Mobile Computing Chapter 2: Wireless Transmission

The Graduate School of Information Technology and TelecommunicatThe Graduate School of Information Technology and Telecommunications, INHA Universityions, INHA Universityhttp://http://multinet.inha.ac.krmultinet.inha.ac.kr

Multimedia Network Lab.Multimedia Network Lab.

Mobile Computing

Chapter 2: Wireless Transmission

Prof. Sang-Jo Yoo

2The Graduate School of Information Technology and TelecommunicatThe Graduate School of Information Technology and Telecommunications, INHA Universityions, INHA University

http://multinet.inha.ac.kr Multimedia Network LabMultimedia Network Lab..

ContentsFrequencies for communications

Signals

Antennars

Signal propagation

Multiplexing

Modulation

Spread spectrum technologies

Cell structure



1. Frequencies for communication

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

Frequency and wave length:λ = c/f wave length λ, speed of light c ≅ 3x108m/s, frequency f

1 Mm300 Hz

10 km30 kHz

100 m3 MHz

1 m300 MHz

10 mm30 GHz

100 µm3 THz

1 µm300 THz

visible lightVLF LF MF HF VHF UHF SHF EHF infrared UV

optical transmissioncoax cabletwisted pair



Frequencies for communicationLF (Low Frequency)

Penetrate water and can follow the earth’s surface: used by submarines

MF & HF (Medium and High Frequency)Typical for transmission of radio stations AM: 520kHz – 1065.5kHzSW (short wave): 5.9MHz – 26.1MHz (amateur radio transmission)FM: 87.5MHz - 108MHz

VHF & UHFAnalog TV: 174 – 230 MHz, 470 – 790 MHzDAB: 223 – 230MHz, 1452 – 1472 MHzDigital TV: 470 – 862 MHzAnalog mobile phone: 450 – 465MHzDigital GSM: 890 – 960MHz, 1710 – 1880MHz3G UMTS: 1900 – 1980MHz, 2020-2025MHz, 2110-2190MHz



Frequencies for communication



Frequencies for mobile communicationVHF-/UHF-ranges for mobile radio

simple, small antenna for carsdeterministic propagation characteristics, reliable connections

SHF and higher for directed radio links, satellite communicationsmall antenna, focusinglarge bandwidth available

Wireless LANs use frequencies in UHF to SHF spectrumsome systems planned up to EHFlimitations due to absorption by water and oxygen molecules (resonance frequencies)

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



Frequencies and regulationsITU-R holds auctions for new frequencies, manages frequency bands worldwide (WRC, World Radio Conferences) Europe US A Japan

Cellu lar Phones

G SM 450-457, 479-486/460-467,489-496, 890-915/935-960, 1710-1785/1805-1880 UM TS (FD D) 1920-1980, 2110-2190 UM TS (TD D) 1900-1920, 2020-2025

AM PS , TDM A , CD M A 824-849, 869-894 TD M A , C DM A , G SM 1850-1910, 1930-1990

PDC 810-826, 940-956, 1429-1465, 1477-1513

Cordless Phones

CT1+ 885-887, 930-932 CT2 864-868 DECT 1880-1900

P AC S 1850-1910, 1930-1990 P AC S-U B 1910-1930

PHS 1895-1918 JCT 254-380

W ireless LAN s

IEEE 802.11 2400-2483 HIPER LAN 2 5150-5350, 5470-5725

902-928 IEEE 802.11 2400-2483 5150-5350, 5725-5825

IEEE 802.11 2471-2497 5150-5250

O thers RF-C ontrol 27, 128, 418, 433, 868

RF-Contro l 315, 915

RF-Contro l 426, 868



2. Signals physical representation of data

function of time and location

signal parameters: parameters representing the value of data

classificationcontinuous time/discrete timecontinuous values/discrete valuesanalog signal = continuous time and continuous valuesdigital signal = discrete time and discrete values

signal parameters of periodic signals: period T, frequency f=1/T, amplitude A, phase shift ϕ

sine wave as special periodic signal for a carrier:

s(t) = At sin(2 π ft t + ϕt)



Fourier representation of periodic signals

)2cos()2sin(2

1)(

11

nftbnftactgn

nn

n ππ ∑∑∞

=

∞

=

++=

1

0

1

0

t t

ideal periodic signal real composition(based on harmonics)

It is possible to construct every periodic signal g by using only sine and cosine functions.

Periodic signal - analog or digital signal pattern that repeats over times(t +T ) = s(t )



Different representations of signals amplitude (amplitude domain)frequency spectrum (frequency domain)phase state diagram (amplitude M and phase ϕ in polar coordinates)

Composed signals transferred into frequency domain using Fouriertransformation

Digital signals needinfinite frequencies for perfect transmission (in reality, limited) modulation with a carrier frequency for transmission (analog signal!)

