l3 radio channel 2010v1

8/6/2019 L3 Radio Channel 2010v1

http://slidepdf.com/reader/full/l3-radio-channel-2010v1 1/69

UEET4563Mobile and SatelliteMobile and Satellite

CommunicationsCommunications

L3 – Mobile Radio

Channel and Fading



SubSub--topicstopics• Path loss

• Propagation mechanisms• Field strength prediction models

• Multipath propagation problem

• Rayleigh & Rician effects• Fading effects

• Doppler effects

• Propagation channel characteristics

2UTAR 2010



Radio wave propagationRadio wave propagation• Radio waves (signals) propagate outwards from a transmitting antenna in a straight line (for

simplicity in illustration) and are reflected in the same way that light is reflected.

• n ree space a x an x s ou e n ne o s g t , .e., must see eac ot er w t out

blockage.

• The RF signals are affected by terrain, atmospheric

conditions, and natural and/or artificial objects.

• Depending on frequency, there are 3

basic types of propagation of RF waves:

– ground wave (LF)

– skywave propagation (long distance HF)

– line-of-sight propagation (VHF/UHF)

3UTAR 2010



Ground wave propagationGround wave propagation• Ground wave is a surface wave that propagates close to the surface of the Earth and is

directly affected by terrain (buildings, hills and vegetation). Its propagation follows the

curvature of the Earth.• Ideal for relatively short distance propagation on these frequencies during the daytime.

• Mostly used for military communications.

• Frequency < 2 MHz (e.g. AM radio).

4UTAR 2010



SkywaveSkywave propagationpropagation• Propagation of radio waves is ‘bent’ back to the Earth’s surface by the ionosphere (‘bounced off’ the

ionosphere).

• Can be up to 3000 km.• It depends on the Earth's ionosphere, it changes with the weather and time of day.

• Frequency 2-30 MHz (e.g. ham/ amateur radio).

5UTAR 2010



Ionosphere DIonosphere D--layer layer • During sunlight hours the ionosphere separates into 3 layers.

– The lowest layer (D-layer) does not refract radio waves but unfortunately attenuates the wave as it travels through.

The lower the frequency the greater the attenuation. This encourages HF operators to use the highest frequency

possible to make reliable contact with other stations.

– The ‘E-layer’ refracts radio waves and is used for daylight communications up to approximately 500 km. Frequency

selection is important as if the frequency is too high it will pass through the E-Layer and be refracted by the next

higher layer and will not return to earth within the 500 km desired working distance.

– The ‘F-layer’ separates during daylight hours into the indistinct F1 and F2 layers. These layers also refract radio waves

and are used for ‘long-haul’ daytime communications over 500 km.

• Once the sun has set the D-layer decays quite rapidly.

• Frequency selection is an important factor that will determine the

success of communications. Generally speaking - the higher the

sun - the higher the frequency to be used.

• Based on this rule, a lower frequency should be use during early

morning, late afternoon and early evening to communicate over

the same distance as the frequency selected at mid day. Even

lower frequencies should be selected for late night operation.

6UTAR 2010



LOS propagationLOS propagation• Require a path where both transmitter and receiver are visible to one another without any obstruction.

• It is sometimes called space waves or tropospheric propagation.

• Limited by the curvature of the Earth for ground-based stations (50 km, from horizon to horizon).• Reflected waves can cause problems. Examples of line of sight propagation are: FM radio, microwave and

satellite.

• VHF and UHF communication typically use this path (FM radio, microwave and satellite).

• Frequency >30 MHz.

7UTAR 2010

YouneedgoodLOSforthesedishes.



Mobile radio signalsMobile radio signals• Mobile radio signals undergo a variety of alterations as they travel from transmitter to receiver. These

alterations are as follows:

– signal attenuation that increases with distance – signal distortions due to signals taking multiple paths to the receiver

8UTAR 2010



Mobile radio signalsMobile radio signals• Large scale fading

– path loss models

– predict mean signal strength over a long distance – shadowing

• Small scale fading

– rapid fluctuations of the signal strength over a short distance, short time duration, or short

frequency samples

– flat fading vs. frequency selective fading

– fast fading vs. slow fading

9UTAR 2010



Free space propagationFree space propagation• The free space propagation model is used to predict Pr when the Tx and Rx have a clear, unobstructed LOS

path between them.

• This model applies to both microwave and satellite systems.• The free space propagation model predicts that Pr decays as a function of the distance, d , between Tx and

Rx.

