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Greg Davis Chris Johnson Scott Hambleton Jon Holton Chris Monfredo

P14251 Underwater Acoustic Communication. Greg Davis Chris Johnson Scott Hambleton Jon Holton Chris Monfredo. 10/29/13. Rochester Institute of Technology. 1. Underwater Acoustic Communication. Agenda Brief Review of Project Subsystems Analysis: CE Software Subsystems

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Greg Davis Chris Johnson Scott Hambleton Jon Holton Chris Monfredo

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  1. P14251 Underwater Acoustic Communication Greg DavisChris Johnson Scott HambletonJon HoltonChris Monfredo 10/29/13 Rochester Institute of Technology 1

  2. Underwater Acoustic Communication • Agenda • Brief Review of Project • Subsystems Analysis: • CE Software Subsystems • CE Hardware Subsystems • EE Subsystems • Communications • Power Systems • ME Subsystems • Box Subsystem • Thermal Subsystem 10/29/13 Rochester Institute of Technology 2

  3. Underwater Acoustic Communication Customer Requirements • Two-way communication at 15 kb/s of data • 15 Watts of power • Operating depth of 10m • Max operating temperature of 85 deg F 10/29/13 3 Rochester Institute of Technology

  4. Underwater Acoustic Communication • Software Subsystems • Communication Protocol • Control Unit • Receivers and Transmitters • Compression/Decompression • Encryption/Decryption • Data Framing • Error Checking and Correction 10/29/13 Rochester Institute of Technology 4

  5. Underwater Acoustic Communication Communication Protocol: CSMA/CA (Carrier Sense Multiple Access/Collision Avoidance) • Little Noise • High Throughput via CA • Functions with Swarm Expansions • Little Overhead (“11” for RTS, wait for “00” for CTS) 10/29/13 Rochester Institute of Technology 5

  6. Underwater Acoustic Communication • Control Unit • The main program running on the microcontroller:It initializes all of the other software modules and manages them • Receives data, decides what to do with it, and sends it off to its destination 10/29/13 Rochester Institute of Technology 6

  7. Underwater Acoustic Communication • Receivers and Transmitters • Four total software modules: • Incoming Message Receiver (Rx Hardware to MC) • Outgoing Message Transmitter (MC to Tx Hardware) • PC Receiver (PC to MC) • PC Transmitter (MC to PC) • Rx modules send incoming data to the control unit • Control unit sends outgoing data to the Tx modules 10/29/13 Rochester Institute of Technology 7

  8. Underwater Acoustic Communication • Compression/Decompression Module • One method for compression, one for decompression • Each method takes in and returns a bit array • From engineering analysis, some level of compression is needed to achieve a 15kbit data rate (timing analysis done later) • Most lossless compression algorithms can achieve a 2:1 compression ratio 10/29/13 Rochester Institute of Technology 8

  9. Underwater Acoustic Communication • Encryption/Decryption • One method for encryption, one for decryption • Each method takes in and returns a bit array • Encryption will be implemented as time allows, therefore: • Assume a publically shared key • No need to worry about timing analysis for now 10/29/13 Rochester Institute of Technology 9

  10. Underwater Acoustic Communication • Data Framing • A frame contains data being transmitted, and provides some additional information about the data as well • More frames = Easier to correct errors (Less data to check over at a time) • Start with a 7.5kbit frame, this can be lowered as needed • Frame header will start with a “11” to signify the beginning, followed by the amount of bits contained (a 14-bit number) • Frame footer will simply have a “00” to signify the end of the frame. 10/29/13 Rochester Institute of Technology 10

  11. Underwater Acoustic Communication 10/29/13 Rochester Institute of Technology 11

  12. Underwater Acoustic Communication • User Interface • Data is sent and received using Rx and Tx Modules • A Text-based Interface (i.e. unix terminal, cmd prompt) is sufficient for sending and receiving messages • If time allows, a more user-friendly GUI may be implemented 10/29/13 Rochester Institute of Technology 12

  13. Underwater Acoustic Communication Software Architecture 10/29/13 Rochester Institute of Technology 13

  14. Underwater Acoustic Communication • Microcontroller: Raspberry Pi • Widely-used, cost-effective microprocessor • Price: About $50 after tax + shipping • 700MHz ARM11-based processor (CPI is just over 1) • 512 MiB SDRAM • Easy to interface with (multiple serial, i2c ports) • Numerous written resources and strong developer community 10/29/13 Rochester Institute of Technology 14

  15. Underwater Acoustic Communication • Phase Shift Keying • Digital modulation scheme that stores data by modulating the phase of the carrier frequency • The modulation will allow each phase to represent a unique pattern of bits, with each phase containing the same number of bits • There are two main ways of demodulating a PSK signal • By viewing the phase itself as conveying info • By viewing a change of phase as conveying info 10/29/13 Rochester Institute of Technology 15

