Electromagnetic Radio in the Sea: Is it more than boiling water?Petar DjukicResearch Scientist
(jew♦kitch)
Joint work with Mylène Toulgoat
Networking in Seawater Knowledge of oceans important
Environmental reasons (tsunamis) Security reasons (the North passage)
Current communications technology based on acoustics Unreliable (environmental impact) Low rates (100 bps) Long propagation times (1500 m/s) But long range (>1 km)
Can we replace acoustics with more reliable EM technology? Less susceptible to environmental noise Higher rates (1000s bps) Shorter range (50-100 m)
Must use multi-hop networking! What kind of MAC? What is end-to-end throughput? What is end-to-end delay?
Before answering above need information about the physical layer (1) Find the SNR to get (2) the rates to get (3) throughput and delay
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Previous Research in Seawater EM
Focus on communication with submarines Surface to seawater Very long-range links
Extremely Low Frequency (ELF) technology 76 Hz carrier frequency 2 sites 148 miles apart (WI and MI) 22 km antenna (buried electrodes in the bedrock establish the antenna) 5 MW of power
Use for paging the submarine to the surface At surface use kHz link to base “Star topology”
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
How does the physical layer affect the MAC layer?
Transmission rate The higher the better! Well, not always. Especially if packets are small.
Transmission range The longer the better! Well, not always. Longer distance → lower transmission rate or higher packet error.
Propagation delay The shorter the better! Not an issue in terrestrial wireless networks Issue in long-distance wired networks, satellite networks (1000 km distances) Issue in acoustic networks due to low propagation speed (1500 m/s) Is it an issue in EM underwater networks?
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Conduction current signaling in seawater
Traditional antennas do not work in seawater Energy gets absorbed by the sea close to antenna Magnetic induction or isolated antennas may work
But, one can also use the sea itself as an antenna Modeled as a dipole between the two electrodes Can find channel response with a 2-port network
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
I
Electrode
EM Radiation
Electrical field induces voltage and current in the sea
Apply voltage
Measure voltage
Seawater
2-Port network model of conduction signaling
Need to find Ztt = Vt/It and Zrt = Vr/It to get the channel
H=Vr/Vt= Zrt / Ztt If Ztt and Zrt are known we have the transfer function
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Vt Vr
It→ Ir→
Ir←It←
+ +
- -
Vr=ZttIt+ZtrIr
Vt=ZrtIt+ZrrIr
A slide with a lot of equations
C. Burrows, “Radio communication within the earth’s crust,” IEEE Transactions on Antennas and Propagation, vol. 11, no. 3, pp. 311 – 317, May 1963:
Almost any physics textbook:
Propagation constant:
Intrinsic impedance
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Another slide with a lot of equations
From previous results:
Absorption
Wavelength
Propagation speed
Skin depth
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Seawater attenuation is very different from terrestrial
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
0 200 400 600 800 1000 1200 1400 1600 1800 2000-80
-75
-70
-65
-60
-55
-50
-45
Atte
nuat
ion
(dB
)
Frequency (Hz)
@20 m
@30 m@40 m
@50 m
10 dB loss due to frequency choice
18 dB loss over 30 m
Physics are great, but what now? Previous observations still hold
Attenuation huge due to absorption Attenuation increases with frequency Conventional wisdom: use very low frequencies for long range
Can we use higher frequencies and shorter range? For indication of MAC performance need to know:
Transmission range Transmission rate Propagation delay
Next calculate: Received signal strength SNR at the receiver Propagation speed/delay
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
5 10 20 30 40 50 70 100-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Received Power vs. Distance (fc=1000.0 Hz, W=100.0 Hz)
r (m)
Pr
(dB
m)
Tx Power 0.005 W
Tx Power 0.5 WTx Power 5 W
Effect of Transmit Power on Transmission Range
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Potentially expensiveto implement receiver
More reasonablereceiver
5 10 20 30 40 50 70 100-120
-110
-100
-90
-80
-70
-60
-50
-40
-30
-20
-10
0
Received Power vs. Distance (Pt=500.0 mW, W=100.0 Hz)
r (m)
Pr
(dB
m)
fc = 200 Hz
fc = 400 Hz
fc = 1000 Hz
fc = 2000 Hz
Effect of Carrier Frequency on Transmission Range
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
25 m gain
10 m gain
Effect of Seawater on Noise(from ITU Recommendation P372)
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
101
102
103
104
-180
-160
-140
-120
-100
-80
-60
-40
-20
0
Frequency (Hz)
Noi
se P
ower
(dB
m/H
z)
Noise Power vs. Frequency
At the surface d=0 m
At depth of d=25 mAt depth of d=50 m
At depth of d=75 m
ITU-P372(atmosphere)Refraction
Refraction &Absorption
Finally, the SNR!
