1

Wireless SensornetworksConcepts, Protocolls and Applications

PHYsical layer

(Wireless communication)

Goals of this chapter

• Get an understanding of the peculiarities of wireless communication

• “Wireless channel” as abstraction of these properties – e.g., bit error patterns

• Focus is on radio communication

• Impact of different factors on communication performance

• Frequency band, transmission power, modulation scheme, etc.

• Some brief remarks on transceiver design

• Here, differences between ad hoc and sensor networks mostly in the required performance

• Larger bandwidth/sophisticated modulation for higher data rate/range

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Wireless networks vs. fixed networks

• Higher loss-rates due to interference and other mechanisms in wireless transmission• emissions of, e.g., engines, lightning (only in lower frequency range up to

200 MHz)• mainly interference with other users in the higher frequency range

As a rule of thumb: BER (wired) 10-11 – 10-13 ; BER (wireless) 10-4 – 10-6

• Restrictive regulations for use of frequencies• frequencies have to be coordinated, useful frequencies are almost all

(statically) occupied• New approaches for „cognitive radio“ is in research and development

(overlay/ underlay approaches)

• Transmission rates typically lower than in wired networks• local >>100 Mbit/s, cellular currently up to 30 (HSPA+)• In the new generation of cellular networks the speed difference between

Local and regional will not play any more any role. E.G. LTE will support up to 100 Mb/s

• Wired fiber based communication > 10Gb/s

Wireless networks in comparison to fixed networks

• Higher delays, higher delay-jitter• connection setup time with GSM in the second range, several

hundred milliseconds for other wireless systems• Jitter and delay may impact the QoS (quality of service) but with LTE

low latency services that will be as good as in wired networks

• Lower security, simpler active attacking• radio interface accessible for everyone, base station can be

simulated, • Security is an important issue of research in wireless systems

• Always shared medium• The capacity of a network is divided between the participants. This

is a growing concern for the further development of wireless systems.

Early history of wireless communication

• Many people in history used light for communication

• heliographs, flags („semaphore“), ...

• 150 BC smoke signals for communication;(Polybius, Greece)

• 1794, optical telegraph, Claude Chappe

• Here electromagnetic waves areof special importance:

• 1831 Faraday demonstrates electromagnetic induction

• J. Maxwell (1831 - 1879): theory of electromagnetic fields, wave equations (1864)

• H. Hertz (1857 - 1894): demonstrateswith an experiment the wave characterof electrical transmission through space

History of wireless communication I

• 1896 Guglielmo Marconi• first demonstration of wireless

telegraphy (digital!, UWB puls-transmission)

• long wave transmission, hightransmission power necessary (> 200 kW)

• 1907 Commercial transatlantic connections• huge base stations (30*100 m high antennas)

• 1915 Wireless voice transmission New York - San Francisco

• 1920 Discovery of short waves by Marconi• reflection at the ionosphere (radio works arround the spheric-shape of the earth)

• smaller transmitter and receiver, possible due to the invention of the vacuum tube which allows to build amplifiers for signals (LNA Low Noise amplifier; PA power amplifier) (1906, Lee DeForest and Robert von Lieben)

• 1926 Train-phone on the line Hamburg – Berlin• wires parallel to the railroad track

History of wireless communication II

• 1928 many TV broadcast trials (across Atlantic, color TV, TV news)

• 1933 Frequency modulation (E. H. Armstrong)

• 1958 A-Netz in Germany

• analog, 160 MHz, connection setup only from the mobile station, no handover, 80 % coverage, 1971 11000 customers

• 1972 B-Netz in Germany

• analog, 160 MHz, connection setup from the fixed network too (but location of the mobile station has to be known)

• available also in A, NL and LUX, 1979 13000 customer in D

• 1979 NMT (Nordic Mobile Telecommunication) at 450 MHz (in Scandinavian countries)

• 1982 Start of GSM-specification

• goal: pan-European digital mobile phone system with roaming

• 1983 Start of the American AMPS (Advanced Mobile Phone System, analog)

