CS 647 2.1

CS647: Advanced Topics in Wireless Networks

Basics of Wireless TransmissionPart II

Drs. Baruch Awerbuch & Amitabh MishraComputer Science Department

Johns Hopkins University

CS 647 2.2

For a circular reflector antennaG = η ( π D / λ )2

η = net efficiency (depends on the electric field distribution over the antenna aperture, losses such as ohmic heating , typically 0.55)D = diameter, thus,

G = η (π D f /c )2, c = λ f (c is speed of light)

Example:

Antenna with diameter = 2 m, frequency = 6 GHz, wavelength = 0.05 mG = 39.4 dB

Frequency = 14 GHz, same diameter, wavelength = 0.021 mG = 46.9 dB

* Higher the frequency, higher the gain for the same size antenna

Antenna Gain

CS 647 2.3

Definition of path loss LP :

Path Loss in Free-space:

Lf = (4π d/λ) 2 = (4π f cd/c )2

where fc is the carrier frequency

This shows greater the fc , more is the loss.

,r

tP P

PL =

),(log20)(log2045.32)( 1010 kmdMHzfdBL cPF ++=

Path Loss (Free-space)

CS 647 2.4

Example of Path Loss (Free-space)

Path Loss in Free-space

70

80

90

100

110

120

130

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s L f

(dB

)

fc=150MHzfc=200MHzfc=400MHzfc=800MHzfc=1000MHzfc=1500MHz

CS 647 2.5

Land Propagation

The received signal power:

L is the propagation loss in the channel, i.e.,L = LP LS LF

LPGGP trt

r =

Path loss

Fast fadingSlow fading (Shadowing)

CS 647 2.6

Slow Fading (Long-term

fading)

Propagation Loss

Signal Strength

(dB)

Distance

Path Loss

Fast Fading (Short-term

fading)

CS 647 2.7

Path Loss (Land Propagation)

Simplest Formula:

Lp = A d-α

where A and α: propagation constantsd : distance between transmitter and receiverα : value of 3 ~ 4 in typical urban area

CS 647 2.8

Path Loss (Urban, Suburban and Open areas)

Urban area:

where

Suburban area:

Open area:

[ ][ ] )(log)(log55.69.44

)()(log82.13)(log16.2655.69)(

1010

1010

kmdmhmhmhMHzfdBL

b

mbcPU

−+−−+= α

[ ][ ] [ ]

[ ][ ]

≥−

≤−

−−−

=citymediumsmallfor

MHzfformh

MHzfformh

cityelforMHzfmhMHzf

mh

cm

cm

cmc

m &,400,97.4)(75.11log2.3

200,1.1)(54.1log29.8

arg,8.0)(log56.1)(7.0)(log1.1

)(2

10

210

1010

α

4.528

)(log2)()(2

10 −

−=

MHzfdBLdBL cPUPS

[ ] 94.40)(log33.18)(log78.4)()( 102

10 −+−= MHzfMHzfdBLdBL ccPUPO

CS 647 2.9

Path Loss

Path loss in decreasing order:Urban area (large city) Urban area (medium and small city)Suburban areaOpen area

CS 647 2.10

Example of Path Loss (Urban Area: Large City)

Path Loss in Urban Area in Large City

100

110

120

130

140

150

160

170

180

0 10 20 30

Distance d (km)

Path

Los

s L p

u(d

B) fc=200MHz

fc=400MHzfc=800MHzfc=1000MHzfc=1500MHzfc=150MHz

CS 647 2.11

Example of Path Loss (Urban Area: Medium and Small Cities)

Path Loss in Urban Area for Small & Medium Cities

100110

120130

140150

160170

180

0 10 20 30

Distance d (km)

Path

Los

s Lp

u(d

B)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

CS 647 2.12

Example of Path Loss (Suburban Area)

Path Loss in Suburban Area

90

100

110

120

130

140

150

160

170

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s Lp

s(d

B)

fc=150MHz

fc=200MHz

fc=400MHz

fc=800MHz

fc=1000MHz

fc=1500MHz

CS 647 2.13

Example of Path Loss (Open Area)

Path Loss in Open Area

8090

100

110120

130

140

150

0 5 10 15 20 25 30

Distance d (km)

Path

Los

s L p

o(d

B)

fc=150MHzfc=200MHzfc=400MHzfc=800MHzfc=1000MHzfc=1500MHz

CS 647 2.14

Slow Fading

The long-term variation in the mean level is known as slow fading (shadowing or log-normal fading). This fading caused by shadowing.Log-normal distribution:- The pdf of the received signal level is given in decibels by

where M is the true received signal level m in decibels, i.e., M=10log10m, is the area average signal level, i.e., the mean of M,

σ is the standard deviation in decibels

( )( )2

2212

M M

p M e σ

πσ

−−

=

M

CS 647 2.15

Log-normal Distribution

M

2σ

M

p(M)

The pdf of the received signal level

CS 647 2.16

Fast Fading

The signal from the transmitter may be reflected from objects such as hills, buildings, or vehicles.