Signals

f [Hz]

A [V]

ϕ

I= M cos ϕ

Q = M sin ϕ

ϕ

A [V]

t[s]



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

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

3. Antennas: isotropic radiator

zy

x

z

y x idealisotropicradiator



Antennas: simple dipolesReal antennas are not isotropic radiators but, e.g., dipoles with lengths λ/4 on car roofs or λ/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)

side view (xy-plane)

x

y

side view (yz-plane)

z

y

top view (xz-plane)

x

z

simpledipole

λ/4 λ/2



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 mobilephones (e.g., radio coverage of a valley)

directedantenna

sectorizedantenna: typically applied incellular systems



Antennas: diversityGrouping of 2 or more antennas

multi-element antenna arrays

Antenna diversityswitched diversity, selection diversity

receiver chooses antenna with largest outputdiversity combining

combine output power to produce gaincophasing needed to avoid cancellation

+

λ/4λ/2λ/4

ground plane

λ/2λ/2

+

λ/2



4. Signal propagation ranges

distance

sender

transmission

detection

interference

Transmission rangecommunication possiblelow error rate

Detection rangedetection of the signal possibleno communication possible

Interference rangesignal may not be detected signal adds to the background noise cell



Signal propagationPropagation in free space always like light (straight line)

Straight line between a sender and a receiver: line-of-sight (LOS)

Free space loss: receiving power proportional to 1/d²(d = distance between sender and receiver) : inverse square law

Path loss and attenuationThe atmosphere heavily influences transmission over long distancesAir, rain, snow, fog, dust particles, smog, etc..

Rain can absorb much of the radiated energy.

Radio waves can also penetrate objects.The lower the frequency, the better the penetration

η

κ ⎟⎠⎞

⎜⎝⎛⋅⋅=d

PP tr

1



Signal propagation

Path loss formula

( )( )

spacefreed

GG

P

PdPL rt

r

t ,4

log10log1022

2

πλ

−==



Signal propagationThree fundamental propagation behaviors

Ground wave (< 2MHz)Follow the earth’s surface and propagate long distancesSubmarine communication or AM radio

Sky wave (2-30 MHz)Reflected at the ionosphere, traveling around the world.International broadcasts and amateur radio

Line-of-sight (> 30MHz)The emitted waves follow a straight line of sight.Mobile phone systems, satellite systems, cordless telephones, etc.



Signal propagationReceiving power additionally influenced by

fading (frequency dependent)shadowingreflection at large obstacles

Objects can absorb some of the signal’s power.Helps transmitting signals as soon as no LOS exists.

refraction depending on the density of a mediumscattering at small obstacles

If the size of an obstacle is in the order of the wavelength or less.diffraction at edges

It is very difficult to predict the precise strength of signals.

reflection scattering diffractionshadowing refraction



Real world example



Signal can take many different paths between sender and receiverdue to reflection, scattering, diffraction

Lead to the most severe radio impairment: multi-path propagation

Signals arrive at the receiver at different times: delay spreadTypical values for delay spread: 3 us – 12 usGSM can tolerate up to 16 us (5km path difference): equalizer

Multipath propagation

signal at sendersignal at receiver

LOS pulsesmultipathpulses



Effects of the delay spread on the signals

Time dispersion: signal is dispersed over timeinterference with “neighbor” symbols, Inter Symbol Interference (ISI)The higher the symbol rate to be transmitted, the worse the effects of ISI will be.ISI limits the bandwidth of a radio channel.

Compensation at the receiverSender first transmit a training sequence (known by the receiver).Receiver knows the delay pattern, and programs an equalizer.

The signal reaches a receiver directly and phase shifteddistorted signal depending on the phases of the different parts

Multipath propagation



Effects of mobilityThe situation (delay spread) is even worse if receiver/sender moves.

Channel characteristics change over time and location signal paths changedifferent delay variations of different signal partsdifferent phases of signal partsquick changes in the power received (short term fading)

Additional changes indistance to senderobstacles further awayslow changes in the average power received (long term fading)

short term fading

long termfading

t

power



Multiplexing in 4 dimensionsspace (si)time (t)frequency (f)code (c)

Goal: multiple use of a shared mediumHow several users can share a medium with minimum or no interference.with maximum medium utilization.

5. Multiplexing



The channel k1, k2, k3 can be mapped onto the three spaces ‘s1 to s3’ which clearly separate channels and prevent the interference ranges from overlapping.

Guard space is needed.

space division multiplexingused with TDM, SDM, and CDM

Space division multiplexing

s2

s3

s1f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

Interference range



Frequency division multiplexingSeparation 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 necessarybetween sender and receiverworks also for analog signals

Disadvantages:waste of bandwidth if the traffic is distributed unevenlyinflexibleguard spaces

to avoid frequency band overlapping (adjacent channel interference)

k2 k3 k4 k5 k6k1

f

t

c



f

t

c

k2 k3 k4 k5 k6k1

Time division multiplexingA channel gets the whole spectrum for a certain amount of time

Co-channel interferenceIf two transmissions which use the same frequency spectrum overlap in time.In cellular, the interference between signals from co-channel cells.