• 1984 CT-1 (Cordless Telephony) standard (Europe) for home use

History of wireless communication III

• 1986 C-Netz in Germany• analog voice transmission, 450 MHz, hand-over possible, digital signaling, automatic location

of mobile device

• was in use until 2000, services: FAX, modem, X.25, e-mail, 98 % coverage

• 1988 first discussion of UMTS networks as a solution of a worldwide wireless communication system (later known as „IMT-2000“)

• 1991 Specification of DECT• Digital European Cordless Telephone (today: Digital Enhanced Cordless Telecommunications)

• 1880 – 1900 MHz, ~100 – 500 m range, 120 duplex channels, 1.2 Mbit/s data transmission, voice encryption, authentication, up to several 10000 user/km2, used in more than 50 countries

• 1992 Start of GSM commercial operation• in D as D1 and D2, fully digital, 900 MHz, 124 channels

• automatic location, hand-over, cellular

• roaming in Europe - now worldwide in more than 200 countries

• services: data with 9.6 kbit/s, FAX, voice, ...

History of wireless communication IV

• 1994 E-Netz in Germany

• GSM with 1800 MHz, smaller cells

• As Eplus in D (1997 98 % coverage of the population)

• 1996 HiperLAN (High Performance Radio Local Area Network) as part of BRAN (Broadband Radio Access Network)

• ETSI, standardization of type 1: 5.15 - 5.30 GHz, 23.5 Mbit/s

• recommendations for type 2 and 3 (both 5 GHz) and 4 (17 GHz) as wireless ATM-networks (up to 155 Mbit/s)

• 1997 Wireless LAN - IEEE802.11

• IEEE standard, 2.4 - 2.5 GHz and infrared, 2 Mbit/s

• already many (proprietary) products available at the beginning

• 1998 Specification of GSM successors

• for UMTS (Universal Mobile Telecommunication System) as European proposals for IMT-2000

• 1998 Iridium

• 66 satellites (+6 spare), 1.6 GHz to the mobile phone

History of wireless communication V

• 1999 Standardization of additional wireless LANs• IEEE standard 802.11b, 2.4 - 2.5 GHz, 11 Mbit/s

• Bluetooth for piconets, 2.4 Ghz, < 1 Mbit/s

• Decision about IMT-2000• Several “members” of a “family”: UMTS, cdma2000, DECT, …

• Start of WAP (Wireless Application Protocol) and i-mode• First step towards a unified Internet/mobile communication system

• Access to many services via the mobile phone

• 2000 GSM with higher data rates• HSCSD offers up to 57.6 kbit/s

• First GPRS trials with up to 50 kbit/s (packet oriented!)

• UMTS auctions/beauty contests• Hype followed by disillusionment (100 B DM paid in Germany for 6 licenses!)

• 2001 Start of 3G systems (Release-99)• CDMA 2000 in Korea, UMTS tests in Europe, FOMA (almost UMTS) in Japan

• 2003 3GPP took over further international standardization of cellular radio networks

• 2010 Roll out of first LTE systems in Germany

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Radio spectrum for communication

• Which part of the electromagnetic spectrum is used for communication

• Not all frequencies are equally suitable for all tasks – e.g., wall penetration, different atmospheric attenuation (oxygen resonances, …)

• 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 Mm

300 Hz

10 km

30 kHz

100 m

3 MHz

1 m

300 MHz

10 mm

30 GHz

100 m

3 THz

1 m

300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted

pair

© Jochen Schiller, FU Berlin

Frequencies for mobile communication

• VHF-/UHF-ranges for mobile radio

• simple, small antennas

• deterministic propagation characteristics, reliable connections

• SHF and higher for directed radio links, satellite communication

• small antenna, focusing

• large bandwidth available

• Wireless LANs use frequencies in UHF to SHF spectrum

• some systems planned up to EHF

• E.g. IHP working on systems for 100 Gb/s at 250 GHz

• limitations due to absorption by water-, oxygen- and other gas- molecules (resonance frequencies) (Application Resonance Spectroscopy

• weather dependent fading, signal loss caused by heavy rainfall etc.

