The success of multicarrier modulation in the form of orthogonal-frequency-division-modulation (OFDM) in radio channels illuminates a clear path one could take towards high-rate underwater acoustic communications.
Our research aims to make OFDM work in challenging underwater enviroments and build a practical multicarrier modem prototype.
Aqua-fModem, as shown in the following two figures, is our OFDM modem prototype, implemented on a TMS320C6713 DSP board. It uses QPSK modulation, rate-1/2 channel coding (both convolutional and nonbinary low-density-parity-check (LDPC) codes). With a bandwidth 5.5 kHz, the achieved spectral efficiency is 0.56 bits/s/Hz, and the data rate is 3.1 kb/s.
It has the following features:
- Real time decoding is achieved. In a system with one transmitter and one receiver, only 40 ms (with convolutional coding) or 50 ms (with nonbinary LDPC coding) is needed to decode an OFDM block of duration 210 ms.
- Easy to interface with networking protocols. (following the NEMA standard).
Aqua-Modem has been demonstrated in the WUWNet workshop, Berkeley, CA, Nov. 2, 2009.
We have been engaged in acoustic modem prototype development.
- The PC-based and DSP-based modem prototypes were demonstrated at WUWNet, Montreal, Sept. 2007, as shown in the below two figures.
- The PC-based and DSP-based (2 × 2) MIMO-OFDM Acoustic Modem Prototypes are shown in the below two figures. The MIMO-OFDM modem prototypes were demonstrated at WUWNet, San Francisco, CA, Sept. 2008 and won the first-prize in demo category, as voted by workshop participants. With two parallel data streams transmitted, the data rate is 6.2 kb/s, with the other parameters the same as preivous modems.
- DSP Implementation of SISO and MIMO OFDM Acoustic Modems, H. Yan, S. Zhou, Z. Shi, J.-H. Cui, L. Wan, J. Huang, and H. Zhou, Proc. of OCEANS, Sydney, May 2010.
- A DSP Implementation of OFDM Acoustic Modem, H. Yan, S. Zhou, Z. Shi, and B. Li, WUWNet, Montréal, Québec, Canada, September 14, 2007.
- Office of Naval Research
- National Science Foundation