Advantages:only one carrier in themedium at any timethroughput high even for many users

Disadvantages:precise synchronization necessary

to avoid co-channel interference



f

Time and frequency multiplexCombination of both methods

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

Example: GSM

Advantages:better protection against tappingprotection against frequency selective interferencehigher data rates compared tocode multiplex

but: precise coordinationrequired

t

c

k2 k3 k4 k5 k6k1



Code division multiplexingEach channel has a unique code (language)

All channels use the same spectrum at the same time

Advantages:bandwidth efficient (code space is huge)no coordination and synchronization necessarygood protection against interference and tapping

Disadvantages:more complex signal regenerationPrecise power control is required.

Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c



6. ModulationDigital modulation

digital data is translated into an analog signal (baseband)ASK, FSK, PSK - main focus in this chapterdifferences in spectral efficiency, power efficiency, robustness

Analog modulationshifts center frequency of baseband signal up to the radio carrier

Motivation of analog modulationsmaller antennas (e.g., λ/4)frequency division multiplexingmedium characteristics

path loss, penetration , reflection, scattering, diffraction depend on the wavelength of the signal

Basic schemesAmplitude Modulation (AM)Frequency Modulation (FM)Phase Modulation (PM)



Modulation and demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter



Digital modulationModulation of digital signals known as Shift Keying

Amplitude Shift Keying (ASK):very simplelow bandwidth requirementsvery susceptible to interference

Frequency Shift Keying (FSK):needs larger bandwidthReceiver uses bandpass filters

and comparator (to compare the signal levels of the filter output).less susceptible to errors

Phase Shift Keying (PSK):Receiver must synchronize in frequency and phase with the transmitter with phase local loop (PLL).more complexrobust against interference

1 0 1

t

1 0 1

t

1 0 1

t



Advanced Frequency Shift KeyingSpecial pre-computation avoids sudden phase shifts

Sudden changes in phase cause high frequencies undesired side-effect

MSK (Minimum Shift Keying)bit separated into even and odd bits, the duration of each bit is doubled depending on the bit values (even, odd) the higher or lower frequency, original or inverted is chosenthe frequency of one carrier is twice the frequency of the otherequivalent to offset QPSKeven higher bandwidth efficiency using a Gaussian low-pass filter

GMSK (Gaussian MSK), used in GSM



Example of MSK

data

even bits

odd bits

1 1 1 1 000

t

low frequency

highfrequency

MSKsignal

bit

even 0 1 0 1

odd 0 0 1 1

signal h n n hvalue - - + +

h: high frequencyn: low frequency+: original signal-: inverted signal

No phase shifts!



Advanced Phase Shift Keying

BPSK (Binary Phase Shift Keying):bit value 0: sine wavebit value 1: inverted sine wavevery simple PSKlow spectral efficiencyrobust, used e.g. in satellite systems

QPSK (Quadrature Phase Shift Keying):2 bits coded as one symbolsymbol determines shift of sine wavefrom the reference signalneeds less bandwidth compared to BPSKmore complex

Incoming signal has to be compared with the reference signal.

11 10 00 01

Q

I01

Q

I

11

01

10

00

A

t



Advanced Phase Shift Keying

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

Phase shift relative to the phase of the previous two bits (not to a reference signal)

Quadrature Amplitude Modulation (QAM)combines amplitude and phase modulationIt is possible to code n bits using one symbol2n discrete levels, n=2 identical to QPSKBit error rate increases with n.16 QAM (4 bits per symbol)

3 different amplitudes and 12 anglesused in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

φ

a



Hierarchical Modulation

DVB-T modulates two separate data streams onto a single DVB-T stream

High Priority (HP) embedded within a Low Priority (LP) stream

QPSK, 16 QAM, 64QAM

Example: 64QAMgood reception: resolve the entire64QAM constellationpoor reception, mobile reception: resolve only QPSK portion6 bit per QAM symbol, 2 mostsignificant determine QPSKHP service coded in QPSK (2 bit), LP uses remaining 4 bit

Q

I

00

10

000010 010101



Multi-carrier modulationMCM splits the high bit rate stream into many lower bit rate streams.

Higher bit rates are more vulnerable to ISI.Each stream sent using an independent carrier frequency (orthogonal).IEEE 802.11a = 48 sub-carriers

Efficient IFFT structure at transmitterReverse structure (with FFT) at receiver

Subcarrier orthogonality must be preserved

x

cos(2πfct)

R bpsQAM

Modulator

Serial To

ParallelConverter

IFFT

X0

XN-1

x0

xN-1

Add cyclicprefix and

ParallelTo SerialConvert

D/A



7. Spread spectrum technologySpread the narrow band signal into a broad band signal using a special code.

protection against narrow band interference Frequency dependent narrow band fading can wipe out narrow band signals for duration of the interference.

