1. Underwater Acoustic Communication1
P14251
Underwater Acoustic Communication
CE Overview
Software Architecture
Control unit flowchart and pseudo-code
Error Detection/Correction
Framing Information
Data Rate Analysis
11/26/13
Rochester Institute of Technology 1

2. Underwater Acoustic Communication2
P14251
Underwater Acoustic Communication
Software Architecture
11/26/13
Rochester Institute of Technology 2

3. Underwater Acoustic Communication3
P14251
Underwater Acoustic Communication
Control Unit Pseudo-Code
11/26/13
Rochester Institute of Technology 3

4. Underwater Acoustic Communication4
P14251
Underwater Acoustic Communication
Control Unit Pseudo-Code
11/26/13
Rochester Institute of Technology 4

5. Underwater Acoustic Communication5
P14251
Underwater Acoustic Communication
Control Unit Pseudo-Code
11/26/13
Rochester Institute of Technology 5

6. Underwater Acoustic Communication6
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Underwater Acoustic Communication
Error Detection and Correction
Hybrid Scheme: ECC and ARQ
EEC Implementation: BCH vs. Reed Solomon
BCH is easier to implement, but requires a large amount of redundancy
Reed-Solomon is more complex, but overall much better and requires only 20% redundancy (to correct 10% of errors)
11/26/13
Rochester Institute of Technology 6

7. Underwater Acoustic Communication7
P14251
Underwater Acoustic Communication
Error Detection and Correction
Encoding: LFSR based implementation is extremely simple and fast. Will translate well to C
11/26/13
Rochester Institute of Technology 7

8. Underwater Acoustic Communication8
P14251
Underwater Acoustic Communication
Error Detection and Correction
Decoding is more complex, but efficient algorithms exist that will help significantly
11/26/13
Rochester Institute of Technology 8

9. Underwater Acoustic Communication9
P14251
Underwater Acoustic Communication
Frame Sentinels
Sentinels are a unique pattern of bits that signify the start and end of the frame i.e –data– 01110
When preparing the frame for transmission, if the pattern appears anywhere in the data, bit stuffing is used to eliminate it (i.e > )
Commonly used sentinel which we’ll use is
Frame Header
1 bit to signify the type of frame: control or message
1 bit that gets flipped each time a new frame is sent
11/26/13
Rochester Institute of Technology 9

10. Underwater Acoustic Communication
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P14251
Underwater Acoustic Communication
Control frame formats
Since it’s especially important to interpret control frames correctly, 4 bits are used to display the unique patterns
0000 – Request to Send (RTS)
0110 – Clear to Send (CTS)
1001 – Acknowledgement (ACK)
1111 – Done (Signifies that all frames have been sent)
11/26/13
Rochester Institute of Technology 10

11. Underwater Acoustic Communication
1111
P14251
Underwater Acoustic Communication
Frame Sizes
- Control Frame: 18 (overhead) + 4 (type) = 22 bits - Message Frame: 18(overhead) + X (message) = 18+X bits
Propagation delay (20ms for 30m distance) limits the number of frames that can be sent.
For 15kb/s, the maximum number of message frames = 9 frames, each containing 1k encoded information bits
Increase to ~1023 bits for RS(1023, 853)
11/26/13
Rochester Institute of Technology 11

12. Underwater Acoustic Communication
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Underwater Acoustic Communication
Data Rate Analysis
f c =37kHz, t c =81μs (3 periods per 2 bits)
Frames that get transmitted for a message:
Send RTS – 22 bits
Receive CTS – 22 bits
Send all message frames – 1042 bits/frame
Receive all acknowledgements – 22 bits/frame
Send Done – 22 bits
Receive Acknowledgement – 22 bits
Total transferred bits = 9664 bits
Total transfer time = 0.831s (about 170ms left for code)
11/26/13
Rochester Institute of Technology 12