Greg DavisChris JohnsonScott HambletonJon HoltonChris Monfredo
10/29/13 Rochester Institute of Technology 1
P14251Underwater Acoustic Communication
Underwater Acoustic Communication
10/29/13 Rochester Institute of Technology 2
AgendaBrief Review of ProjectSubsystems 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 3
Underwater Acoustic Communication
Customer Requirements
Customer Rqmt. #
Importance Description
CR1 9 Send signalCR2 9 Send signal at a rate in kb/sCR3 9 System must function underwaterCR4 9 Reliable communication schemeCR5 9 Communications must resist frequency contaminationCR6 9 Must have 2-way communication capabilitiesCR7 9 Must be able to operate in frequency contaminated environments
Two-way communication at 15 kb/s of data15 Watts of powerOperating depth of 10mMax operating temperature of 85 deg F
Underwater Acoustic Communication
Rochester Institute of Technology 410/29/13
Software Subsystems-Communication Protocol-Control Unit-Receivers and Transmitters-Compression/Decompression-Encryption/Decryption-Data Framing-Error Checking and Correction
Underwater Acoustic Communication
Rochester Institute of Technology 510/29/13
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)
StartAssemble a
Frame of DataChannel
Idle?
Back off for random amount
of time
Transmit Request to Send (RTS)
Clear to Send (CTS) Received?
Transmit Data End
NO
NO
YES
YES
Underwater Acoustic Communication
Rochester Institute of Technology 610/29/13
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
Underwater Acoustic Communication
Rochester Institute of Technology 710/29/13
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
Underwater Acoustic Communication
Rochester Institute of Technology 810/29/13
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
Underwater Acoustic Communication
Rochester Institute of Technology 910/29/13
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
Underwater Acoustic Communication
Rochester Institute of Technology 1010/29/13
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.
Underwater Acoustic Communication
Rochester Institute of Technology 1110/29/13
Encoder
k data bits
n encoded bits
Underwater Acoustic Communication
Rochester Institute of Technology 1210/29/13
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
Underwater Acoustic Communication
Rochester Institute of Technology 1310/29/13
Software ArchitecturePC
(User Interface)
RX (PC)
TX (PC)
Control Unit (Microcontroller)
RX (Incoming Message)
TX (Outgoing Message)
DSP
Error Handler (Encoding, Decoding, Correcting)
Encryptor/Decryptor
Compressor/Decompressor
Underwater Acoustic Communication
Rochester Institute of Technology 1410/29/13
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
Phase Shift Keying
Digital modulation scheme that stores data by modulating the phase of the carrier frequencyThe modulation will allow each phase to represent a unique pattern of bits, with each phase containing the same number of bitsThere are two main ways of demodulating a PSK signal
1. By viewing the phase itself as conveying info2. By viewing a change of phase as conveying info
Underwater Acoustic Communication
Rochester Institute of Technology 1510/29/13
Underwater Acoustic Communication
Rochester Institute of Technology 1610/29/13
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.
Underwater Acoustic Communication
Rochester Institute of Technology 1710/29/13
Spectral Efficiency
Specifies the information rate that can be sent over a given bandwidthBeing that PSK is a double-sideband modulation scheme, the symbol rate W cannot exceed the N (bit/s)/HzSince we are considering QPSK to be our modulation scheme, we have an alphabet of M = 4 symbolsFrom 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
Underwater Acoustic Communication
Rochester Institute of Technology 1810/29/13
• 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
Underwater Acoustic Communication
Rochester Institute of Technology 1910/29/13
Demodulation: 2 Schemes
AMPBPF
sin(2πfct)
cos(2πfct)
ADC
ADC
To MC
To MC
si(t)
Underwater Acoustic Communication
Rochester Institute of Technology 2010/29/13
Mixing:
Underwater Acoustic Communication
Rochester Institute of Technology 2110/29/13
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
Underwater Acoustic Communication
Rochester Institute of Technology 2210/29/13
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
AMP BPF si(t) ADC To MC
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Underwater Acoustic Communication
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Transmitter Amplifier StageVoltage Gain: Non-inverting Op-Amp Circuit
Current Gain: Class B or AB Amplifier
(5V )(AV )=12V
AV =2.4V /V
-Class B uses less power-Class AB has lower distortion
Underwater Acoustic Communication
Rochester Institute of Technology 2510/29/13
Common Mode Choke:
These are very useful for removing electromagnetic interference and radio frequency interference from the power supply linesA 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 currentTherefore, alternating currents athigher frequencies are resisted muchmore than current changes at lowfrequenciesThis is to say that the chokes impedance increases with freq.
