Apparatus, systems and methods for enhanced multi-carrier based underwater acoustic communications

• US 7,859,944 B2
• Filed: 06/18/2008
• Issued: 12/28/2010
• Est. Priority Date: 06/18/2007
• Status: Active Grant

• Current Assignee: Massachusetts Institute of Technology, University of Connecticut (State of Connecticut), Woods Hole Oceanographic Institution
• Original Assignee: Massachusetts Institute of Technology, University of Connecticut (State of Connecticut), Woods Hole Oceanographic Institution
• Inventors: Li, Baosheng, Zhou, Shengli, Willett, Peter, Freitag, Lee, Stojanovic, Milica
• Primary Examiner(s): Lobo; Ian J
• Application Number:
  US12/141,527
• Publication Number:
  US 20090129204A1
• Time in Patent Office: 923 Days
• Field of Search: 367/134
• US Class Current: 367/134
• CPC Class Codes: G01H 3/08 Analysing frequencies present in complex vibrations, e.g. comparing harmonics present acoustic presence detection G01V1/001
• CPC Class Codes: H04B 11/00 Transmission systems employing sonic, ultrasonic or infrasonic waves

Abstract

Advantageous OFDM-based underwater acoustic (UWA) apparatus, systems and methods are provided according to the present disclosure. In general, OFDM transmissions over UWA channels encounter frequency-dependent Doppler drifts that destroy the orthogonality among OFDM subcarriers. The disclosed apparatus, systems, and methods use a two-step approach to mitigate frequency-dependent Doppler drifts for zero-padded OFDM transmissions over fast-varying channels: (1) non-uniform Doppler compensation via resampling to convert a “wideband” problem into a “narrowband” problem; and (2) high-resolution uniform compensation on the residual Doppler. The disclosed apparatus, systems and methods are based on block-by-block processing and do not rely on channel dependence across OFDM blocks. Thus, the disclosed apparatus, systems and methods are advantageously applicable for fast-varying UWA channels.

Claims

1. A method for mitigating Doppler distortions in a received multi-carrier based underwater acoustic (UWA) signal, the method comprising the steps of:

(a) receiving a multi-carrier based UWA signal in a distorted condition, wherein the multi-carrier based UWA signal includes one or more packets and wherein each of the one or more packets includes one or more data blocks; and

(b) processing the received multi-carrier based UWA signal to compensate for the distorted condition and restore the multi-carrier based UWA signal to an approximate original condition, wherein processing of the received multi-carrier based UWA signal includes;

i. estimating the Doppler scale for each packet,

ii. using the Doppler scale estimations to resample the multi-carrier based UWA signal for non-uniform Doppler compensation,

iii. estimating the carrier frequency offset (CFO) on a block by block basis to generate a plurality of different CFO estimations, and

iv. using the plurality of different CFO estimations for high-resolution uniform compensation of residual Doppler effects for inter-carrier interference (ICI) reduction.

2. The method according to claim 1, wherein each of the one or more packets includes a preamble and a postamble, and wherein the Doppler scale for each packet is estimated by synchronizing the preamble and the postamble of that packet.

3. The method according to claim 1, wherein zero-padded orthogonal frequency division multiplexing (ZP-OFDM) of the UWA signal is used to save transmission power on the long guard interval.

4. The method according to claim 3, wherein null subcarriers in the received UWA signal are used to estimate the CFO on a block by block basis to generate the plurality of different CFO estimations.

5. The method according to claim 4, wherein each CFO is estimated using one of:

(i) a one dimensional search, (ii) a standard Gradient method, and (iii) a coarse-grid followed by a bisectional search.

6. The method according to claim 4, wherein a two dimensional analysis of the null subcarriers is used to jointly estimate the Doppler scale and the each CFO on a block by block basis.

7. The method according to claim 1, wherein processing of the received UWA signal further includes an initial step of bandpass filtering of the received UWA signal to suppress out-of-band noise.

8. The method according to claim 1, wherein the received signal is converted to baseband before processing.

9. The method according to claim 1, wherein processing of the received UWA signal further includes estimating the channel frequency response for each subcarrier using pilot tones.

10. The method according to claim 1, wherein processing of the received UWA signal further includes Viterbi algorithm (VA) decoding.

