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«2.1 Time Synchronization The actual need for time synchronization within an underwater acoustic network is not always present. It can be argued that ...»

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In terrestrial wireless communication systems, ACM has been in widespread use for some time. But as is pointed out in [11], the key difference between a terrestrial radio-based communications system and an underwater acoustic communication system is the large propagation delay, low bandwidth and high bit-error-rate. Their conclusion is that a direct adaptation of a terrestrial radio protocol may not provide acceptable results, and that protocols for this purpose need to be developed from the ground up.

The research into adaptive data rates in underwater communication systems seems to be in its infancy, and in order to move this research forward and in the end create a successful system employing ACM the following factors needs to be

addressed:

The special requirements for an underwater acoustic communication system need to be taken into account already at the design stage. There are currently no existing solutions that can directly be applied.

Measures for estimating the (strongly time varying) channel impulse response needs to be built into the burst structure in order to reduce measurement latency and overhead in the communication link.

In order to have maximum flexibility in the selection of possible signalling waveforms to use at any given time, a software defined radio (SDR) approach is necessary. In any case, lessons learnt in the SDR arena should be taken into consideration in a new design.

In order to be able to realize a fully software defined radio, the hardware platform need to be as flexible as possible, and that means that all the necessary functions must be realized in digital domain. Especially important in this respect is to ensure that the digital-to-analog (DAC) and analog-to-digital (ADC) conversion subsystems have the speed and precision needed.

Fortunately, the latter is steadily improving with the advances in ADCs, DACs, field-programmable-gate-arrays (FPGA) and microcontrollers (uC), and in the end the transducers and the required signal conditioning circuitry will be the limiting factors.

References

1. Syed AA, Heidemann J (2006) Time synchronization for high latency acoustic networks. In:

Proceedings of 25th IEEE international conference on computer communications INFOCOM, Barcelona, Spain, pp 1–12

2. Chirdchoo N, Soh W-S, Chua KC (2008) Mu-sync: a time synchronization protocol for underwater mobile networks. In: Proceedings of ACM WUWNet, San Francisco, CA, USA

3. Liu J, Zhou Z, Peng Z, Cui JH (2010) Mobi-sync: efficient time synchronization for mobile underwater sensor networks. Technical report UbiNet-TR10-01, UCONN CSE

4. Lu F, Mirza D, Schurgers C (2010) D-sync: doppler-based time synchronization for mobile underwater sensor networks. In: Proceedings of ACM WUWNet, Woods Hole, MA, USA 18 2 Topics Bordering the Physical Layer

5. Jarvis S, Janiesch R, Fitzpatrick K, Morrissey R (1997) Results from recent sea trials of the underwater digital acoustic telemetry system. In: Proceedings of 7th international conference on electronic engineering in oceanography, Southampton, UK, Halifax, Nova Scotia, pp 186–192

6. Smith KB, Larazza A, Kayali B (2001) Scale model analysis of full-duplex communications in an underwater acoustic channel. In: Proceedings of MTS/IEEE Oceans, Honolulu, HI, USA, (4):2250–2255

7. Xie GG, Gibson JH, Bektas K (2004) Evaluating the feasibility of establishing full-duplex underwater acoustic channels. In: Proceedings of 3rd annual Meditteranean ad hoc networking workshop (MedHoc), Bodrum, Turkey, pp 449–459

8. Gibson J, Xie G, Kaminski A (2005) Demand assigned channel allocation applied to full duplex underwater acoustic networking. J PACON International, pp 81–95

9. Stojanovic M, Catipovic J, Proakis JG (1993) Adaptive multichannel combining and equalization for underwater acoustic communications. J Acoust Soc Am 94(3):1621–1631

10. Benson A, Proakis J, Stojanovic M (2000) Towards robust adaptive acoustic communications.

In: Proceedings of MTS/IEEE Oceans, Providence, RI, USA, (2):1243–1248

11. Chitre M, Shahabudeen S, Stojanovic M (2008) Underwater acoustic communications and networking: recent advances and future challenges. Marine Technol Soc J 42(1):103–116 http://www.springer.com/978-3-642-25223-5



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