• Free space power received by Rx antenna (Pr ) which is separated from radiating Tx antenna by a distance

d can be calculated by Friis free space equation:

10UTAR 2010

Ld

GGPd P r t t

r 22

2

)4()(

π

λ =

Pt - transmittedpower

Pt (d )- receivedpowerasafunctionofdistanced Gt - TxantennagainGr - Rxantennagain

L - systemlossfactornotduetopropagation,e.g.,filterloss,transmissionlineattenuation,antennalosses λ - wavelength



Free space received power Free space received power Antenna gain is related to its effective aperture ( Ae) by

where Ae is related to the physical size of the antenna and λ is related to the carrier

frequency, f c, as λ =c/f , c =3×108ms-1 is the speed of light.

Power flux density,

2

4

λ

π e AG =

2

GPS t t =

Received power at distance d ,

Including L for effective system loss,

11UTAR 2010

2

4)(

=

d L

GGPd P r t t

r π

λ

( )

2

2

4

4

=

=

=

d GGP

GS

SAd P

r t t

r

er

π

λ

π

λ



FrissFriss equationequation• Values for Pt and Pr (d)must be in the same units, antenna gain G is unit-less and L=1 implies loss

hardware in the system.

• The Friis equation shows that Pr (d)decays as the square of the distance which implies that Pr (d)decays ata rate of 20 dB per decade.

• An isotropic radiator is an ideal antenna, which radiates the power with unit gain uniformly in all

directions, and is often used to reference antenna gains in wireless systems.

• e e ec ve so rop c ra a e power s = t t .

– It represents the maximum radiated power available from a transmitter in the direction of maximum antenna gain, ascompared to an isotropic radiator.

• Effective radiated power (ERP) is often used instead of EIRP and compares output to that of a half-wave

dipole antenna instead of a isotropic antenna.

• Gain [dBi] = Gain [dBd] + 2.14

• Antenna gains are often expressed in dBi unit, which means dB difference from an isotropic antenna or in

dBd unit, which means dB difference from a half-wave dipole antenna.

12UTAR 2010



Path lossPath loss• Path loss measures signal attenuation as a positive quantity in dB and is defined as the difference between

received power and transmitted power and usually includes antenna gain, thus

−==

22

2

)4(log10log10)(

d

GG

P

PdBPL r t

r

t

π

λ

• Note that Friis free space model is only a predictor for Pr

(d)which is in the far-field of the transmitted

signal.

13UTAR 2010



Far field regionFar field region• The far-field or Fraunhofer region of a transmitting antenna is the region beyond the far-field distance d f

which is related to the largest linear dimension of the antenna aperture ( D) and the carrier wave length ( λ)

by

• D is the largest physical linear dimension of the antenna.

Dd d Dd f f f >>>>= ,,2 2λ

λ

• It is obvious that Friss equation does not hold when d= 0. Therefore our models must use some close-in

distance d 0 as a known Rx power reference point.

• The Pr (d)will thus be related to Pr (d 0), where d>d 0.

• Usually, Pr (d 0)is determined experimentally by taking many observations at distances d 0 from the Tx.

14UTAR 2010



Far field reference distanceFar field reference distance• The reference distance d 0must be in the antenna far-field. Thus d 0 ≥ d f , and

• We often use dBm or dBW to represent Pr (d) because of the large fluctuation over several kms. Thus

f r r d d d d d d Pd P ≥≥

= 0

2

00 ;)()(

• Pr (d 0) is measured in Watts. The usual reference distance for systems with low-gain antennas in the 1-2

GHz range is d 0 = 1 m indoor and d 0 = 0.1-1 km outdoor.

15UTAR 2010

+

=

d d P

r

r

00

log20watt001.0log10dBm)(



ExampleExample• Find the far field region for an antenna with maximum dimension of 1 m and operating frequency of 900

MHz.

m633.0

)1(22

m33.010900

103MHz900,m1

22

6

8

===

=×

×==

==

λ

λ

Dd

f

c f D

f

• If a transmitter produce 50 W of power, express the transmit power in dBm and dBW.

16UTAR 2010

[ ]

[ ] dBW1750log10W1

)W(log10)dBW(

dBm471050log10mW1

)mW(log10)dBm(

3

==

=

=×=

=

t t

t t

PP

PP



ExampleExample• If 50 W is applied to unity gain antenna with a 900 MHz carrier frequency, compute the Rx power in dBm

at free space distance of 100 m from the antenna. What is Pr (10km)? Assume unity gain for the Rx

antenna and a lossless system.

