electromagnetic radio in the sea: is it more than boiling water? petar djukic research scientist...

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”


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?


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


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


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


Another slide with a lot of equations

From previous results:

Absorption

Wavelength

Propagation speed

Skin depth


Seawater attenuation is very different from terrestrial


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


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


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


25 m gain

10 m gain

Effect of Seawater on Noise(from ITU Recommendation P372)


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!


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


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


256-QAM 7/8

64-QAM 2/3

Realistic range

“Fancy” range

Actual rates


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


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


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


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)


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


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


5.0 W, 5 nodes


Multi-hop Throughput (ideal TDMA)


21% Gain

29% Gain

12% Gain

Multi-hop Delay (ideal TDMA)


0 500 1000 15000

10

20

30

40

50

60

70Delay vs. Distance (depth 25 m)

Distance (m)

Del

ay (s

)


0.1 W, 20 nodes

0.5 W, 5 nodes


5.0 W, 5 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


Thank you!


electromagnetic radio in the sea: is it more than boiling water? petar djukic research scientist...

Documents