• WLAN uses unlicensed spectrum in ISM-bands (Industrial, Scientific, Medical) in the 2.4 GHz and 5.2 to 5.8 GHz range)

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Frequency allocation

• Some frequencies are allocated to specific uses

• Cellular phones, analog television/radio broadcasting, DVB-T, radar, emergency services, radio astronomy, …

• Particularly interesting: ISM bands (“Industrial, scientific, medicine”) – license-free operation

• However, ISM bands are regulated (TX power, duty cycle, etc.

Some typical ISM bands

Frequency Comment

169 MHz Europe

433 – 464 MHz Europe

868 - 869 MHz Europe

900 – 928 MHz Americas

2,4 – 2,5 GHz WLAN/WPA

N

5,725 – 5,875 GHz WLAN

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Frequency allocation

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Modulation

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Transmitting data using radio waves

• Basics: Transmitter can send a radio wave, receiver can detect whether such a wave is present and also its parameters

• Parameters of a wave = sine function:

• Parameters: amplitude A(t), frequency f(t), phase (t)

• Manipulating these three parameters allows the sender to express data; receiver reconstructs data from signal

• Simplification: Receiver “sees” the same signal that the sender generated –not true, see later!

• Different representations of signals

• amplitude (amplitude domain)

• frequency spectrum (frequency domain)

• phase state diagram (amplitude M and phase in polar coordinates)

• Composed signals transferred into frequency domain using Fourier transformation

• Digital signals need

• infinite frequencies for perfect transmission

• modulation to a carrier frequency for transmission (analog signal!)

Signals

f [Hz]

A [V]

I= M cos

Q = M sin

A [V]

t[s]

Antennas: isotropic radiator

zy

x

z

y xideal

isotropic

radiator

• Radiation and reception of electromagnetic waves, coupling of wires to space for radio transmission

• Antennas are resonant structures thus they are limited to certain frequency ranges

• Isotropic radiator: equal radiation in all directions (three dimensional) – only a theoretical reference antenna

• Real antennas always have directive effects (vertically and/or horizontally)

• Radiation pattern: measurement of radiation around an antenna

• is used as reference for measuring of antennas (EIRP = Equivalent Isotropic Radiated Power)

Antennas: simple dipoles

• Real antennas are not isotropic radiators but, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole shape of antenna proportional to wavelength

• Example: Radiation pattern of a simple Hertzian dipole

• Gain: maximum power in the direction of the main lobe compared to the power of an isotropic radiator (with the same average power)

• Gain measure in dBi ( 10*log10Pmax/Pi) (there might be other reference Antennas like e.g. dipole etc.)

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simple

dipole

/4 /2

Metallic

Surface

Antennas: directed and sectorized

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

top view, 3 sector

x

z

top view, 6 sector

x

z

• Often used for microwave connections or base stations for mobile phones (e.g., radio coverage of a valley)

directed

antenna

sectorized

antenna

• Grouping of 2 or more antennas

• multi-element antenna arrays

• Antenna diversity and passive

• Active (e.g. MIMO)

• switched diversity, selection diversity

• receiver chooses antenna with largest output

• diversity combining

• combine output power to produce gain

• co-phasing needed to avoid cancellation

Antennas: diversity

+

/4/2/4

ground plane

/2

/2

+

/2

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Signal propagation ranges

distance

sender

transmission

detection

interference

• Transmission range

• communication possible

• low error rate

• Detection range

• detection of the signal possible

• no communication possible

• Interference range

• signal may not be detected

• signal adds to the background noise

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Signal propagation

• Propagation in free space always like light (straight line)

• Receiving power proportional to 1/d² (d = distance between sender and receiver)

• Receiving power additionally influenced by

• fading (frequency dependent; H2O resonance at 2.5 GHz; O2 Resonance at 60 GHz)

