Electromagnetic Radio in the Sea: Is it more than boiling water? Petar Djukic Research 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 ( 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 Z tt = V t /I t and Z rt = V r /I t to get the channel H=V r /V t = Z rt / Z tt  If Z tt and Z rt are known we have the transfer function P. Djukic, EM Networking in the Sea: Is it more than boiling water? VtVt VrVr It→It→ Ir→Ir→ Ir←Ir← It←It← V r =Z tt I t +Z tr I r V t =Z rt I t +Z rr I r

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? 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?

Effect of Transmit Power on Transmission Range P. Djukic, EM Networking in the Sea: Is it more than boiling water? Potentially expensive to implement receiver More reasonable receiver

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? ITU-P372 (atmosphere) Refraction Refraction & Absorption

Finally, the SNR! P. Djukic, EM Networking in the Sea: Is it more than boiling water? 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? 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? Conventional wisdom is wrong!

Propagation Time P. Djukic, EM Networking in the Sea: Is it more than boiling water? ν=1.6∙10 4 m/s ν=5.0∙10 4 m/s 50 m UW~300 km wire

Single-hope Theoretical Performance P. Djukic, EM Networking in the Sea: Is it more than boiling water? 18.6% ALOHA 80 % CSMA

Single-hop Theoretical Performance (acoustic) P. Djukic, EM Networking in the Sea: Is it more than boiling water? 18.6% ALOHA Low due to CTS-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 A B B C C D D A→B B→C C→D D→C C→B B→A t prop A A B B C C D D A→B B→C C→D D→C C→B B→A

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? Linear Increase With # of hops Exponential increase due to lower SNR

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 each  1 km requires 20 EM 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?