Architectures and Applications for Wireless Sensor

Networks (01204525)

Physical Layer

Chaiporn Jaikaeochaiporn.j@ku.ac.th

Department of Computer EngineeringKasetsart University

Materials taken from lecture slides by Karl and Willig

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Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

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RF Spectrum

VLF = Very Low Frequency UHF = Ultra High Frequency LF = Low Frequency SHF = Super High Frequency MF = Medium Frequency EHF = Extra High Frequency HF = High Frequency UV = Ultraviolet Light VHF = Very High Frequency

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 m3 THz

1 m300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair

© Jochen Schiller, FU Berlin

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Frequency Allocation Some frequencies

are allocated to specific uses E.g., Cellular

phones, analog radio broadcasting

ISM bands Industrial,

Scientific, Medical License-free

operation

Some typical ISM bands

Frequency Comment

13.553 - 13.567 MHz26.957 – 27.283 MHz40.66 – 40.70 MHz

433 – 464 MHz Europe

900 – 928 MHz Americas

2.4 – 2.5 GHz WLAN/WPAN

5.725 – 5.875 GHz WLAN

24 – 24.25 GHz

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Example: US Freq Allocation

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Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

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Signals General form of signal

Parameters A(t) – amplitude f(t) – frequency (t) – phase

When the above parameters are all constant, the signal becomes a basic sine wave

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Modulations Process of encoding data into signal

By changing the three parameters Different ways of setting parameters

Set the parameter to an arbitrary value analog modulation

Choose values from a finite set of legal values digital modulation (keying)

Resulting signal requires a certain bandwidth to be transmitted Centered around frequency of the basic sine

wave

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Modulation/Keying Examples

© Tanenbaum, Computer Networks

Amplitude Shift Keying (ASK)

Frequency Shift Keying (FSK)

Phase Shift Keying (PSK)

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QAM QAM – Quadrature Amplitude

Modulation A combination of ASK and PSK

© Forouzan, Data communications and Networking

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Receiver: Demodulation Receiver matches signal with

corresponding data bits Problems

Carrier synchronization: frequency may vary

Bit (symbol) boundary Frame boundary Received signal is not the transmitted

signal!

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Dealing with Synchronization E.g., Mica motes

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Overview Frequency bands Modulation Signal distortion – wireless

channels From waves to bits

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Signal Distortion Wireless transmission distorts

transmitted signal Wireless channel abstract model describes

these distortion effects Sources of distortion

Attenuation Reflection/refraction Diffraction Scattering Doppler fading

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Suitable Frequency Attenuation depends

on the used frequency Can result in a

frequency-selective channel with large enough

bandwidth span

© http://w

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.itnu.de/radargrundlagen/grundlagen/gl24-de.html

© http://141.84.50.121/iggf/Multimedia/Klimatologie/physik_arbeit.htm

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Path Loss Friis free-space equation:

Other than free-space

Ptx – Transmission PowerGt – Tx antenna gainGr – Rx antenna gainL 1 – Loss in circuitry2 6 – Path-loss exponent

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Non-Line-Of-Sight (NLOS) Paths Different paths have

different propagation times Results in delay spread

of the wireless channel Closely related to

frequency-selective fading properties of the channel

With movement: fast fading

LOS path

NLOS path

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Signal Strength in Multi-Path Environment Brighter color = stronger

signal Simple free-space

attenuation formula is not sufficient

© Jochen Schiller, FU Berlin

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> 2 is the path-loss exponent

Rewrite in logarithmic form (in dB):

Take obstacles into account by a random variation Add a Gaussian random variable with 0 mean,

variance 2 to dB representation

General Path-Loss Formula

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Overview Frequency bands Modulation Signal distortion – wireless channels From waves to bits

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Noise and interference Noise

Due to effects in receiver electronics Depends on temperature

Interference from third parties Co-channel interference Adjacent-channel interference

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Symbols and Bit Errors Error ratio depends on signal strength

compared to noise Captured by signal to noise and

interference ratio (SINR)

SINR allows to compute bit error rate (BER) Depends on modulation and symbol rate E.g., formula for BPSK

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SINR vs. BER Example

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SINR vs. BER Example

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Summary Wireless radio communication introduces

many uncertainties and vagaries into a communication system

Handling the unavoidable errors will be a major challenge for the communication protocols