• shadowing

• reflection at large obstacles

• refraction depending on the density of a medium

• scattering at small obstacles

• diffraction at edges

reflection scattering diffractionshadowing refraction

• Signal can take many different paths between sender and receiver due to reflection, scattering, diffraction

• Time dispersion: signal is dispersed over time (delay spread)

interference with “neighbor” symbols, Inter Symbol Interference (ISI)

• The signal reaches a receiver directly and phase shifted distorted signal depending on the phases of the different parts

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Multipath propagation

signal at sender

signal at receiver

LOS pulsesmultipath

pulses

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Wireless signal strength in a multi-path environment

• Brighter color = stronger signal

• Obviously, simple (quadratic) free space attenuation formula is not sufficient to capture these effects

© Jochen Schiller, FU Berlin

• Multiplexing in 4 dimensions

• space (si)

• time (t)

• frequency (f)

• code (c)

• Goal

• multiple use of a shared medium

• Increase capacity

• Important: guard spaces needed in space, time, frequency and code!

Multiplexing

Space Multiplexing

• Separation of space into differentspatial regions

• The different channels can useany frequency at any time if thespatial separation is good

• Spatial guards have to beprovided to allow interferencefree operation

• Advantage:

• Works with any technology

• Simple to use

• Disadvantage:

• Not very flexible

• Expensive due to severalantennas

• Use in segmented cells forcellular radio systems

s2

s3

s1f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

SM

Frequency multiplex

• Separation of the whole spectrum into smaller frequency bands

• A channel gets a certain band of the spectrum for the whole time

• Advantages:

• no dynamic coordination necessary

• works also for analog signals

• Disadvantages:

• waste of bandwidth if the traffic is distributed unevenly

• inflexible

• guard spaces

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

k2 k3 k4 k5 k6k1

Time multiplex

• A channel gets the whole spectrum for a certain amount of time

• Advantages:

• only one carrier in themedium at any time

• throughput high even for many users

• Disadvantages:

• precise synchronization necessary

f

Time and frequency multiplex

• Combination of both methods

• A channel gets a certain frequency band for a certain amount of time

• Example: GSM

• Advantages:

• better protection against tapping

• protection against frequency selective interference

• higher data rates as compared tocode multiplex

• but: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1

Code multiplex

• Each channel has a unique code

• All channels use the same spectrum at the same time

• Advantages:• bandwidth efficient

• no coordination and synchronization necessary

• good protection against interference and tapping

• Disadvantages:• lower user data rates

• more complex signal regeneration

• Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

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c

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Transmitted signal <> received signal!

• Wireless transmission distorts any transmitted signal

• Received <> transmitted signal; results in uncertainty at receiver about which bit sequence originally caused the transmitted signal

• Abstraction: Wireless channel describes these distortion effects

• Sources of distortion (already discussed)

• Attenuation – energy is distributed to larger areas with increasing distance

• Reflection/refraction – bounce of a surface; enter material

• Diffraction – start “new wave” from a sharp edge

• Scattering – multiple reflections at rough surfaces

• Doppler fading – shift in frequencies (loss of center)

• Multi-path fading

• Assumptions:

• Free Space, vacuum + far-field (idealized conditions)

• lossless antenna (loss-coefficient η=1):

Ptx = Pr (r for radiated)

Pa = Prx (a for absorbed)

• Distance between antennas is d

• The far-field condition is assumed in a distance of > 10

TX Antenne RX AntennePtx

Pr Pa

Prx

d

Range Equation according to Friis I

Pr/Ptx = ηtx und Prx/Pa = ηrx

• Antenna gain G combines beam-characteristic & efficiency :

G (gain): G = η * D (D: directivity)

• The receive power Prx :

Prx = Ptx * (λ/4πd)² * Grx * Gtx

• In logarithmic form:

P|dBm = 10 * lg(P/1 mW)Lpath|dB= 20 * lg(4πd/ λ)

Prx|dBm = Ptx|dBm + Gtx|dBi + Grx|dBi – Lpath|dB

Range Equation according to Friis II

TX Antenne RX AntennePtx

Pr Pa

Prx

d

ηtx ηrx

Other path-loss models

• The Friis formula is based on a physical propagation model

• Today very much statistical propagation models are used.