( )

( )( )( )

( ) ( ) ( )mW105.3W105.3

11004

311150

4

36

22

2

22

2

×=×===−−r t t

r Ld

GGPP

π π

λ

17UTAR 2010

[ ]

dBm5.6410000

100

log20)100()km10(

km10

m100

dBm5.24)mW(log10)dBm(

0

−=

+=

=

=

−==

r r

r r

PP

d

d

PP



Propagation mechanismPropagation mechanism• Mobile radio channel places fundamental limitations on the performance of wireless communication

systems.

• The mechanisms behind radio wave propagation are diverse, but generally can be attributed to: – Reflection

– Diffraction

– Scattering

18UTAR 2010



ReflectionReflection• Reflection occurs when a propagating EM wave impinges upon an object

which has very large dimension when compared to the wavelength of the

propagating wave

– from earth surface

– building and wall

• Ground reflection (two-ray) model

19UTAR 2010

λ r t r t r t t

r

hhd

d

hhGGPP

20,

4

22

>≈



DiffractionDiffraction• Diffraction occurs when radio path between the Tx and Rx is obstructed by a surface that has sharp

irregularities (edge).

• The secondary wave present throughout the space and even behind the obstacle, giving rise to a bending

of waves around the obstacle, even when LOS does not exist between Tx and Rx.• The resulted waves depend on object geometry, amplitude, phase, and polarisation at point of diffraction.

• The concept of diffraction loss as a function of the path difference around an obstruction is explained by

Fresnel zones.

• Fresnel zones represent successive regions where secondary waves have a path length from the Tx to Rx

which are n λ 2 greater than the total path length of a LOS path.

• Dimensionless Fresnel-Kirchoff diffraction parameter ν is given by

20UTAR 2010

( )21

212d λd d d hv +=



KnifeKnife--edge diffractionedge diffraction• When there is a Tx and Rx, blocked by an obstacle, we can apply the following equation for the ν parameter:

( )21

212

d d λ

d d αv

+

=

• Estimating the signal attenuation caused by diffraction of radio waves over hills and buildings is essential in predicting the

field strength in a service area

• Attenuation caused by diffraction can be estimated by treating the obstruction as a diffracting knife edge

• Diffraction gain due to the presence of a knife edge is a function of ν:

21UTAR 2010

( )

( )( )( )

( )

>

≤≤

−−−

≤≤−

≤≤−−

−≤

=

4.2,225.0

log20

4.211.038.01184.04.0log20

10,95.0exp5.0log20

01,62.05.0log20

1,0

dB 2

vv

vv

vv

vv

v

Gd



ScatteringScattering• Scattering occurs when the medium through which the wave travels consists of object with

dimension that are small compared to the wavelength, and when the number of obstacles

per unit volume is large.• Produced by rough surface, small objects, other irregularities in the channel.

• Street signs, lamp post induce scattering in mobile communication system.

22UTAR 2010



Field strength predictionField strength prediction• Deterministic channel models based on ray-tracing technique relies on site-specific geometry,

material properties such as reflection coefficient and permittivity, amount of scatterers,

degree of reflections and diffractions of the propagation paths, number of multipaths, etc. – must be validated through measurements

• Another way is to develop stochastic channel model by utilising measurement data collected

– mathematical equations and formulations are developed using statistical way to produce response that will provide

best match to the real response of the environment

– e.g. propagat on mo e

• Propagation models have traditionally focused on predicting the average received signalstrength at a given distance from the transmitter, as well as the variability of the signal

strength in close spatial proximity to a particular location.

• Path loss measures signal attenuation as a positive quantity in dB and is defined as the

difference between Rx power and Tx power.

23UTAR 2010



British urban path lossBritish urban path loss( ) H L f hhd r t 34.018.04020log20log40PL −+++−=

f= frequency in MHz

L = land usage factor, a percentage of the test area covered by buildings of any type (0-100%)

H = terrain height difference between Tx and Rx

25UTAR 2010



HataHata modelmodel• The Hata model is an empirical formulation of the graphical path loss data provided by

Okumura and is valid from 150 MHz to 1500 MHz.