  16. Underwater Acoustic Communication • Quadrature Phase Shift Keying (QPSK) • Each point in the constellation represents a 2 bit binary number based on the in phase and quadrature components the signal • 00 = A*cos(2πfct) • 01 = A*sin(2πfct) • 10 = -A*cos(2πfct) • 11 = A*cos(2πfct) • Initializing the constellation in this manner is known as Gray Coding. This allows for a lower bit error rate due to only one bit changing per 90 degree shift in phase. 10/29/13 Rochester Institute of Technology 16

  17. Underwater Acoustic Communication • Spectral Efficiency • Specifies the information rate that can be sent over a given bandwidth • Being that PSK is a double-sideband modulation scheme, the symbol rate W cannot exceed the N (bit/s)/Hz • Since we are considering QPSK to be our modulation scheme, we have an alphabet of M = 4 symbols • From this we know: • N = log2(M) = 2, and thus we cannot exceed 2 (bit/s)/Hz • From our Engineering Specs. our data rate must be 15 k(bit/s) • 15k(bit/s) <= 2x(bit/s)/Hz • Therefore our Bandwidth, x, must be at least 7.5kHz • To account for coding overhead and non-perfect signals, we have decided to set our bandwidth to 10kHz 10/29/13 Rochester Institute of Technology 17

  18. Underwater Acoustic Communication • Being that the frequency range of our speaker is 2kHz to 15 kHz, we will center our bandwidth around 8 kHz • This will give us an overall bandwidth ranging from 3kHz to 13kHz 10/29/13 Rochester Institute of Technology 18

  19. Underwater Acoustic Communication • Demodulation: 2 Schemes cos(2πfct) ADC To MC si(t) BPF AMP To MC ADC sin(2πfct) 10/29/13 Rochester Institute of Technology 19

  20. Underwater Acoustic Communication • Mixing: 10/29/13 Rochester Institute of Technology 20

  21. Underwater Acoustic Communication • This modulation scheme will take in the signal, si(t), bandpass filter the signal around the frequencies contained in our bandwidth, and then pass that signal to the in phase and quadrature phase branches • In each branch, the signal will either get mixed with a cosine or a sine to leave only the part of the signal which is in phase with the mixing signal • Each modified signal will now be passed to an analog to digital converter and fed to pins on the micro controller • Pros: 1) we only need to differentiate between two signals on each pin • Cons: 1) requires more components and therefore space • 2) slightly more complicated than other demodulation schemes 10/29/13 Rochester Institute of Technology 21

  22. Underwater Acoustic Communication • This scheme is similar to the first scheme only without the phase discriminant part of the circuit. • Pros: 1) requires fewer components • Cons: 1) more error prone due to comparing between 4 signals vs. 2 signals si(t) ADC BPF To MC AMP 10/29/13 Rochester Institute of Technology 22

  23. Underwater Acoustic Communication 10/29/13 Rochester Institute of Technology 23

  24. Underwater Acoustic Communication Transmitter Amplifier Stage Voltage Gain: Non-inverting Op-Amp Circuit Current Gain: Class B or AB Amplifier -Class B uses less power -Class AB has lower distortion 10/29/13 Rochester Institute of Technology 24

  25. Underwater Acoustic Communication • Common Mode Choke: • These are very useful for removing electromagnetic interference and radio frequency interference from the power supply lines • A CMC is composed of either 2 windings around a magnetic core or a ferrite bead. • A CMC is essentially 2 inductors in series, and just like any otherinductor, resist changes to current • Therefore, alternating currents athigher frequencies are resisted muchmore than current changes at lowfrequencies • This is to say that the chokes impedance increases with freq. 10/29/13 Rochester Institute of Technology 25

  26. Underwater Acoustic Communication • Specifying a CMC: • The cutoff frequency of the CMC can be derived to be fc = 1/(2π*(2L/R)) • where L is the value of the inductor in Henneries and R is the value of the load resistor, determined by the speaker, in Ohms • Since we are working with very low frequencies we are able to the allow the CMC to filter any frequency above 50 kHz without much worry • fc = 50 kHz (L) = R/(4π*50k) 10/29/13 Rochester Institute of Technology 26

  27. Underwater Acoustic Communication Signal Power Transformations Electrical Power to Acoustic Power 10/29/13 Rochester Institute of Technology 27

  28. Underwater Acoustic Communication Signal Power Transformations Acoustic Power to Transmitting Sound Pressure Level (SPL) 10/29/13 Rochester Institute of Technology 28

  29. Underwater Acoustic Communication Signal Power Transformations Transmission Losses 1. Spreading 2. Absorption 10/29/13 Rochester Institute of Technology 29

  30. Underwater Acoustic Communication Signal Power Transformations Transmitting to Receiving SPL 10/29/13 Rochester Institute of Technology 30

  31. Underwater Acoustic Communication Signal Power Transformations Receiving SPL to Hydrophone Voltage @0m @30m 10/29/13 Rochester Institute of Technology 31