When MS far from BS, the envelope distribution of received signal is Rayleigh distribution. The pdf is

where σ is the standard deviation and r is the envelope of fading signal.

Middle value rm of envelope signal within sample range to be satisfied by

We have rm = 1.777

( ) 0,22

22 >=

−rerrp

rσ

σ

.5.0)( =≤ mrrPσ

CS 647 2.17

Rayleigh Distribution

The pdf of the envelope variation

r2 4 6 8 10

P(r)

0

0.2

0.4

0.6

0.8

1.0

σ=1

σ=2

σ=3

CS 647 2.18

Fast Fading (Continued)

When MS is close to BS, the envelope distribution of received signal is Rician distribution. The pdf is

where σ is the standard deviation,I0(x) is the zero-order Bessel function of the first kind,α is the amplitude of the direct signal

( ) 0,02

22

22

≥

=

+−

rrIerrpr

σα

σσα

CS 647 2.19

Rician Distribution

r

Pdfp

(r)

r 86420

0.6

0.5

0.4

0.3

0.2

0.1

0

α = 2α = 1α= 0 (Rayleigh)

σ = 1

α = 3

The pdf of the envelope variation

CS 647 2.20

Characteristics of Instantaneous Amplitude

Level Crossing Rate:Average number of times per second that the signal envelope crosses the level in positive going direction.

Fading Rate:Number of times signal envelope crosses middle value in positive going direction per unit time.

Depth of Fading:Ratio of mean square value and minimum value of fading signal.

Fading Duration:Time for which signal is below given threshold.

CS 647 2.21

Doppler ShiftDoppler Effect: When a wave source and a receiver are moving towards each other, the frequency of the received signal will not be the same as the source.

When they are moving toward each other, the frequency of the received signal is higher than the source.When they are opposing each other, the frequency decreases.

Thus, the frequency of the received signal is

where fC is the frequency of source carrier, & fD is the Doppler frequency.

Doppler Shift in frequency:

where v is the moving speed, λ is the wavelength of carrier.

θλcosvf D =

DCR fff −=

Moving speed v

MS

Signal

θ

CS 647 2.22

Delay Spread

When a signal propagates from a transmitter to a receiver, signal suffers one or more reflections.

This forces signal to follow different paths.

Each path has different path length, so the time of arrival for each path is different.

This effect which spreads out the signal is called “Delay Spread”.

CS 647 2.23

Moving Speed Effect

Time

V1 V2 V3 V4

Sign

al st

reng

th

CS 647 2.24

Delay Spread

Delay

Sign

al S

tren

gth

The signals from close by reflectors

The signals from intermediate reflectors

The signals from far away reflectors

CS 647 2.25

Intersymbol Interference (ISI)

Caused by time delayed multipath signals

Has impact on burst error rate of channel

Second multipath is delayed and is received during next symbol

For low bit-error-rate (BER)

R (digital transmission rate) limited by delay spread τd.

d

Rτ21

<

CS 647 2.26

Intersymbol Interference (ISI)

Time

Time

Time

Transmission signal

Received signal (short delay)

Received signal (long delay)

1

0

1

Propagation time Delayed signals

CS 647 2.27

Coherence Bandwidth

Coherence bandwidth Bc:Represents correlation between 2 fading signal envelopes at frequencies f1 and f2.Is a function of delay spread.Two frequencies that are larger than coherence bandwidth fade independently.Concept useful in diversity reception

Multiple copies of same message are sent using different frequencies.