Underwater Acoustic Communication
Rochester Institute of Technology 2610/29/13
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 OhmsSince 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)
Underwater Acoustic Communication
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Signal Power TransformationsElectrical Power to Acoustic Power
P ac=P elec Eff speaker
P ac=(10W )(0.01)
P ac=0.1W
Underwater Acoustic Communication
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Signal Power TransformationsAcoustic Power to Transmitting Sound Pressure Level (SPL)
SPL=√((P ac Z )
A)
SPL= 1216 Pa
Z =1.48 MNs /m 3
P ac=0.1W
A=1ft2=0.1m2
LT=20log(SPL1uPa
)
LT=181 dB
Underwater Acoustic Communication
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Signal Power TransformationsTransmission Losses1. Spreading
2. Absorption
TLS=20 log R
TL A=1dB / km
TL( A@ 30m )=0.03dB
TLS=20 log30mTLS=29.54 dB
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Signal Power TransformationsTransmitting to Receiving SPL
LR= LT −TL S−TL A
LR=181dB−29.54dB− .03 dB
LR=151.43 dB
SPL10W=35.5Pa
SPL2.5W=18.7Pa
Underwater Acoustic Communication
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Signal Power TransformationsReceiving SPL to Hydrophone Voltage
@0m @30m
Hydrophone Sensitivity=20 log( (? uV /Pa )(1V /uPa )
)=−190dB
Hydrophone Sensitivity= 316uV / Pa
V R=(316uV /Pa )(1216Pa )
V R=.381V
V R=(316uV /Pa )(35.5Pa )
V R=.0112V
Underwater Acoustic Communication
Rochester Institute of Technology 3210/29/13
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)
Underwater Acoustic Communication
Rochester Institute of Technology 3310/29/13
Bandpass Filter Simulation:
Parameters:RL = 5kΩCL = 2.3nF
RH = 10kΩCH = 8nF
Results:wcL = 8.696e+4 rad/secwcH = 1.25e+4 rad/sec
Underwater Acoustic Communication
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Receiver Amplifier GainPSKAutomatic 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
AMMaximum Gain
Resulting Gain of lowest voltage @30m
ADC Level Division
AV=5.381
=13
(18.7Pa )(316uV / Pa )(13 )= .074 V
5
210=.005V
Underwater Acoustic Communication
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Noise Squelch
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Signal to Noise
NL10k=NL1k−17log(f
1000)
NL10k=70 dB−17log( 100001000
)dB
NL10k=53dB
SPLnoise=450uPa
SNR=SPLSig
SPLnoise
SNR= (35.5Pa )(450 uPa)
=98dB
Underwater Acoustic Communication
Rochester Institute of Technology 3710/29/13
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…)
Underwater Acoustic Communication
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Level ShiftingOnly necessary is hydrophone voltage varies between a positive and negative voltage
Underwater Acoustic Communication
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Interfacing with Modulation/Demodulation SchemesModulation:•AD9835 Direct Digital Synthesizer, Waveform Generator•Takes 16-bit commands, can store phases and frequencies•Outputs an analog signal based on selected phase and frequencyDemodulation:•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
Underwater Acoustic Communication
Rochester Institute of Technology 4010/29/13
Communication Hardware Diagram
Microcontroller
Demodulation Chip/Circuit
Modulation Chip
PC
Voltage/Current Amplifier
Common Mode Choke
Speaker
HydrophoneBandpass
FilterAutomatic
Gain Amplifier
Underwater Acoustic Communication
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System Power
Underwater Acoustic Communication
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Buck Converter Buck-Boost Converter
D=V 0
V S
D=V 0
(V S+ V 0)
L=(1−D )2 R2f
L=(1−D) R2f
C= D
(R (ΔV 0
V 0
) f )C= (1−D)
(8L(ΔV 0
V 0
) f 2)
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Design Specifications
Buck Specifications Buck-Boost Specifications
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Battery SelectionBattery Voltage:12 VoltsSystem Power: 15 WattsSystem Current: 1.25 AmpsBattery Lifetime > 1 HourBattery Energy > 1.25 Ah
Data Rate Analysis
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Underwater Acoustic Communication
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Underwater Acoustic Communication
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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
Underwater Acoustic Communication
Rochester Institute of Technology 4810/29/13
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)
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Thermal Analysis
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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
Underwater Acoustic Communication
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Sheet Metal Design•Uses Corrosive resistant sheet metal and rubber gaskets
•Metal bending and welding techniques
•Low cost of material
•Quick local fabrication vendor
•Interchangeable front and back panels for future integrations
•Watertight connectors on back
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Structural Analysis
Using stainless steel, ¼” thickSalt water density is 1035Pressure requirement is increased
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Final Tests Overview•Mounting in pool
• Base structures with cables for manipulating•Test data transfer rate•Test error correction
• Test with/without•Test range
• Test at 30 m distance•Test operating temperature range
• Hot tub for hot range, ice bath for cold•Test pressure resistance
• Lower box to 10 m depth
Underwater Acoustic Communication
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Cost Analysis
Estimate is under budgetLeaves room for any surprises later on in the project
Item Qty Cost/item Total CostRaspberry Pi 2 50 100Speakers 2 100 200Hydrophone 2 200 400Electronics 2 100 200Box 2 300 600Material & equipment tests 1 80 80
Total System 1580
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Material Testing (B117-11)
Samples are sprayed continuously with salt solution for 1 weekSamples are weighed before and after test5-6 Samples of 316 Stainless, 464 Naval Brass, ASC Plastic, and 6061 Aluminum
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Questions?