Greg Davis Chris Johnson Scott Hambleton Jon Holton Chris Monfredo 10/29/13Rochester Institute of Technology1 P14251 Underwater Acoustic Communication

10/29/13Rochester Institute of Technology2 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 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

Underwater Acoustic Communication Rochester Institute of Technology410/29/13 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 Technology510/29/13 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)

Underwater Acoustic Communication Rochester Institute of Technology610/29/13 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 Technology710/29/13 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 Technology810/29/13 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 Technology910/29/13 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 Technology1010/29/13 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 Technology1110/29/13

Underwater Acoustic Communication Rochester Institute of Technology1210/29/13 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 Technology1310/29/13 Software Architecture

Underwater Acoustic Communication Rochester Institute of Technology1410/29/13 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 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 1.By viewing the phase itself as conveying info 2.By viewing a change of phase as conveying info Underwater Acoustic Communication Rochester Institute of Technology1510/29/13

Underwater Acoustic Communication Rochester Institute of Technology1610/29/13 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(2πf c t) 01 = A*sin(2πf c t) 10 = -A*cos(2πf c t) 11 = A*cos(2πf c t)  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 Technology1710/29/13 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 = log 2 (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 Technology1810/29/13 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 Technology1910/29/13 Demodulation: 2 Schemes AMP BPF sin(2πf c t) cos(2πf c t) ADC To MC s i (t)

Underwater Acoustic Communication Rochester Institute of Technology2010/29/13 Mixing:

Underwater Acoustic Communication Rochester Institute of Technology2110/29/13  This modulation scheme will take in the signal, s i (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 Technology2210/29/13  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 s i (t) ADC To MC

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Underwater Acoustic Communication 10/29/13Rochester Institute of Technology24 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

Underwater Acoustic Communication Rochester Institute of Technology2510/29/13 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 other inductor, resist changes to current  Therefore, alternating currents at higher frequencies are resisted much more than current changes at low frequencies  This is to say that the chokes impedance increases with freq.

Underwater Acoustic Communication Rochester Institute of Technology2610/29/13 Specifying a CMC:  The cutoff frequency of the CMC can be derived to be f c = 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 f c = 50 kHz (L) = R/(4π*50k)

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology27 Signal Power Transformations Electrical Power to Acoustic Power

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology28 Signal Power Transformations Acoustic Power to Transmitting Sound Pressure Level (SPL)

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology29 Signal Power Transformations Transmission Losses 1. Spreading 2. Absorption

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology30 Signal Power Transformations Transmitting to Receiving SPL

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology31 Signal Power Transformations Receiving SPL to Hydrophone Voltage

Underwater Acoustic Communication Rochester Institute of Technology3210/29/13 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 f c =1/(2π*RC) f cLP = 13kHz= 1/(2π*RC) RC = 1/(2π*13k) f cHP = 2kHz = 1/(2π*RC) RC = 1/(2π*2k)

Underwater Acoustic Communication Rochester Institute of Technology3310/29/13 Bandpass Filter Simulation: Parameters: R L = 5kΩ C L = 2.3nF R H = 10kΩ C H = 8nF Results: w cL = 8.696e+4 rad/sec w cH = 1.25e+4 rad/sec

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology34 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 AM Maximum Gain Resulting Gain of lowest ADC Level Division

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology35 Noise Squelch

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology36 Signal to Noise

Underwater Acoustic Communication Rochester Institute of Technology3710/29/13 Symbol Error Rate P s = 2Q[(E s /N o ) (1/2) ]-{Q[(E s /N o ) (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  E s /N o = signal to noise ration of each symbol (in dB) P s = 2Q[(98) (1/2) ]-{Q[(98) (1/2) ]} 2 = e-23 (That’s pretty low…)

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology38 Level Shifting Only necessary is hydrophone voltage varies between a positive and negative voltage

Underwater Acoustic Communication Rochester Institute of Technology3910/29/13 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

Underwater Acoustic Communication Rochester Institute of Technology4010/29/13 Communication Hardware Diagram

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology41 System Power

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology42 Buck ConverterBuck-Boost Converter

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology43 Design Specifications Buck SpecificationsBuck-Boost Specifications

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology44 Battery Selection Battery Voltage:12 Volts System Power: 15 Watts System Current: 1.25 Amps Battery Lifetime > 1 Hour Battery Energy > 1.25 Ah

Data Rate Analysis Underwater Acoustic Communication Rochester Institute of Technology4510/29/13

Underwater Acoustic Communication Rochester Institute of Technology4610/29/13

Underwater Acoustic Communication Rochester Institute of Technology4710/29/13 Data Rate Analysis (Compression/Decompression) FLZP compressor was chosen for reference analysis (selected a compression algorithm with a low rating) Can compress at kbps and decompress at kbps on a 2.9 GHz processor (2:1 ratio) RPi is roughly 24% as fast. Slowdown translates to kbps compression and kbps 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 Technology4810/29/13 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)

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology49 Thermal Analysis

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology50 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 10/29/13Rochester Institute of Technology51 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

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology52 Structural Analysis Using stainless steel, ¼” thick Salt water density is 1035 Pressure requirement is increased

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology53 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 10/29/13Rochester Institute of Technology54 Cost Analysis  Estimate is under budget  Leaves room for any surprises later on in the project

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology55 Material Testing (B117-11)  Samples are sprayed continuously with salt solution for 1 week  Samples are weighed before and after test  5-6 Samples of 316 Stainless, 464 Naval Brass, ASC Plastic, and 6061 Aluminum

Underwater Acoustic Communication 10/29/13Rochester Institute of Technology56 Questions?