• The standard formula for median path loss in urban areas is given by:

( ) ( ) d hh Ah f t r t loglog55.69.44log82.13log16.2655.69PL −+−−+=

f – between150and1500MHz

• For a small or medium sized city:

• For a large city and f> 400 MHz

26UTAR 2010

t –

d – between1and20km A(hr ) – correctionfactorformobileantennaheight

( ) ( ) ( )

[ ]m10,1

8.0log56.17.0log1.1

∈

−−−=

r

r r

h

f h f h A

( ) ( )[ ] 97.475.11log2.32−= r r hh A



ExtendedExtended HataHata modelmodel• Extension of Hata model to 1800 MHz band

C M =0dBformediumsizecityandsuburbanarea

C M =3dBformetropolitancentre f – between1500and2000MHz

( ) ( ) M t r t C d hh Ah f +−+−−+=

loglog55.69.44log82.13log91.333.46PL

27UTAR 2010

ht – etween30an 200m

hr – between1and10md – between1and20km



Example path loss predictionExample path loss prediction

100

120

140

160

l o s s [ d B ]

Suburbanmacrocell

Urbanmacrocell

Urbanmicrocell

28UTAR 2010

0 200 400 600 800 1000 1200 1400 1600 1800 200040

60

80

Distance[m]

P a t h

BasedonCOST231Walfish-Ikegamimodel;2.5GHz;NLOScondition;BSheights(32mformacro,12mformicro);MSheight1.5m



Frequency domainFrequency domain• Fundamental frequency - when all frequency components of a signal are integer multiples of

one frequency, it’s referred to as the fundamental frequency

• Spectrum - range of frequencies that a signal contains• Absolute bandwidth - width of the spectrum of a signal

• Effective bandwidth (or just bandwidth) - narrow band of frequencies that most of the

signal’s energy is contained in

• ny e ec romagne c s gna can e s own o cons s o a co ec on o per o c ana og s gna s

(sine waves) at different amplitudes, frequencies, and phases• The period of the total signal is equal to the period of the fundamental frequency

• The greater the bandwidth, the higher the information-carrying capacity

• Conclusions

– Any digital waveform will have infinite bandwidth

– But the transmission system will limit the bandwidth that can be transmitted

– And, for any given medium, the greater the bandwidth transmitted, the greater the cost

– However, limiting the bandwidth creates distortions

29UTAR 2010



SmallSmall--scale fading scale fading• Small-scale fading describes the rapid fluctuations of the amplitudes, phases, or multipath

delays of a radio signal over a short period of time, short frequency window, or travel

distance, so that large-scale path loss effects may be ignored

• Fading is caused by superposition between two or more versions of the transmitted signal

which arrive at the receiver at slightly different times

– these multi ath waves combine at the receiver antenna to ive a resultant si nal which can var widel in am litude

and phase

• 3 most important small-scale fading effects:

– Rapid changes in signal strength over a small travel distance or time interval

– Random frequency modulation due to varying Doppler shifts on different multipath signals

– Time dispersion (echoes) caused by multipath propagation delays

• Multiple signals arriving a different times. When added together at the antenna, signals are spread out in time. This cancause a smearing of the signal and interference between bits that are received.

30UTAR 2010



SmallSmall--scale fading scale fading• Fading signals occur due to reflections from ground & surrounding buildings (clutter) as well

as scattered signals from trees, people, towers, etc.

– often an LOS path is not available so the first multipath signal arrival is probably the desired signal(the one which travelled the shortest distance)

– NLOS allows service even when Rx is severely obstructed by surrounding clutter

– multipath signals have randomly distributed amplitudes, phases, & direction of arrival

• vector summation of (A ∠θ) @ Rx of multipath leads to constructive/destructive interference as mobile Rx

moves n space w t respect to t me

• Even stationary Tx/Rx wireless links can experience fading due to the motion of objects (cars,

people, trees, etc) in surrounding environment off of which come the reflections

31UTAR 2010



SmallSmall--scale fading scale fading• Fading occurs around received signal strength predicted from large-scale path loss models.

• If a user stops at a deeply faded point, the signal quality can be quite bad.

• However, even if a user stops, others around may still be moving and can change the fadingcharacteristics.

• And if we have another antenna, say only 7 to 10 cm separated from the other antenna, that

signal could be good.

32UTAR 2010

0 1 2 3 4 5 6-70

-65

-60

-55

-50

-45

-40

-35

-30

-25

Time[s]

G a i n [ d B ]

pedestrianwalkingspeedacross6sspatialfading



SmallSmall--scale fading scale fadingPhysical factors influencing fadingPhysical factors influencing fading

• Multipath propagation – the presence of reflecting objects and scatterers, the result of

multiple versions of the transmitted signal that arrive at the receiving antenna

– number and strength of multipath signals

– time delay of signal arrival

• large path length differences → large differences in delay between signals

– urban area with many buildings distributed over large spatial scale

• large number of strong multipath signals with only a few having a large time delay

– suburb with nearby office park or shopping mall

• moderate number of strong multipath signals with small to moderate delay times

– rural → few multipath signals (LOS + ground reflection)

• Speed of the mobile

– relative motion between base station & mobile causes random frequency modulation due to Doppler shift ( f d )

– different multipath components may have different frequency shifts.