• Statistical models recognize certain scenarios as additional attenuation

• The path loss is no longer calculated per path but as a statistical function

• In an room scenario the attenuation coefficient might be as high as 4

• In a town scenario even values of 5-6 are in use

Modulation

• Digital modulation• digital data is translated into an analog signal (baseband: without a specific carrier)• The properties of parts of the signal are changed to represent the information• We try to modulate as much information as possible on 1 Hz of bandwidth. The spectral

efficiency (b/s/Hz) is a very important property of the radio system• ASK, FSK, PSK and their combinations are main focus in this chapter• differences in spectral efficiency, power efficiency, robustness

• Analog modulation• shifts center frequency of baseband signal up to the radio carrier

• Motivation• smaller antennas (e.g., /4)• Frequency Multiplexing• medium characteristics

• Basic schemes• Amplitude Modulation (AM)• Frequency Modulation (FM)• Phase Modulation (PM)

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Modulation and demodulation

synchronization

decision

digital

dataanalog

demodulation

radio

carrier

analog

baseband

signal

101101001 radio receiver

digital

modulation

digital

data analog

modulation

radio

carrier

analog

baseband

signal

101101001 radio transmitter

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Modulation (keying!) examples

• Use data to modify the amplitude of a carrier frequency ! Amplitude Shift Keying

• Use data to modify the frequency of a carrier frequency ! FrequencyShift Keying

• Use data to modify the phase of a carrier frequency ! Phase Shift Keying

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Advanced Phase Shift Keying

• BPSK (Binary Phase Shift Keying):• bit value 0: sine wave

• bit value 1: inverted sine wave

• very simple PSK

• low spectral efficiency /1b/s/Hz

• robust, used e.g. in satellite systems

• QPSK (Quadrature Phase Shift Keying):• 2 bits coded as one symbol

• Spectral efficiency 2 b/s/Hz

• symbol determines shift of sine wave

• needs less bandwidth compared to BPSK

• more complex

• Often also transmission of relative, not absolute phase shift: DQPSK -Differential QPSK (IS-136, PHS)

11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t

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Receiver: Demodulation

• The receiver looks at the received wave form and matches it with the data bit that caused the transmitter to generate this wave form

• Necessary: one-to-one mapping between data and wave form

• Because of channel imperfections, this is at best possible for digital signals, but not for analog signals

• Problems caused by

• Carrier synchronization: frequency can vary between sender and receiver (drift, temperature changes, aging, …)

• Bit synchronization (actually: symbol synchronization): When does a symbol representing a certain bit start/end? (OOK)

• Frame synchronization: When does a packet start/end? (bit stuffing)

• Biggest problem: Received signal is not the (identical to) transmitted signal!

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Signal distortion – wireless channels

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Attenuation results in path loss

• Effect of attenuation: received signal strength is a function of the distance d between sender and transmitter

• Captured by Friis free-space equation

• Describes signal strength at distance d relative to some reference distance d0 < d for which strength is known

• d0 is far-field distance, depends on antenna technology

• d0 ~ 1m for WLAN; d0 ~1 km for GSM

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Suitability of different frequencies –Attenuation

• Attenuation depends on the used frequency

• Can result in a frequency-selective channel

• If bandwidth spans frequency ranges with different attenuation properties

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Friis free-space equation in logarithmic form

• Prcvd(d)= Ptx+Gt+Gr+PL in dB

• PL= 10*log10(/4*p*d)2 path loss in free space

• First Fresnel Zone considerations for antenna highs and reference distance

Link Budget Calculation

• Prcvd(d)= Ptx+Gt+Gr+PL

• PL= ( L0+ L1)

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• To take into account stronger attenuation than only caused by distance (e.g., walls, …), use a larger exponent > 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