CS 647 2.28

Cochannel Interference

Cells having the same frequency interfere with each other.rd is the desired signalru is the interfering undesired signal

β is the protection ratio for which rd ≤ βru (so that the signals interfere the least)

If P(rd ≤ βru ) is the probability that rd ≤ βru ,Cochannel probability Pco = P(rd ≤ βru )

CS 647 2.29

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

Goal: multiple use of a shared medium

Important: guard spaces needed!

s2

s3

s1

Multiplexing

f

t

c

k2 k3 k4 k5 k6k1

f

t

c

f

t

c

channels ki

CS 647 2.30

Frequency multiplex

Separation of the whole spectrum into smaller frequency bandsA channel gets a certain band of the spectrum for the whole timeAdvantages:no dynamic coordination necessaryworks also for analog signals

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

k2 k3 k4 k5 k6k1

f

t

c

CS 647 2.31

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 timethroughput high even for many users

Disadvantages:precise synchronization necessary

CS 647 2.32

f

Time and frequency multiplex

Combination of both methodsA channel gets a certain frequency band for a certain amount of timeExample: 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

CS 647 2.33

Code multiplex

Each channel has a unique code

All channels use the same spectrum at the same timeAdvantages:

bandwidth efficientno coordination and synchronization necessarygood protection against interference and tapping

Disadvantages:lower user data ratesmore complex signal regeneration

Implemented using spread spectrum technology

k2 k3 k4 k5 k6k1

f

t

c

CS 647 2.34

Modulation

Digital modulationdigital 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

Motivationsmaller antennas (e.g., λ/4)Frequency Division Multiplexingmedium characteristics

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

CS 647 2.35

Modulation and demodulation

synchronizationdecision

digitaldataanalog

demodulation

radiocarrier

analogbasebandsignal

101101001 radio receiver

digitalmodulation

digitaldata analog

modulation

radiocarrier

analogbasebandsignal

101101001 radio transmitter

CS 647 2.36

Digital modulation

Modulation of digital signals known as Shift KeyingAmplitude Shift Keying (ASK):

very simplelow bandwidth requirementsvery susceptible to interference

Frequency Shift Keying (FSK):needs larger bandwidth

Phase Shift Keying (PSK):more complexrobust against interference

1 0 1

t

1 0 1

t

1 0 1

t

CS 647 2.37

Advanced Frequency Shift Keying

bandwidth needed for FSK depends on the distance between the carrier frequenciesspecial pre-computation avoids sudden phase shifts

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 QPSK

even higher bandwidth efficiency using a Gaussian low-pass filter GMSK (Gaussian MSK), used in GSM

CS 647 2.38

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!

CS 647 2.39

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 waveneeds less bandwidth compared to BPSKmore 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

CS 647 2.40

Quadrature Amplitude Modulation

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, but less errors compared to comparable PSK schemes

Example: 16-QAM (4 bits = 1 symbol)Symbols 0011 and 0001 have the same

phase φ, but different amplitude a. 0000 and 1000 have different phase, but same amplitude.

used in standard 9600 bit/s modems

0000

0001

0011

1000

Q

I

0010

φ

a

CS 647 2.41

Hierarchical Modulation

DVB-T modulates two separate data streams onto a single DVB-T streamHigh Priority (HP) embedded within a Low Priority (LP) streamMulti carrier system, about 2000 or 8000 carriersQPSK, 16 QAM, 64QAMExample: 64QAM

good 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

CS 647 2.42

Spread spectrum technology

Problem of radio transmission: frequency dependent fading can wipe out narrow band signals for duration of the interferenceSolution: spread the narrow band signal into a broad band signal using a special code

protection against narrow band interference

protection against narrowband 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

CS 647 2.43

Effects of spreading and interference

dP/df

fi)

dP/df

fii)

sender

dP/df

fiii)

dP/df

fiv)

receiverf

v)

user signalbroadband interferencenarrowband interference

dP/df

CS 647 2.44

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

CS 647 2.45

DSSS (Direct Sequence Spread Spectrum) I

XOR of the signal with pseudo-random number (chipping sequence)

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

reduces frequency selective fadingin cellular networks

base stations can use the same frequency rangeseveral base stations can detect and recover the signalsoft handover

Disadvantagesprecise power control necessary

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 periodtc: chip period

CS 647 2.46

DSSS (Direct Sequence Spread Spectrum) II

Xuser data

chippingsequence

modulator

radiocarrier

spreadspectrumsignal

transmitsignal

transmitter

demodulator

receivedsignal

radiocarrier

X

chippingsequence

lowpassfilteredsignal

receiver

integrator

products

decisiondata

sampledsums

correlator

CS 647 2.47

FHSS (Frequency Hopping Spread Spectrum) I

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

Two versionsFast Hopping: several frequencies per user bitSlow 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

CS 647 2.48

FHSS (Frequency Hopping Spread Spectrum) II

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

CS 647 2.49

FHSS (Frequency Hopping Spread Spectrum) III

modulatoruser data

hoppingsequence

modulator

narrowbandsignal

spreadtransmitsignal

transmitter

receivedsignal

receiver

demodulatordata

frequencysynthesizer

hoppingsequence

demodulator

frequencysynthesizer

narrowbandsignal