Wireless Sensor System Design

Wireless Sensor System Design

A Joint Course of the University of South Florida

and Tennessee Technological University

Spring 2002

Lecture 11 – Multipath / Course Wrap-up

Tennessee TechUNIVERSITY

USF EEL 4935/TTU ECE 4720SPECIAL TOPIC: WIRELESS SENSOR SYSTEMS


Course Introduction Analog and Digital Modulation Methods (1/11) Fundamentals of Antennas and Propagation (1/18) Signal Processing Techniques (1/25) Microwave Systems: Communications Hardware, Noise, Linearity (2/1) System Test, Evaluation and Documentation / Effective Presentation Styles (2/8) Preliminary Design Review (student presentations*) (2/15) Microwave Sensor Technology (2/22) @ TTU Modern Wireless Communication Systems (3/1) Microwave Proximity Sensors (3/8) Microelectromechanical Systems for Communications (RF - MEMS) (3/22) Internal CDR (4/5) Wireless Sensor System Research (Paul Flikkema from NAU) (4/12) Multipath / Course Wrap-up (4/19)

Weekly Lecture Topics

* On-site internal reviews/preparation will precede inter-university presentations.



Motivation for Today’s Talk

Source: University of Kansas' Information & Telecommunications Technology Center and Kansas Applied Remote Sensing Program

Free space propagation is not reality!

Note: Path loss is directionally dependent



Motivation for Today’s Lecture

Free space propagation is not (virtual) reality!

Dr. Sarabandi’s WAMI Forum presentation



Propagation Loss – Two Categories

Large scale path loss – predicts mean signal strength from transmitter to receiver (T-R) Distance, Reflection, Diffraction and Scattering

Small scale path loss – rapid changes in signal strength over a small travel distance or time interval Multipath Doppler



Multipath

The environment produces “echoes” The signal received gets spread out in time

From Fourier analysis: Spreading in time Reduction in Frequency Filtering of baseband signal (information)!

The environment changes due to Moving transmitter or receiver Dynamic environment



$1 Animation

T



$10 Animation

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$100 Animation

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$1000 Animation

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$10000 Animation

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$100000 Animation

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$1M Animation

Large scale effects

Small scale effects



So what happens?

Fading Flat or Frequency-

Selective Fast or Slow

p. 211 of Rappaport

Speed: 75 mphWavelength: 0.33 m



Why is this bad?

Signals corrupted by multipath are more susceptible to channel noise

Degradation in BER (digital) SNR (analog)

Bottom Line – Information quality is compromised



Multipath Effects on Time Domain Data

W/O multipath W/ multipath

I

Q



So what can be done?

Equalization

Diversity

Channel Coding



Equalization

Equalization “undoes” the multipath filtering effect Filtering will induce intersymbol interference (ISI) Equalization makes pulses look “rectangular” again

Two classes of algorithms Those requiring training sequence Those that don’t: Blind techniques

channel

Equalizer

time freq time time



Equalization with a Training Sequence

A known sequence will be sent at regular intervals

The equalizer will adapt to minimize the error between the known signal and recovered signal (LMSE optimization)

Advantage: minimizes computation Disadvantage: utilizes bandwidth resources



Equalization of Training Data

Known Desired Received

EqualizerLSMEAdaptive

Algorithm

New coefficients

Equalized outputequals known

I

Q

I

Q



Blind Equalizations

Equalizer continuously adjusts based on the statistics of the received signal

Advantage: no training sequence

Disadvantage: more computationally intensive

e.g., 16-QAM

I

Q



Blind Equalization Process

Equalizer

AdaptiveAlgorithm

Statistics of received data

Statistics of for 16-QAM

I

Q

I

Q



What is needed?

