1 An Underwater Acoustic Telemetry Modem for Eco-Sensing * Ronald A. Iltis, Ryan Kastner, and Hua Lee Daniel Doonan, Tricia Fu, Rachael Moore and Maurice Chin Department of Electrical and Computer Engineering University of California, Santa Barbara, CA 93106-9560 {iltis,kastner,lee}@ece.ucsb.edu * This work was supported in part by the W.M. Keck Foundation and UCSB Marine Sciences Institute.
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1 An Underwater Acoustic Telemetry Modem for Eco-Sensing * Ronald A. Iltis, Ryan Kastner, and Hua Lee Daniel Doonan, Tricia Fu, Rachael Moore and Maurice.
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1
An Underwater Acoustic Telemetry Modem for Eco-Sensing*
Ronald A. Iltis, Ryan Kastner, and Hua Lee Daniel Doonan, Tricia Fu, Rachael Moore and Maurice Chin
Department of Electrical and Computer EngineeringUniversity of California,
Santa Barbara, CA 93106-9560{iltis,kastner,lee}@ece.ucsb.edu
*This work was supported in part by the W.M. Keck Foundation and UCSB Marine Sciences Institute.
2
Dock
AquaNodes with acoustic modems/routers, sensors.
Dockside acoustic/RF comms and signal processing.
Cabled hydrophone array
Wi-Fi or Wi-Max link
CTD, currents, nutrient data to Internet. Adaptive sampling commands to AquaNodes.
WetNet Implemented with AquaNodes
Applications: Santa Barbara Channel LTER, Moorea Coral Reef LTER and many more.
– Too expensive, power hungry for Eco-Sensing. Proprietary algorithms, hardware.– M-FSK (Scussel, Rice 97, Proakis 00) does use frequency diversity, but requires coding to
erase/correct fades.• Navy modems:
– Need open architecture for international LTER community – precludes military products.• Direct-sequence, QPSK, QAM, coherent OFDM
– Great deal of work on DS, QPSK for underwater comms. But equalization, channel estimation are difficult. (Stojanovic 97, Freitag, Stojanovic 2001, 2003.)
• MicroModem: – Best available solution for WetNet. FSK/Freq. Hopping relies on coding to correct bad hops.– But can we do better? Less power? Wider bandwidth for lower uncoded symbol error rate
estimation.– Uses per-symbol frequency diversity.– Motivated by 802.15.4 (Zigbee), 802.11b m-ary quasi-orthogonal waveforms.– Achieves 133 bps data rate without need for equalization, accurate carrier phase tracking.– Battery life in months for reasonable transmit duty cycles.– Matching Pursuits only assumes channel constant over 22 msec.– Far superior to FSK in slow fading (Doppler spread < .1 Hz) scenario using Kalman filter
channel tracking.
4
Multipath/Doppler Spread References
• Long range shallow water multipath spread ~100 msec. (Kilfoyle and Baggeroer 00)
• 120 msec. spread at 48 nm, 5 msec. at 2nm shallow water (Stojanovic et. al. 94)
• Significant delay spread ~10 msec. 2-6 meter depth, 400-500m range. (Freitag et. al. JOE 01)
• 2.5 msec multipath spread, .5 Hz Doppler spread at 3km (6 to 30m depth) in Baltic. (Sozer et. al. 99)
Wideband for Acoustic Comms -- Frequency Diversity Argument
• B Hz. Wideband signal can resolve multipaths Ts = 1/2B apart.• Classical RAKE receiver.
)(nr
k
L
kk
L
kks nE
1
*
1
2|2
*1 *
2 *3 *
L
sT sT sT
)(ns
8
Channel, Walsh/m-sequence Spectra
0 500 1000 1500 2000 25000
0.2
0.4
0.6
0.8
1
1.2
1.4
1.6
1.8
2
Frequency (Hz.)
|H(f
)|
Channel Response
Walsh/m-seq. spectrumFSK Tones
9
Auto-Cross Correlation of Walsh/m-seq.
0 20 40 60 80 100 120-0.5
0
0.5
1
Delay (Samples)
Wa
lsh
/m-s
eq
ue
nce
Co
rre
latio
nS
1,S
2 cross-correlation
S1 auto-correlation
10
Un-coded SER Improvement with Diversity
FSK in Rayleigh Fading
Binary Orthogonal with L-degree Diversity in i.i.d. Rayleigh Fading. Ideal RAKE – zero cross-correlation.