• Equivalent to multiplying with a lognormal distributed r.v. in metric units ! lognormal fading

Generalizing the attenuation formula

From waves to bits

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Noise and interference

• So far: only a single transmitter assumed

• Only disturbance: self-interference of a signal with multi-path “copies” of itself

• In reality, two further disturbances

• Noise – due to effects in receiver electronics, depends on temperature

• Typical model: an additive Gaussian variable, mean 0, no correlation in time

• N0 = K*T K: Bolzmann Constant = 1.38*10-23 J/K

• Interference from third parties

• Co-channel interference: another sender uses the same spectrum

• Adjacent-channel interference: another sender uses some other part of the radio spectrum, but receiver filters are not good enough to fully suppress it

• Effect: Received signal is distorted by channel, corrupted by noise and interference

• What is the result on the received bits?

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Symbols and bit errors

• Extracting symbols out of a distorted/corrupted wave form is fraught with errors

• Depends essentially on strength of the received signal compared to the corruption

• Captured by signal to noise and interference ratio (SINR)

• Demodulating a bit depends on the energy per bit Eb in relation to the noise energy N0

Eb/N0 =SINR*1/R

• Bandwidth Efficiency describes efficiency pf modulation scheme• : hBW = R/W (measured in bits/s/Hz) with W Bandwidth/Hz and R bitrate in bits/s

Eb/N0 =SINR 1/ W*hBW

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Transceiver design

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Some transceiver design considerations

• Strive for good power efficiency at low transmission power

• Some amplifiers are optimized for efficiency at high output power

• To radiate 1 mW, typical designs need 30 - 100 mW to operate the transmitter (efficiency is very low; 1 – 10 % is a good guess)

• WSN nodes: 20 mW for 1mW emitted power (mica motes)

• Receiver can use as much or more power as transmitter at these power levels (that is true also for the idle mode)

! Sleep state is important

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Some transceiver design considerations

• Startup energy/time penalty can be high

• Examples take 0.5 ms and approximately 60 mW to wake up

• The power management has to be carefully chosen

• Exploit communication/computation tradeoffs

• Might payoff to invest in rather complicated coding/compression schemes

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Choice of modulation

• One exemplary design point: which modulation to use?

• Consider: required data rate, available symbol rate, implementation complexity, required BER, channel characteristics, …

• Tradeoffs: the faster one sends, the longer one can sleep

• Power consumption can depend on modulation scheme

• Tradeoffs: symbol rate (high?) versus data rate (low)

• Use m-ary transmission to get a transmission over with ASAP

• But: startup costs can easily void any time saving effects

• Adapt modulation choice to operation conditions

• Akin to dynamic voltage scaling, introduce Dynamic Modulation Scaling

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AFE Components

• Antenna:

• Gain and directivity; Bandwidth; Quality Factor

• Amplification stage

• PA (Power Amplifier)

• Linearity; Power Control; Efficiency

• LNA (Low Noise Amplifier)

• Noise Suppression; Bandwidth; Dynamic Range

• Down/Up conversion stage

• Local Oscillator

• Stability; Noise; Power consumption; Stabilization time

• Mixer

• Gain; Quality; Intermixing; Low-suppression; Noise

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Transceiver Building Blocks

Antenna

Interface

LNA

PA

LO

Down/Up conversion stage

Amplification stage

Receive/Transmit resonator

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Digital Components

• AD/DA Conversion

• Dynamic Range; Efficiency

• Modulation/Demodulation

• Modulation Form

• Single Carrier/ Multi Carrier

• Channel Coding

• Hamming Distance; Code selection; Interleaving

• Spreading

• Narrow band distortion suppression

• Synchronization

• Synchronization time for locking

• Synchronization Framing; Synchronization Efficiency

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

• Dealing with limited bandwidth in an energy-efficient manner is the main challenge

• MANET and WSN are pretty similar here

• Main differences are in required data rates and resulting transceiver complexities (higher bandwidth, spread spectrum techniques)

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… see you !

Thanks for your attention !

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