Side effects:coexistence of several signals without dynamic coordinationtap-proof

Alternatives: Direct Sequence, Frequency Hopping

detection atreceiver

interference spread signal

signal

spreadinterference

f f

power power



Effects of spreading and interference

dP/df

f

i)

dP/df

f

ii)

sender

dP/df

f

iii)

dP/df

f

iv)

receiverf

v)

user signalbroadband interferencenarrowband interference

dP/df



Spreading and frequency selective fading

frequency

channelquality

1 23

4

5 6

narrow bandsignal

guard space

22

22

2

frequency

channelquality

1

spreadspectrum

narrowband channels

spread spectrum channels



DSSS (Direct Sequence Spread Spectrum)XOR of the signal with pseudo-random number (chipping sequence)

many chips per bit (e.g., 128) result in higher bandwidth of thesignal

Advantagesreduces frequency selective fadingin cellular networks

terminals can use the same frequency rangebase station detects each stationwith an unique PN code.

Disadvantagesprecise power control necessary

Wireless LAN (802.11)use 11 chips per bit10110111000 (Barker code)

user data

chipping sequence

resultingsignal

0 1

0 1 1 0 1 0 1 01 0 0 1 11

XOR

0 1 1 0 0 1 0 11 0 1 0 01

=

tb

tc

tb: bit period tc: chip periods (spreading factor)=tb/tc



DSSS (Direct Sequence Spread Spectrum)

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator



DSSS (Direct Sequence Spread Spectrum)

SynchronizationThe sender and receiver have to be precisely synchronized.

Multi-path propagationseveral paths with different delays.rake receiver

uses n correlators for the n strongest paths.Each correlator is synchronized to the transmitter plus the delay on that specific path.The output of the correlators are then combined and fed into the decision unit.



FHSS (Frequency Hopping Spread Spectrum)

Discrete changes of carrier frequencysequence of frequency changes determined via pseudo random number sequence

Two versionsFast Hopping: several frequencies per user bit

better overcoming the narrow band interferenceSlow Hopping: several user bits per frequency

Advantagesfrequency selective fading and interference limited to short periodsimple implementationuses only small portion of spectrum at any time

Disadvantagesnot as robust as DSSSsimpler to detect (security)

Bluetooth1600 hops per second, 79 hop carriers spaced with 1MHz in 2.4 GHz ISM band.




user data

slowhopping(3 bits/hop)

fasthopping(3 hops/bit)

0 1

tb

0 1 1 t

f

f1

f2

f3

t

td

f

f1

f2

f3

t

td

tb: bit period td: dwell time




modulatoruser data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal



8. Cell structureImplements space division multiplex: base station covers a certain transmission area (cell)

tens of meters: buildingshundreds of meters: citiestens of kilometers : countryside

Mobile stations communicate only via the base stationAdvantages of cell structures:

higher capacity, higher number of users : frequency reusesmall cells: higher capacity (many base stations)

less transmission power needed : important for mobile stationsmore robust, decentralizedbase station and mobile stations deal with local interference (short distance)

Problems:fixed network needed for the base stations

antennars, switches, location registers, etc..handover (changing from one cell to another) necessaryfrequency planning : interference with other cells

Frequencies have to be distributed carefully.



Frequency planning Frequency reuse only with a certain distance between the base stations

Two possible models for minimal interferenceClusters: all cells within a cluster use disjointed set of frequencies.

Standard model using 7 frequencies:Sectorized antennas

f1f2

f3f2

f1

f1

f2

f3f2

f3f1

f2f1

f3f3

f3f3

f3

f4f5

f1f3

f2

f6

f7

f3f2

f4f5

f1f3

f5f6

f7f2

f2

3 cell cluster 7 cell cluster

f1f1 f1f2f3

f2f3

f2f3

h1

h2

h3g1

g2

g3

h1

h2

h3g1

g2

g3g1

g2

g3

3 cell clusterwith 3 sector antennas



Frequency planning Fixed frequency assignment: FCA (fixed channel allocations)

certain frequencies are assigned to a certain cellproblem: different traffic load in different cells

Dynamic frequency assignment: BCA (borrowing channel allocations)

base station chooses frequencies depending on the frequencies already used in neighbor cellsmore capacity in cells with more trafficassignment can also be based on interference measurements



Cell breathingCDM systems: cell size depends on current load

Additional traffic appears as noise to other userscarefull power control

If the noise level is too high users drop out of cells

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Mobile Computing Chapter 2: Wireless Transmission