11. The method according to claim 1, wherein the multi-carrier based UWA signal is received by multi-channel reception using maximum-ratio-combining (MRC).

12. An underwater acoustic (UWA) receiver unit for mitigating Doppler distortions in a received multi-carrier based underwater acoustic (UWA) signal, the unit comprising:

(a) at least one receiving element for receiving a multi-carrier based UWA signal in a distorted condition, wherein the multi-carrier based UWA signal includes one or more packets and wherein each of the one or more packets includes one or more data blocks; and

(b) a processor for processing the received multi-carrier based UWA signal to compensate for the distorted condition and restore the multi-carrier based UWA signal to an approximate original condition, wherein the processor is programmed to;

i. estimate the Doppler scale for each packet,

ii. use the Doppler scale estimations to resample the multi-carrier based UWA signal for non-uniform Doppler compensation,

iii. estimate the carrier frequency offset (CFO) on a block by block basis to generate a plurality of different CFO estimations, and

iv. use the plurality of different CFO estimations for high-resolution uniform compensation of residual Doppler effects for inter-carrier interference (ICI) reduction.

13. The unit according to claim 12, wherein each of the one or more packets includes a preamble and a postamble, and wherein the Doppler scale for each packet is estimated by synchronizing the preamble and the postamble of that packet.

14. The unit according to claim 12, wherein zero-padded orthogonal frequency division multiplexing (ZP-OFDM) of the UWA signal is used to save transmission power on the long guard interval.

15. The unit according to claim 14, wherein null subcarriers in the received UWA signal are used to estimate the CFO on a block by block basis to generate the plurality of different CFO estimations.

16. The unit according to claim 15, wherein each CFO is estimated using one of:

(i) a one dimensional search, (ii) a standard Gradient method, and (iii) a coarse-grid followed by a bisectional search.

17. The unit according to claim 15, wherein a two dimensional analysis of the null subcarriers is used to jointly estimate the Doppler scale and each CFO on a block by block basis.

18. The unit according to claim 12, wherein the processor is programmed to estimate the channel frequency response for each subcarrier using pilot tones in the UWA signal.

19. The unit according to claim 12, wherein the processor is programmed to decode the received UWA signal using Viterbi algorithm (VA) decoding.

20. The unit according to claim 12, wherein the one or more receiving elements are adapted for multi-channel signal reception of the UWA signal.

21. An underwater acoustic (UWA) communications system, comprising:

(a) a transmitter unit adapted to transmit a multi-carrier based UWA signal in an original condition, the multi-carrier based UWA signal including one or more packets and each of the one or more packets including one or more data blocks;

(b) one or more receiving elements adapted to receive the multi-carrier based UWA signal in a distorted condition; and

(c) a processor in association with the one or more receiving elements, the processor being programmed to compensate for the distorted condition of the received multi-carrier based UWA signal and to restore the multi-carrier based UWA signal to an approximate original condition by;

i. estimating the Doppler scale for each packet,

ii. using the Doppler scale estimations to resample the multi-carrier based UWA signal for non-uniform Doppler compensation,

iii. estimating the carrier frequency offset (CFO) on a block by block basis to generate a plurality of different CFO estimations, and

iv. using the plurality of different CFO estimations for high-resolution uniform compensation of residual Doppler effects for inter-carrier interference (ICI) reduction.

22. The system according to claim 21, wherein each of the one or more packets includes a preamble and a postamble and wherein the Doppler scale for each packet is estimated by synchronizing the preamble and the postamble of that packet.

23. The system according to claim 21, wherein zero-padded orthogonal frequency division multiplexing (ZP-OFDM) of the UWA signal is used to save transmission power on the long guard interval.

24. The system according to claim 23, wherein null subcarriers in the received UWA signal are used to estimate the CFO on a block by block basis to generate the plurality of different CFO estimations.

25. The system according to claim 21, wherein the processor is programmed to estimate the channel frequency response for each subcarrier using pilot tones in the UWA signal.

26. The system according to claim 21, wherein the processor is programmed to decode the received UWA signal using Viterbi algorithm (VA) decoding.

27. The system according to claim 21, wherein the one or more receiving elements are adapted for multi-channel signal reception of the UWA signal.