33UTAR 2010



SmallSmall--scale fading scale fadingPhysical factors influencing fadingPhysical factors influencing fading

• Speed of surrounding objects

– also influence Doppler shifts on multipath signals

– dominates small-scale fading if speed of objects > mobile speed, otherwise ignored

• The transmission bandwidth of the signal – the mobile radio channel is modelled as filter with specific bandwidth (BW)

– ,

determine:

• if small-scale fading is significant

• if time distortion of signal leads to inter-symbol interference (ISI)

– an radio channel can cause distortion/ISI or small-scale fading, or both.

34UTAR 2010



Doppler effect Doppler effect • Consider a mobile moving at a constant velocity v, along a path segment having length

between points X and Y , while it receives signals from a remote source S.

• The difference in path lengths travelled by the wave from source S to the mobile at points X and Y is

where ∆t is the time required for the mobile to travel from X to Y , and θ is assumed to be the

θ ν θ coscos t d l ∆==∆

.

35UTAR 2010



Doppler shift Doppler shift • The phase change in the received signal due to the difference in path lengths is therefore

• The apparent change in frequency, or Doppler shift, is given by f d where

θ

λ

π v ∆t

λ

π∆l ∆ cos

22==φ

θ . λ

v

∆t

∆.

π f d cos

2

1==

φ

• f d relates the Doppler shift to the mobile velocity (v) and the spatial angle between thedirection of motion of the mobile and the direction of the wave.

• If the mobile is moving toward the direction of arrival of the wave, the Doppler shift is

positive (i.e. the apparent received frequency is increased)

• If the mobile is moving away from the direction of arrival of the wave, the Doppler shift is

negative (i.e. the apparent received frequency is decreased).

36UTAR 2010



Doppler shift Doppler shift • Such motion of the antenna leads to (time varying) phase shifts of individual reflected waves.

• Two Doppler shifts to consider

– the Doppler shift of the signal when it is received at the car – the Doppler shift of the signal when it bounces off the car and is received somewhere else

• Multipath signals will have different f d ’s for constant v because of random arrival directions

– many waves arrive, all with different shifts

• Thus their relative hases chan e all the time and so it affects the am litude of the resultin

composite signal

• The Doppler effects determine the rate at which the amplitude of the resulting composite

signal changes

• What matters with Doppler shift is not the absolute frequency, but the shift in frequency

relative to the bandwidth of a channel

– e.g., a shift of 166 Hz may be significant for a channel with a 1 kHz bandwidth.

– in general, low bit rate (low bandwidth) channels are affected by Doppler shift

37UTAR 2010



Impulse responseImpulse response• Model the channel as a linear filter with a time varying characteristics

• Vector summation of random amplitudes & phases of multipath signals results in a ‘filter’

• That is to say, the channel takes an original signal and in the process of sending the signalproduces a modified signal at the receiver

• Time variation due to mobile motion → time delay of multipath signals varies with location of

Rx

• an e oug as a oca on vary ng er

– as mobile moves with time, the location changes with time; hence, time-varying characteristics

• The channel has a fundamental bandwidth limitation → model as a band pass filter

38UTAR 2010



Real measurement exampleReal measurement example

39UTAR 2010

x y

X(f) H(f) Y(f)

( ) ( ) ( ) ( ) ( ) ( ) f H f X f Y t ht xt y ⋅=⊗=



Impulse responseImpulse responseHow to obtain it? How to obtain it? • Channel sounding – channel measurement → delay profile, no longer in ‘impulse’ form

• Transmit short time duration pulse (not exactly an impulse, but with certain BW) and record multipath echoes @ Rx

40

LOS



TimeTime--variant impulse responsevariant impulse response• Amplitude and delay time of multipath change:

– as mobile moves

– across time (due to movements in surrounding)

( ) max, τ τ >∆t t h

41UTAR 2010

measureddynamicresponseinanindoorcorridorenvironmentinfinitebandwidthassumption



COST207 modelCOST207 model• Rural Area

• Typical Urban Area

≤<−

=

elsewhere0

µs7.00),2.9exp()(

τ τ τ P

≤<−

=µs70),exp(

)(τ τ

τ P

• Bad Urban Area

• Hilly terrain

42UTAR 2010

≤<−

≤<−

=

elsewhere0

µs105),5exp(5.0

µs50),exp(

)( τ τ

τ τ

τ P

≤<−

≤<−

=

elsewhere0

µs2015),15exp(1.0

µs20),5.3exp(

)( τ τ

τ τ

τ P

Di t i lDi t i l



Discrete impulse responseDiscrete impulse responseMathematical representationMathematical representation

Model multipath as a sum of unit impulses

a(·)=amplitude

θ = phase

( ) ( )( )

( )n

t j N

n

nn

et at h τ τ δ τ τ τ θ

−⋅⋅=∑=,

1,,

N= total number of multipath components

Note: a(·)may be relatively constant over a local area, but θ will change significantly due to the

different path lengths of multipath components

43UTAR 2010



Time dispersion parametersTime dispersion parameters• To quantify any multipath channel impulse response, we can calculate the maximum excess delay and root

mean square (RMS) delay spread of the multipath channel.