  32. Underwater Acoustic Communication • Bandpass Filtering: • Simplest implementation, is an RC high pass filter followed by an RC low pass filter in series • The cutoff frequency is defined as fc=1/(2π*RC) • fcLP = 13kHz= 1/(2π*RC) • RC = 1/(2π*13k) • fcHP = 2kHz = 1/(2π*RC) • RC = 1/(2π*2k) 10/29/13 Rochester Institute of Technology 32

  33. Underwater Acoustic Communication • Bandpass Filter Simulation: • Parameters: • RL = 5kΩ • CL = 2.3nF • RH = 10kΩ • CH = 8nF • Results: • wcL = 8.696e+4 rad/sec • wcH = 1.25e+4 rad/sec 10/29/13 Rochester Institute of Technology 33

  34. Underwater Acoustic Communication AM Maximum Gain Resulting Gain of lowest voltage @30m ADC Level Division Receiver Amplifier Gain PSK Automatic Gain Amplifier -amplifies the input signal such that the RMS voltage matches a reference voltage -allows for amplification to same voltage level independent of distance -can be used because information is not stored in amplitude 10/29/13 Rochester Institute of Technology 34

  35. Underwater Acoustic Communication Noise Squelch 10/29/13 Rochester Institute of Technology 35

  36. Underwater Acoustic Communication Signal to Noise 10/29/13 Rochester Institute of Technology 36

  37. Underwater Acoustic Communication • Symbol Error Rate • Ps = 2Q[(Es/No)(1/2)]-{Q[(Es/No)(1/2)]}2 • Q[x] = Q-function or the tail probability: Gives the probability that a normal, random Gaussian variable will be larger than x • Es/No = signal to noise ration of each symbol (in dB) • Ps = 2Q[(98)(1/2)]-{Q[(98)(1/2)]}2 = 4.183825607779467e-23 • (That’s pretty low…) 10/29/13 Rochester Institute of Technology 37

  38. Underwater Acoustic Communication Level Shifting Only necessary is hydrophone voltage varies between a positive and negative voltage 10/29/13 Rochester Institute of Technology 38

  39. Underwater Acoustic Communication • Interfacing with Modulation/Demodulation Schemes • Modulation: • AD9835 Direct Digital Synthesizer, Waveform Generator • Takes 16-bit commands, can store phases and frequencies • Outputs an analog signal based on selected phase and frequency • Demodulation: • Still looking at potential chips • If no chip can be found, an ADC can just pass the input wave to the microcontroller and DSP can be performed 10/29/13 Rochester Institute of Technology 39

  40. Underwater Acoustic Communication Communication Hardware Diagram 10/29/13 Rochester Institute of Technology 40

  41. Underwater Acoustic Communication System Power 10/29/13 Rochester Institute of Technology 41

  42. Underwater Acoustic Communication Buck ConverterBuck-Boost Converter 10/29/13 Rochester Institute of Technology 42

  43. Underwater Acoustic Communication Design Specifications Buck SpecificationsBuck-Boost Specifications 10/29/13 Rochester Institute of Technology 43

  44. Underwater Acoustic Communication Battery Selection Battery Voltage:12 Volts System Power: 15 Watts System Current: 1.25 Amps Battery Lifetime > 1 Hour Battery Energy > 1.25 Ah 10/29/13 Rochester Institute of Technology 44

  45. Underwater Acoustic Communication Data Rate Analysis 10/29/13 Rochester Institute of Technology 45

  46. Underwater Acoustic Communication 10/29/13 Rochester Institute of Technology 46

  47. Underwater Acoustic Communication • Data Rate Analysis (Compression/Decompression) • FLZP compressor was chosen for reference analysis (selected a compression algorithm with a low rating) • Can compress at 171968kbps and decompress at 674608kbps on a 2.9 GHz processor (2:1 ratio) • RPi is roughly 24% as fast. Slowdown translates to 41272 kbps compression and 161905kbps decompression. • 15kbits can be compressed in roughly 360µs, 7.5kbits can be decompressed in roughly 46µs • The time needed for compression and decompression is negligible, even for a poorly rated compressor 10/29/13 Rochester Institute of Technology 47

  48. Underwater Acoustic Communication • Data Rate Analysis (Encoding/Decoding) • Using Reed-Solomon error correcting codes,a 206MHz processor can correct 10Mbps if 10% error rate • RPi Speedup ≈ 340% • The RPi can theoretically correct 34Mbps if 10% error rate • At most, we are looking to correct 750 bits. Using the above rate, this can be accomplished in 22µs • The time needed to correct errors is negligible for a widely used EEC scheme. • It is assumed that correction is more complex than both encoding and detection (more complex operations) 10/29/13 Rochester Institute of Technology 48

  49. Underwater Acoustic Communication Thermal Analysis 10/29/13 Rochester Institute of Technology 49

  50. Underwater Acoustic Communication • Rapid Prototype Design • Uses Corrosive resistant plastic and rubber O-rings • Rapid Prototyping Machines • Quick build time • Easy implementation • Watertight connectors on back • Interchangeable top and front panels for future integrations • Higher cost 10/29/13 Rochester Institute of Technology 50

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