AGC

High-SpeedAdaptiveEqualizer

Flat Fading Frequency Selective

SlowFading

FastFading

Z-1

Z-1

Z-1

Z-1

1

2

0

3

4

Input

Output

Coefficients

Digital FIR Filter



Antenna Diversity

Idea: if multipath is a small scale random effect that is spatially dependent, then the fading experienced by two antennas spaced a short distance from each other will be uncorrelated

Methods Selection diversity – use the antenna with the

strongest signal Maximal ratio combining – use the antennas as an

array to achieve max power out



Maximal Ratio Combining

1

2

3

m

AdjustableWeightedSummer

Detector

AdaptiveControl

Output



Frequency Diversity

Idea: if multipath is a small scale random effect that is wavelength dependent, then the fading experienced at two frequencies spaced a short distance from each other will be uncorrelated

Buzz words 1:N protection switching Frequency Hopping Spread Spectrum (use: WLAN) Direct Sequence Spread Spectrum (use: CDMA) OFDM (divide bits to modulate many different carriers)



Time Diversity

Idea: if multipath is a small scale random effect that is time dependent, then the fading experienced by the signal at two different points in time will will be uncorrelated

Buzz words RAKE receiver



RAKE Receiver

AdjustableWeightedSummer

Integrator/Detector

AdaptiveControl

Output

Correlator 1

Correlator 2

Correlator 3

Correlator m



Channel Coding (Error Correction Codes)

Add redundancy (extra bits) in data so that information has better chance of being recovered (think parity on steroids)

Costs Complexity Bandwidth

Buzz word Turbo coding (aka Parallel Concatenated

Convolution Codes)



Summary

Multipath effects band limit systems Will definitely be a driving factor in higher data

rate 3G systems

These effects can be counteracted through processing Requirements for DSP in 3G systems will thus be

more demanding

Wireless Sensor Systems Design Course Review

A Joint Course of the

University of South Florida

and

Tennessee Technological University

Spring 2002



Course Objectives

As advertised: USF/TTU - This course satisfies the Senior Design Project

Requirement (3 Credit Hours) Objectives

Hands-on design experience Coverage of emerging wireless and sensor system technologies Interdisciplinary, collaborative project development (USF and

TTU) Putting the E into Experimental



What we hoped you learned

Problems/advantages of a distributed design process Ability to take an idea and make it happen

Design Analysis Implementation Scheduling

Ability to be objective in assessing performance Yours and others

How system level performance can be dependent on subsystem characteristics

Working on a team in an interdisciplinary environment in a relevant area



Hands-On (BBD group)

Sensor Conditioner

Future USF grad student?



Hands On (TX and Power Group)

Sensmitter Power System FM transmitter



Hands On – Literally! (RX group)

PLL

Bit detector



Learned Something About Mixers



Data Visualization (DSP Group)



USF EEL 4935/TTU ECE 499SPECIAL TOPICS: WIRELESS SENSOR SYSTEMS

2.45 GHz Antennas – Rob Harris

Circularly Polarized Antenna (Transmitter)

• T-junction with 90º phase shift

• λ / 4 transformers

• -16.67 dB @ 2.45 GHz

• ƒc~ 2 % design frequency

• Corporate-fed rectangular array.

• .65λ spacing

• -7.56 dB @ 2.45 GHz

• ƒc~ 1% design frequency

4 X 2 Linearly Polarized Antenna Array (Receiver)

55 X 55 mm

280 X 135 mmS11 (reflection coefficient) on VNA

S11 (reflection coefficient) on VNA



Frequency Shift Key (FSK) Bit Detector by Leonard Guerra

LP FilterAmplifier

HP FilterF1 = 220 MHz

LP FilterF2 = 180 MHz

FSK Signal

EnvelopeDetector

EnvelopeDetector

Op-AmpComparator

+

T+Vcc

-Vcc

-

150 200 250 300 350

-40

-30

-20

-10

0

Frequency (MHz)

Atte

nuat

ion

(dB

)

RR1R=100 kOhm

CC1C=680 pF

LL1L=420 nH

DiodeDIODE1

Simulated vs. Actual

300 400 500

0

50

100

150

200

250

300

Time (nsec)

Vol

tage

(mV

)

150 200 250 300 350

-10

-8

-6

-4

-2

0

2

Frequency (MHz)

Atte

nuat

ion

(dB)




Sensor/Microwave Topics





Communication Systems/Signal Processing Topics


Wireless Sensor System Design

Documents

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