• The maximum excess delay is defined as time difference between the first and the last arriving path.

• Typical values of RMS delay spread are on the order of nanoseconds in indoor channels and on the order

of microseconds for outdoor channels.• Mean excess delay, mτ ; and RMS delay spread, τ RMS

– give an indication of how time smearing might occur for the signal

44UTAR 2010

( )

( )

( )

( )∑∑

∫∫

=

===

N

n

n

N

nnn

P

P

d P

d Pm

mas

1

1

0

0

max

τ

τ τ

τ τ

τ τ τ τ

τ

τ

( ) ( )

( )

( ) ( )

( )∑∑

∫∫

=

=

−

=

−

= N

n

n

N

n

nn

P

Pm

d P

d Pm

1

1

2

0

0

2

RMSmax

max

τ

τ τ

τ τ

τ τ τ τ

τ

τ

τ τ



Coherence bandwidthCoherence bandwidth• A statistical measure of the range of frequencies over which the channel can be considered ‘flat’ ( i.e. a

channel which passes all spectral components with approximately equal gain and linear phase)

• Coherence bandwidth Bc is the frequency range over which the frequency components have a strong

potential for amplitude correlation

• Amplitude correlation → multipath signals have similar amplitude → if they are out-of-phase they have

significant destructive interference with each other (deep fades)

• In proper computation, it is the bandwidth over which the frequency correlation coefficient, ρ (.), is above

a predefined threshold, normally 0.9 or 0.5

• Approximation for ρ =0.9:

• Approximation for ρ =0.5:

45UTAR 2010

( )( ) ( )

( ) ( ) ( ) ( )df f f H f f H df f H f H

df f f H f H f

F F

F

∆+∆+

∆+

=∆

∫∫

∫**

*

ρ

( )RMS5

15.0

τ ≈c B

offsetfrequency=∆ f

( )RMS5019.0τ

≈c B



0.8

0.85

0.9

0.95

1AveragedFrequencyCorrelation

c o e f f i c i e n t

Antenna1

Antenna2

Antenna3Antenna4

Antenna5

Antenna6

Antenna7

Antenna8

Real example:Real example: ρ ρ( ( ∆ ∆ f), f),B B c c

0.7

0.8

0.9

1AveragedFrequencyCorrelation

o e f f i c i e n t

Antenna1

Antenna2

Antenna3Antenna4

Antenna5

Antenna6

Antenna7

Antenna8

0 10 20 30 40 50 600.5

0.55

0.6

0.65

0.7

.

Frequencyoffset,MHz

C o r r e

l a t i o n

46UTAR 2010

0 10 20 30 40 50 600.3

0.4

0.5

0.6

Frequencyoffset,MHz

C o r r e

l a t i o n

Which one represents NLOS?Which one represents NLOS?

Bc(0.7) Bc(0.7)



Doppler spreadDoppler spread• Delay spread and coherence bandwidth describe the time dispersive nature of a channel

– they do NOT characterise the time-varying nature of the channel due to the mobility of the mobile and/ or

surrounding objects

– that is to say, Bc and τ RMS characterise the statics – how multipath signals are formed from scattering/ reflections and

travel different distances

• Doppler spread and coherence time describe the time varying nature of the channel

• Doppler spread B D is a measure of the spectral broadening caused by the time rate of change

of the mobile radio channel – due to motion → Doppler shifts

• B D =max Doppler shift =f max =vmax / λ (θ =0 for max Doppler)

• If Tx signal bandwidth ( BS) is large such that BS >> B D then effects of Doppler spread are

NOT important

47UTAR 2010



RealmeasuredDopplerspectrum RealmeasuredDopplerspectrum

-80

-75

-70

-65

-60

-55

[ d B ]

-50

-45

-40

-35

[ d B ]

48UTAR 2010

-80 -60 -40 -20 0 20 40 60 80-110

-105

-100

-95

-90

-85

Dopplerfrequency[Hz]

