Underwater Acoustic Modem :

1
Underwater Acoustic Modem Design and
Implementation Issues
Naval Physical and Oceanographic Laboratory -
Cochin Defense Research and Development
Organization Ministry of Defense Govt. of INDIA
2
1. Acoustic Modem Application Scenarios
Prototype Development Phases
Channel Measurements
Modem Design
Performance Evaluation
Lab Field Testing
2/12
3
2. Problems Solutions for Underwater Channels
Problems Solutions
Existence of Spectral Nulls Existence of Fast Fading Error Coding Interleaving Multi Carrier
Existence of High Delay Spread Complexity of Adaptive Equalization Adaptive Cyclic Prefix Length (Guard Time) Selection
Impracticality of Coherent Modulation Differentially Coherent Modulation
4/12
4
2.1 Test bed and Channel Measurements
3/12
5
3. Modem Design
5/12
6
3.1 Performance Evaluation

• Effect of two key parameters on received images
  in actual scenario are shown for illustration.
• Measured Delay Spread was 30ms (Mini Tank)
• All experiment were carried out at same SNR
  keeping all other parameters same.

6/12
7
4. Field Trials at UARF Kulamavu Idukki
MAJOR EXPERIMENTS CONDUCTED
1. Channel Measurements at various ranges. 2.
Data transmission at various ranges. 3. Study of
Doppler Effect on Data Transmission.
TEST RESULTS
1. Maximum delay spread up to 400ms was
observed 2. Reliable data connectivity achieved
(Text image) up to 1.8km with a max power of
25W. 3. Successfully tested a Doppler
compensation Technique
Current Status Prototype demonstrated
successfully at UARF
7/12
8
5. Doppler Compensation
Doppler Results in Time Scaling
RXR Signal s( 1 ? ? t)
(? 2 v/c)
TXR Signal s(t)
9
5.1 Innovative Method to Compensate Doppler Re
sampling

• The no of samples between the frame start and
  frame end chirps are fixed (N1)
• Compute the difference in the sample indices
  for start and end. (N2)
• Compute the resampling factor based on the
  difference. (N2/N1). Resample the data to
  correct the Doppler.

Naval Physical Oceanographic Laboratory, Kochi.
Actual Re sampling Factor 10021
10
(No Transcript)
11
6. NPOL Acoustic Modem Technology Demonstrator
Technical Specifications Performance Highlights

• Design Philosophy Software Defined Radio
  using MATLAB / LabVIEW ensuring high flexibility.
  
• Data Rate From 1kbps to 5.4 kbps (raw) or 500
  bps to 2.7 kbps (with ½ rate convolutional
  coding).
• Data Transmit Bandwidth 6 KHz to 9 KHz using
  95 carriers with carrier spacing 32 Hz.
• Modulation Format DQPSK over OFDM
  (Orthogonal Frequency Division Multiplexing).
• Error Control Coding Convolutional codes
  with Interleaver and Viterbi Decoding.
• Transmit Power 10 - 15 W (Linear Power
  Amplifier Class AB Developed in NPOL).
• Operation Mode Half Duplex Burst / Frame Mode
  ( Maximum Frame size of 64 kbits).
• Adaptable to Shallow water Multi Path (High
  Delay spread environments).

Other Features
Controllable Parameters Error Correction,
Power Data Rate, Range Cyclic Prefix, No of
Carriers
12
7. State of the Art

• Problems these papers point out have been
  addressed and solved in our work !

Comparison with Commercial Models

• Compared Performance Features (As per
  Brochure).
• NPOL Prototype outperforms the communication
  specs.
• Need to develop Low Power Hardware.
• Need to develop protocols for Networking
• Growing market demand (Defense Civilian)

13
8. Under Water Telephony (UWT) Augmentation

• Specifications of UWT developed by NPOL
• Voice Mode
• NATO Standard
• Carrier Frequency 8.0875 KHz
• Audio Frequency range 300Hz to 3.2KHz
• Modulation SSB (Upper Side Band)
• Pass Band Frequency 8.4 to 11.3 KHz
• EKM Standard
• Carrier Frequency 8.2 KHz
• Audio Frequency range 300Hz to 3.2KHz
• Modulation SSB (Lower Side Band)
• Pass Band Frequency 5 to 7.9 KHz
• Data Mode

Software Tunable Under Water Voice
Communications
Acoustic Modem
8/12
14
9. Future Work Envisaged

• Porting the developed Algorithms to COTS
  Hardware and performance validation.
• Fine tuning of algorithms for low power
  hardware.
• Implementation of Improved FEC Technique (LDPC,
  Turbo Codes)
• Implementation of Space Time Coding (Alamouti
  Scheme, MIMO)
• Design and Evaluation of Communication
  Protocols for Underwater Channels.
• Sensor Networks Project (Project Peer Review
  completed Feb 2006)

Research Proposals for Joint Work
12/12