x n
( )Kx n
2( )x n
LinInterpolate
DownConvert
Down
Convert
DownConvert
Lin
Interpolate
LinInterpolate
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Emerging Technologies
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MIMO-OFDM
OFDM Processing efficiency
PAPR Doppler tracking
MIMO Increased throughput Processing complexity
Size constraints(e.g. ~1.5m spacing @ 10kHz)
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Acoustic Networks
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Acoustic Networks
Seaweb Network,
D. J. Grimmett,
Oceans 2007Challenges
Node energy sustainability Network optimisation
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Physical Layer Network Coding
Without a relay the transmission power must be increasedand there is also a greater delay in exchanging messages.
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PNC for Underwater Relay Networks
PNC
Mapping-1
SISO
Decoder
ModulatorEncoder
Without a relay the transmission power must be increasedand there is also a greater delay in exchanging messages.
A shorter distance means reduced transmissionpower. Employing PNC at the relay reduces thedelay in receiving messages.
Deinterleaver
Interleaver
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Scheduling & Fault Tolerance
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Node 5
(3km)
Scheduling & Fault Tolerance
Half-duplex communications and severe latency
Node 1
(750m)Master
Node
Node 3
(2.25km)
Node 2
(1.5km)
Node 4
(3km)
~20% throughput if all nodes are polled for data
~ 70% throughput by multi-cast data request, selecting nodes to avoid
collisions and hence exploit the channel latency.
Adaptive scheduling is vital to create an efficient subsea network
However, complexity increases with multi-hop routing and dynamic nodes. Redundant nodes can maintain connectivity and coverage in the event of
node or communication failure.
Node 5
(3km)
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Thank You
Sh ifh // i