G a i n

0 1 2 3 4 5 6-65

-60

-55

Timesample[s]

G a i n

3Gmeasurementsconductedatsuburbanenvironment,mobilepedestrianspeedat1m/sover6stimewindow,moderatelypopulatedarea,typical‘mobileusage’configuration



Coherence timeCoherence time• Coherence time T C is the time domain dual of Doppler spread

• Coherence time is a statistical measure of the time duration over which the channel impulse

response is essentially invariant (amplitude & phase of multipath signals ≈ constant), and

quantifies the similarity of the channel response at different times

• T Cpasses all received signals with virtually the same characteristics because the channel has

not changed

–

•Two signals that arrive with a time separation >

T C are affected differently by the channel

since the channel has changed

49UTAR 2010



T T C C calculationcalculation

• The time over which the temporal correlation coefficient, ρ (.), is above a predefined threshold, normally

0.9 or 0.5

• For digital communications coherence time and Doppler spread are related by

( )

( ) ( )

( ) ( ) ( ) ( )dt t t H t t H dt t H t H

dt t t H t H

t T T

T

∆+∆+

∆+

=∆

∫∫

∫**

*

ρ offsetsampletime=∆

t

50UTAR 2010

mm

C f f

T 423.016

9 2 ==π



-95

-90

-85

-80

-75

-70

-65

-60

-55

-50

P a t h g a i n [ d B ]

-60

-55

-50

-45

-40

-35

F r e q u e n c y g a i n [ d B ]

Multi Multi--dimensional fadingdimensional fading

51UTAR 2010

0 1 2 3 4 5 6-100

Delay[µs]

0 2 4 6 8 10 12 14 16 18 20-65

Bandwidth[MHz]

( )τ ,t h ( ) f t H ,



Types of smallTypes of small--scale fading scale fading• RMS delay spread and Doppler spread are channel parameters

• Bandwidth, symbol period, etc. are signal parameters

• Depending on the relation between these two groups of parameters, different transmitted

signals will undergo different types of fading

• Multipath delay spread leads to time dispersion and frequency selective fading

• Doppler spread leads to frequency dispersion and time selective fading

• Fading effects due to multipath time delay spread

– flat fading

– frequency selective fading

• Fading effects due to Doppler spread

– fast fading

– slow fading

52UTAR 2010



Summary of channel fadingSummary of channel fading

53UTAR 2010



Flat fadingFlat fading• If the mobile radio channel has a constant gain and linear phase response over a bandwidth which is

greater than the bandwidth of the transmitted signal, then the received signal will undergo flat fading

– spectral characteristics of the transmitted signal are preserved at the receiver

• A channel that is not a flat fading channel is called frequency selective fading because different

frequencies within a signal are attenuated differently by the channel

– the definition of flat or frequency selective fading is defined with respect to the bandwidth of the signal that is being

transmitted

• Reciprocal bandwidth T S (e.g. symbol period) of the transmitted signal is much larger than the multipath

54UTAR 2010



Flat fadingFlat fading• Flat fading channels are also known as amplitude varying channels (change of amplitude still

occurs in the received signal but the spectrum of the transmission is preserved) or

narrowband channels, since the bandwidth of the applied signal is narrow as compared to

the channel flat fading bandwidth

• Most common amplitude distribution is the Rayleigh distribution

– practical system has limited bandwidth

• A si nal under oes flat fadin if

BS <<BC

T S>>τ RMS – it means all multipaths arrive at Rx during 1 symbol period hence little ISI

• BSis the bandwidth and T S = 1/ BS

• BC is the coherence bandwidth

– range of frequencies over which signals will be transmitted without significant changes in signal strength

– channel acts as a filter depending on frequency – signals with narrow frequency bands are not distorted by the channel

55UTAR 2010



Frequency selective fadingFrequency selective fading• If the mobile radio channel has a constant gain and linear phase response over a bandwidth

which is smaller than the bandwidth of the transmitted signal, then the received signal will

undergo frequency selective fading (exactly opposite to the flat fading conditions)

• Channel impulse response has a multipath delay spread which is greater than the reciprocal

bandwidth of the transmitted signal (exactly opposite from flat fading)

56UTAR 2010



Frequency selective fadingFrequency selective fading• Frequency selective fading is due to time dispersion of the transmitted symbols within the

channel

• Thus the channel induces intersymbol interference (ISI)

• Viewed in the frequency domain, certain frequency components in the received signal

spectrum have greater gains than others

• Frequency selective fading is caused by multipath delays which approach or exceed the

• Frequency selective fading channels are also known as wideband channels since BS is widerthan the bandwidth of the channel impulse response