(J. G. Proakis 7.4.15)
P2 = 12+ E b
N 0
Union Bound
Pe < (Nw ¡ 1)P2¹ =
E bL N 0
2+ E bL N 0
Law of Large Numbers Interpretation – Normalized Channel Energy
E f j®k j2g= 1L
2EsP L
k=1 j®kj2 ! 2EsLE f j®k j2g= 2Es
P2 =
µ1¡ ¹
2
¶L L ¡ 1X
k=0
µL ¡ 1+k
k
¶ µ1+¹
2
¶k
11
Union Bound SER -- Frequency Diversity
10 11 12 13 14 15 16 17 18 19 2010
-7
10-6
10-5
10-4
10-3
10-2
10-1
100
Eb/N
0
Pb
FSK
Orthogonal, diversity order =1
Orthogonal, diversity order =7
Orthogonal, diversity order =13
Orthogonal, diversity order =19
Orthogonal, diversity order =25
12
Received Signal Model
r(t) =N®X
p=1
®p(n)sm(n)(t ¡ ¿p(n)) +n(t)
Transmitted Walsh/m-sequence
Received signal
Nyquist-sampled equivalent vector signal
sm(n)(t) =Nw L pn ¡ 1X
i=0
(dm(n))ig(t ¡ iTc)
r(n) ¼N s ¡ 1X
l=0
f lsm(n)(l) +n(n) = Sm(n)f +n(n)
13
Matching Pursuits Channel Estimation
• Conventional LS estimate is “noisy” for sparse channels (Nf << components of f are nonzero.)
• Assume “minimum underspread” channel. Coefficients f are constant only during one symbol+time guard interval (22.4 msec.)
• Matching Pursuits (Mallat and Zhang, Cotter and Rao 02, Kim and Iltis 04) yields sparse channel estimates and is readily implemented in reconfigurable hardware (Meng et. al. DAC 05.)
r(n) ¼2Tsy
m
s (q1)f̂ q1
s(q2)f̂ q2
s(q3)f̂ q3
s(qN f)f̂ qN f
+
+
+
14
MP Algorithm
• Step 1 – Conventional LS, best fit.
Step k – Cancel previous k-1 detected paths, find qk
f̂ i = s(i)H r=jjs(i)jj2; i = 1;::: ;Ns
q1 = argmini
¯¯¯¯¯r̄ ¡ s(i)f̂ i
¯¯¯¯¯¯2
= argmaxi
jf̂ i j2jjs(i)jj2
rk = r ¡k¡ 1X
i=1
s(qi )f̂ qi
f̂ i = s(i)H rk=jjs(i)jj2
qk = arg mini6=q1 ;:::;qk ¡ 1
¯¯¯¯¯r̄k ¡ s(i)f̂ i
¯¯¯¯¯¯2
= arg maxi6=q1 ;:::;qk ¡ 1
jf̂ i j2jjs(i)jj2
15
GMHT-MP Algorithm• Decision on m(n) using Generalized Multiple Hypothesis Test
(GMHT)
m̂= argminm
½min
f :´ (f )=N f
jjr(n) ¡ Smf jj2¾
Numerosity constraint
GMHT has complexity Nw binom(Ns, Nf) due to numerosity constraint. For Nf = 12 paths, this requires 35x1015 LS channel estimates requiring matrix-vector multiplies!
MP estimation requires Nw Ns!/(Ns-Nf)! = 1.7x1025 least-squares estimates, but these are scalar quanitities. MP also only requires one matrix-vector multiply when implemented using sufficient statistics. GMHT-MP thus has the form
m̂= argminm
njjr(n) ¡ Smf̂M P;mjj2
o
16
Sufficient Statistics MP Implementation(A. Brown and Y. Meng, DAC 05)
• Step 1: Matrix-vector multiply
v1 = SH r(n); A = SH S (look-up table)
q1 = argmaxi
jv1i j
2
A i ;i;
f̂ q1 =v1
i
A i ;i
v2 = v1 ¡ A (:;q1)f̂ q1
Step k: Does not require further matrix-vector multiplies.