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
5 10 20 30 40 50 70-10
-5
0
5
10
15
20
25
30
35
40SNR vs Distance (Tx Power = 27 dBm, Depth = 25 m)
SN
R (d
B)
Distance (m)
Fc=200 Hz, BW=30.1 Hz
Fc=400 Hz, BW=60.2 HzFc=1000 Hz, BW=150.4 Hz
Sweet spot
Theoretically good, But difficult to take advantage of.But, can decrease Tx power
Shouldn’t/can’t use
An abstract view of the rates
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
10 20 30 40 50 60 700
1
2
3
4
5
6
7Spectral Efficiency vs Distance (Tx Power = 27 dBm, Depth = 25 m)
Spe
ctra
l Effi
cien
cy (b
its/s
/Hz)
Distance (m)
Fc=200 Hz, BW=30.1 Hz
Fc=400 Hz, BW=60.2 HzFc=1000 Hz, BW=150.4 Hz
256-QAM 7/8
64-QAM 2/3
Realistic range
“Fancy” range
Actual rates
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
10 20 30 40 50 60 700
200
400
600
800
1000
1200
1400
1600Rate vs Distance (Tx Power = 27 dBm, Depth = 25 m)
Rat
e (b
its/s
)
Distance (m)
Fc=200 Hz, BW=30.1 Hz
Fc=400 Hz, BW=60.2 HzFc=1000 Hz, BW=150.4 Hz
Conventional wisdom is wrong!
5 10 20 30 40 50 70 1000
1
2
3
4
5
6
7Propagation time vs. Distance
Distance (m)
Tim
e (m
s)
fc=100.0 Hz
fc=500.0 Hz
fc=1000.0 Hz
fc=2000.0 Hz
Propagation Time
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
ν=1.6∙104 m/s
ν=5.0∙104 m/s
50 m UW~300 km wire
0 1 2 3 4 5 6 7 80
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
Utilization vs. Load (EM) (L=20 bytes, R=1400 bps, D=50 m, tprop
/T=8.75e-003)
Load (G)
Util
izat
ion
(S)
ALOHA
np-CSMAMACA (uper bound)
Single-hope Theoretical Performance
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
18.6% ALOHA
80 % CSMA
0 1 2 3 4 5 6 7 80
0.02
0.04
0.06
0.08
0.1
0.12
0.14
0.16
0.18
0.2
Utilization vs. Load (Acoustic) (L=20 bytes, R=400 bps, D=1000 m, tprop
/T=1.67)
Load (G)
Util
izat
ion
(S)
ALOHA
MACA (uper bound)
Single-hop Theoretical Performance (acoustic)
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
18.6% ALOHA
Low due toCTS-RTS Overhead
Multi-hop Throughput vs. Latency
Spatial re-use increases throughput at the cost of latency Multiple links can transmit in parallel if not interfering at receiver
e.g. A→B and D→C, B→A and C→D Links have to be specifically ordered to achieve minimum delay
Ordering and spatial re-use sometimes conflict with each other
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
A B C D
A→B
B→C
C→D
D→C
C→B
B→A
tprop
A B C D
A→B
B→C
C→D
D→C
C→B
B→A
0 500 1000 15000
50
100
150
200
250
300
350
400
450
500Rate vs. Distance (depth 25 m)
Distance (m)
Rat
e (b
ps)
0.1 W, 5 nodes0.1 W, 10 nodes
0.1 W, 20 nodes
0.5 W, 5 nodes
0.5 W, 10 nodes0.5 W, 20 nodes
5.0 W, 5 nodes
5.0 W, 10 nodes5.0 W, 20 nodes
Multi-hop Throughput (ideal TDMA)
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
21% Gain
29% Gain
12% Gain
Multi-hop Delay (ideal TDMA)
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
0 500 1000 15000
10
20
30
40
50
60
70Delay vs. Distance (depth 25 m)
Distance (m)
Del
ay (s
)
0.1 W, 5 nodes0.1 W, 10 nodes
0.1 W, 20 nodes
0.5 W, 5 nodes
0.5 W, 10 nodes0.5 W, 20 nodes
5.0 W, 5 nodes
5.0 W, 10 nodes5.0 W, 20 nodes
Linear IncreaseWith # of hops
Exponential increase due to lowerSNR
Conclusions
EM-based radio is a new concept for underwater networks Channel different from terrestrial and acoustic
Rates comparable to acoustic networks are possible Requires multiple-hops Increased delay
More reliable than acoustic Not susceptible to environmental noise More network diversity
Cheaper than acoustic 1 km requires 2 acoustic modems @$30,000 each 1 km requires 20 EM nodes @<1000 each
P. Djukic, EM Networking in the Sea: Is it more than boiling water?
Thank you!
P. Djukic, EM Networking in the Sea: Is it more than boiling water?