Simulation of Communication for Power constrained Embedded Systems

By Samir GovilkarUnder the guidance of Dr. Alex Dean

The RaPTEX Project Rapid Prototyping Tool for Embedded

Communication Systems Aid development of embedded

communication systems by non-specialists

Targeted at study of crabs using acoustic biotelemetry and health monitoring of bridges using wireless sensor networks

Studying Crabs using acoustic biotelemetry

Blue crabs, Callinectes sapidus, are robust enough to carry a transmitter

Allows study of physiological and biological parameters

Power efficiency required because of weight restrictions on the battery

Ideal evaluation platform for RaPTEX

Underwater communication Electromagnetic waves cannot be

used because of a conductive medium and high scattering

Acoustic waves provide a good solution Lesser dissipation Lower scattering Communication over hundreds of

kilometres possible

A simulation environment Testing of underwater communication

systems requires frequent trips to a water body

Simulation environment to cut down on the number of such trips by providing a good estimation to the actual conditions

Provide RaPTEX with performance estimation data

Propagation Losses Spreading Losses

Geometrical divergence loss Effect of the Law of Conservation of Energy Dependent on range

Absorption Losses Viscosity of pure water Molecular relaxation of Magnesium Sulphate and

Boric Acid Dependent on temperature, depth and

frequency of the acoustic wave

Multiple Paths Multiple paths are followed by the acoustic wave from Tx to Rx

Reflections from air-water boundary Reflections from the water body bed

Gives rise to multipath fading Echoes Interference patterns

The delayed paths have lesser power than the LOS component

Transmitter

Receiver

Air

Water

Ground

Modeling Multiple Paths Multipath fading is simulated using a

tapped delay line channel model The first tap is the LOS component The other taps have a gain given by a Rice

processdT’ dT’ dT’

+

XX Xh(0,t)dT’ h(dT’,t)dT’ h(dT’,t)dT’ …

x(t)

y(t)

Ambient Noise

Surface Agitation Noise caused by wind Bursting of bubbles of air at the air-water

boundary Dependent on wind speed and frequency

of the acoustic wave Thermal Noise caused by random

motion of molecules in water Dependent on the frequency

Intermittent Noise Snapping Shrimp cause noise by the

snapping of their claws No mathematical model Model was built using observed data Dependent on frequency

Rain Noise caused by impact of rain drops on surface of water Dependent on rate of rainfall and wind

speed

Sampling rate conversion Enables use of different sources of data For this thesis, two sources are the

simulator and data from the field data capture unit

L DLPF LPF

π/L π/D

x(n) y(n)

L DLPF

π/max(D,L)

x(n) y(n)

Related Work Avrora – AVR Simulator

Cycle accurate simulator for AVR microcontrollers

Highly extensible Relatively fast compared to other AVR

simulators IT++ - Signal Processing Library

Multipath fading channel classes Channel profiles

System Block Diagram

Embedded System

Simulator (ESS)

Water Channel

Simulator (WCS)

Receiver Simulator (RS)

Field Data

Visualization Module (VM)

Embedded System Simulator (ESS)

Based on the Avrora simulator Platform consisting of AVR

microcontroller, DAC and Ultrasonic Transducer

Generates and transmits acoustic signal

Works as a server, to which other programs can connect to, for obtaining data

ESS Block Diagram

Avrora AVR simulator 8 – bit DAC Ultrasonic Transducer

Input is a program in assembly or the output of the avr-objdump facility

Output is streamed over a TCP connection as pairs of data and timing information

Water Channel Simulator (WCS) Attempts to simulate the effects of

propagation losses, noise and multipath fading.

The carrier frequencies are selectively attenuated according to the appropriate noise models

Noise is filtered and added to the carrier frequency components

Multipath fading simulation is done using complex numbers

WCS Block Diagram

Propagation Losses simulator

Noise simulatorMultipath Fading

Channel simulator

The input to the WCS is from the ESS via a TCP connection or from a file

The output is to standard output which can be redirected to a file

The WCS can record data received over the TCP connection for later playback

Receiver Simulator Consists of the Sampling Rate Converter,

Receiver Filter array and the demodulator array

The sampling rate converter will resample the input file to the required sampling frequency

The receiver filters are 6th order elliptic IIR filters with a 2 kHz bandwidth centered around the carrier frequencies

The default demodulation scheme is Amplitude Shift Keying (ASK)

RS Block Diagram

Sampling Rate Convertor

BW = 2 kHzfc = 60 kHz

BW = 2 kHzfc = 65 kHz

BW = 2 kHzfc = 70 kHz

BW = 2 kHzfc = 75 kHz

BW = 2 kHzfc = 80 kHz

fc = 60 kHz

fc = 65 kHz

fc = 70 kHz

fc = 75 kHz

fc = 80 kHz

Receiver Filter Array Demodulator Array

Visualization Module Used to display the RS output waveforms and the

demodulated data Can be launched from the RS via a command line

switch Can be launched independently and file can be

loaded using the GUI

VM Graph Window This window

displays the plots and the corresponding demodulated data

Amplitude Shift Keying (ASK) Simple modulation scheme

Uses amplitude of the carrier wave to encode the binary data

Special case is On-Off Keying (OOK) Uses presence or absence of the carrier wave to

signify a binary ‘1’ and binary ‘0’ respectively. Highly susceptible to noise Simplicity allows for easier debugging of

the system

Implementation Transmission of carrier wave

Uses a timer interrupt based routine in assembly to ensure operation at 5 MHz sampling rate

Profile settings Wind Speed Rainfall Rate Temperature Salinity Depth Range

Multipath profiles Sample underwater

multipath profiles to be used by the tapped delay line model

Underwater 1

Taps 1 2 3 4

Delay(ms) 0 2 4 6

Power (dB) 0 -20 -30 -40

Underwater 2

Taps 1 2 3 4

Delay(ms) 0 2 4 6

Power (dB) 0 -20 -30 -40

Simulation Speed Comparison

0

10

20

30

40

50

60

70

80

90

Live(

Symbo

ls O

nly)

Recor

ded(

Symbo

ls O

nly)

Live(

Full D

ata)

Recor

ded

(Full

Dat

a)

Tim

e (

se

co

nd

s)

Underwater1

Underwater2

Underwater3

Results

Clear advantage observed in using ‘Recorded’ mode for the WCS over the ‘Live’ mode

Correlation observed as expected between the channel profiles and the simulation speeds, based on their computational complexity.

Waveforms and Power Spectra

Observations

Aim of thesis was to provide a simulation solution for underwater acoustic communication by embedded systems

Effect of various factors were explored

Models based on recent research were used to simulate the system

Future Work Integration with RaPTEX needs to be performed in

order to use this system efficiently. Water body profiles need to be built up by performing

measurements of the relevant parameters for the target water bodies

The Visualization Module can be improved to include more information about the received signal, based on the modulation scheme used.

Support for multiple modulation schemes can be added to the receiver, in order to evaluate their pros and cons.

Support for a network of ESS platforms simultaneuously talking to a single WCS.

Thank You