• A signal undergoes frequency selective fading if

BS >BC – certain frequency components of the signal are attenuated much more than others

T S<τ RMS – delayed versions of Tx signal arrive during different symbol periods hence ISI will occur

• Rule of thumb: – flat fading if T S ≥10τ RMS

– frequency selective fading if T S <10τ RMS

57UTAR 2010



Fast fadingFast fading• Depending on how rapidly the transmitted baseband signal changes as compared to the rate

of change of the channel, a channel may be classified as fast fading or slow fading channel

• In a fast fading channel, the channel impulse response changes rapidly within the symbol

duration

• Coherence time of the channel is smaller than the symbol period of transmitted signal

• This causes frequency dispersion (also called time selective fading) due to Doppler spreading

• s gna un ergoes as a ng

– T S

>T C

– channel changes within 1 symbol period, rapid amplitude fluctuations

– BS <B D – Doppler shifts significantly alter spectral BW of TX signal, signal ‘spreading’

• Fast fading only deals with the rate of change of the channel due to motion

• In practice, fast fading only occurs for very low data rates

58UTAR 2010



Channel capacityChannel capacity• Impairments, such as noise, limit data rate that can be achieved

• For digital data, to what extent do impairments limit data rate?

• Channel Capacity – the maximum rate at which data can be transmitted over a given

communication path, or channel, under given conditions

• Data rate – rate at which data can be communicated (bps)

• Bandwidth – the bandwidth of the transmitted signal as constrained by the transmitter and

the nature of the transmission medium (Hz)

• Noise – average level of noise over the communications path

• Error rate – rate at which errors occur

61UTAR 2010



Shannon capacityShannon capacity

• Represents theoretical maximum that can be achieved

• In practice, only much lower rates achieved

– formula assumes white noise (thermal noise)

( )SNR1log2 += BC

– mpu se no se s no accoun e or

– attenuation distortion or delay distortion not accounted for

62UTAR 2010



Bad channel = Good channel ??Bad channel = Good channel ??



Bad channel = Good channel ?? Bad channel = Good channel ?? • The requirement 10 yrs ago was enhanced data rate

– this means high speed transmission over the mobile radio channel

– high speed transmission means signalling bandwidth is greater than coherence bandwidth

– high bandwidth requirement means large delay spread, must live with ISI somehow

• ISI mitigation – 2G uses equalisation technique

– 3G is based on Rake Receiver

– how about 4G?

• On another hand, we always like LOS than NLOS due to SNR vs. BER requirement

– we treat LOS as good channel, NLOS as bad channel

– NLOS usually implies multipath-rich environment

– can transmit faster in LOS

• However, multiple-input multiple-output (MIMO) changes everything!

• For MIMO:

– NLOS can also be a good channel

– in fact, MIMO always prefers multipath channels (under certain SNR condition)

– MIMO can transmit in high speed under NLOS conditions too!

– what’s more? high speed even in narrowband too!

• Ultra high-speed transmission with MIMO means larger bandwidth requirements

– in this case, how about ISI problem?

64UTAR 2010

MIMOMIMO



MIMOMIMO

• Space-time block coding (STBC)

– data stream is replicated and transmitted over multiple antennas

– maximal-ratio combining technique at Rx for robust reception

– to increase radio link quality (enhance coverage) – good for mobility

• Spatial multiplexing

– transmit different data at different antennas in one time-frequency resource

– rich multipath for maximum channel rank

– increase throughput but less robust (sensitive to noise and interference)

• Best performances in micro cellular/indoor/hot spots

– multipath-rich, large angular spread

• Accurate channel knowledge is needed

– channel sounding (normally via preamble and pilot signals)

65UTAR 2010

antennaelementspacingcanbelessifthebasestationislower

MIMOMIMO



MIMOMIMO

10-3

10-2

10-1

100

E R

Nodiversity1x1

Alamouti2x1

Alamouti2x2

MRRC1x2

MRRC1x4 SISOSISO

66UTAR 2010

-20 -10 0 10 20 30 4010

-6

10-5

10-4

SNR[dB]

B

simulationswithnarrowbandRayleighchannel+AWGNTxpowerequallydistributed,BPSK,10mil.trialsperSNR

MIMOMIMO

MIMO advantagesMIMO advantages



MIMO advantagesMIMO advantages

67UTAR 2010

MIMO Mbps coverage rangeMIMO Mbps coverage range



MIMO Mbps coverage rangeMIMO Mbps coverage range

68UTAR 2010

802.11gwithoutMIMO 802.11gwithMIMO

l3 radio channel 2010v1

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