Unit I Introduction of Wirelecc Channel.pdf

V P Shinde
UNIT I
Introduction of
Wireless Channel

UNIT 1: INTRODUCTION OF WIRELESS CHANNEL
Content
1. Propagation models
2. Multipath propagation and Fading
3. Modelling of Wireless Channel
4. Channel estimation techniques
5. Diversity techniques
6. Receiver noise computation
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BASIC TERMS OF WIRELESS COMMUNICATION
 Propagation
 Modes of propagation
 Multipath propagation
 Fading
 Path Loss
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 Propagation Defination-
Propagation is nothing but the movement of radio
wave from transmitter to receiver
Direct Propagation Ground Wave Propagation Sky Wave Propagation
 Modes of propagation
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 Multipath propagation
- In wireless systems, the signal
can reach the receiver via direct,
reflected, and scattered paths as shown
in Figure
- At the receiver, there is a
superposition of multiple copies of the
transmitted signal is called as
Multipath
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 Fading
- The multipath interference, in
turn, results in an amplification or
attenuation of the net received signal
power observed at the receiver,
- this variation in the received
signal strength arising from the
multipath propagation phenomenon is
termed multipath fading or simply
fading
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 Path Loss(PL)
- It refers to the loss or attenuation in electromagnetic signal when it
propagating along its path from transmitter to the receiver.
- The received power level is dependent on factors
1. transmission power
2. antenna gains
3. frequency of operation
4. distance between the
transmitter and the receiver.
-path loss is always expressed
in decibel (dB).
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PROPAGATION MODELS
 As we know, The 3 basic propagation mechanisms which impacts on propagation of electromagnetic
wave.
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PROPAGATION MODELS
1. Reflection :-
It occurs when a propagating electromagnetic wave impinges or collide on an object which
has large dimension as compared to the wavelength (λ) of the propagating wave
One of the application of reflection is fiber optics,
- due to reflection the beam of light travel inside the cable,
- even the cable bend light keep travel inside the cable
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PROPAGATION MODELS
2. Diffraction :-
It occurs when an electromagnetic wave meet to the surface that has sharp edges
- electromagnetic wave meet at knife edge obstacle they have natural tendency to bend
around the tip of the obstacle.
- another definition is bending of waves around an obstacles is called as diffraction.
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PROPAGATION MODELS
3. Scattering :-
Scattering occurs when a propagating electromagnetic wave meet an object which has
small dimension as compared to the wavelength (λ) of the electromagnetic wave
- electromagnetic wave having larger wavelength meet to
> small dust partical,
> rain drop or
> leaves
it will be reflected in many directions called as scattering
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PROPAGATION MODELS
 The performance of the communication system are characterize in terms of the transmitted
power and also the total load in terms of users that can be supported by the network
 from the theory of electromagnetic waves that the strength of the transmitted wireless radio signal
decreases as the distance of propagation increases
 In a typical wireless communication scenario, such as the one shown in Figure
Fig shows Propagation loss for a wireless signal
 Here we characterize the signal strength at the mobile as a function of the distance d
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MODELLING OF WIRELESS CHANNEL
 To develop accurate analytical models to characterize the
performance of wireless communication systems.
 Let us assume initially that the wireless channel is time invariant
 Let us consider a channel with L multipath components
 Each path of the wireless channel basically has two characteristic
properties
1. Delays the signal because of the propagation distance
2. An attenuation of the signal arising because of the scattering
effect
 The impulse response of an LTI system which attenuates a signal
by ai and delays it by τi is given as
 Therefore, a typical Channel Impulse Response (CIR) of a
multipath-scattering based wireless channel is given by the sum of
the above impulse responses corresponding to the individual
model, for a wireless signal
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MODELLING OF WIRELESS CHANNEL
Multipath signal components at the receiver
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PROPAGATION MODELS
 ∴ Propagation Models are traditionally used to traditionally propagation models predict the
average received signal strength at the given distance from the Tx
 So you need to understand what is the received power?
 Propagation models are characterized as
Large-scale propagation models Small-scale propagation models
Propagation models
• Propagation Model which predict the mean
signal strength for an arbitrary Tx & Rx
separation distance.
• Here the T-R separation distance is very large
as off 100 or 1000 meters so called as Large
scale propagation model
• Propagation Model which characterised the rapid
fluctuation of the received signal for very short
travel distances (a few wave length of for short
time duration (on the order of second) called
small scale of adding model
• when the Rx moves rapidly over short distances
the instantaneous signal strength may fluctuate
rapidly which rises the small scale fading
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FREE-SPACE PROPAGATION MODELS
where Pt is the power feed to the Tx antenna,
Gt is the transmit antenna gain,
Gr is the receive antenna gain,
Pr is the received power by Rx antenna
here we consider some assumptions,
1. Antennas are lossless
2. Antennas are Isotropic i.e. it has unit gain at
all directions
3. No obstacles between Tx & Rx
 Consider only Los components,
 obviously this model is mostly suited for satellite communication
 So Free space propagation model is used to modelling satellite communication
 The Free-space propagation model is the simplest large-scale model, quite useful in satellite and microwave
link modeling. It models a single unobstructed path between the transmitter and the receiver.
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 Aim of this derivation is to predict the amount of signal that can be effectively received by the Rx
antenna
 The power density (PD) at Tx side is given by,
 As the antenna is isotropic having area or instead of r we can use D i.e.
Power density is defined as power
received per unit area
Observation at Tx side
PD=
PD=
PD=
 But in practical case the antenna will be dipole antenna , which having higher gain at particular
direction i.e. no unit gain at all directions.
 So by considering gain of the transmitting antenna ,we can write the above equation as
 This is the maximum transmitted power from Tx sides
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 So the amount of power received by the Rx antenna is decided by the Effective Aperture ( ) of the
antenna
 In electromagnetics & antenna Theory antenna aperture ( ) is measure of how effectively an
antenna received the radiated power.
 By putting the value of Ae & PD in above wquation,
Antenna aperture is the ration of
Power received to the Power density
Observation at Rx side
= PD
=
( ) .
=
 As we know the relation between Ae & G
 Here L is the path loss that represents signal attenuation in dB.
=G
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PATH LOSS MODEL FOR FREE-SPACE PROPAGATION
 FSPL is given by
 Gt & Gr measured in dBi or dBd, but path loss always measured in db, So by taking log
=
( ) .
=
Assuming the gains are Unity gain,i.e. Gt & Gr = 1
reciprocal
=
( )
=
( )
=
( )
= log
( ) by solving
= -log
= -log
( )
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PATH LOSS MODEL FOR FREE-SPACE PROPAGATION
1. Friis Free Space Model is a valid predictor for D≠0
2. This Model Assumes only a LOS signal.
=
Where &
So the received power in free space at a distance greater than is given by
)
Where, ≫ ≫
≫ ≫
=
( ) .
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 A two-ray model, which consists of two overlapping waves at the receiver, one direct path and
one reflected wave from the ground so it is called as Two ray model.
 As Free space model is inaccurate when
used alone
GROUND-REFLECTION (TWO RAY) MODEL
 The total received E-field is the result of the direct line of sight component ELOS and the ground
reflected component Eg.
The free space propagating E-field for d>d0 is given by
 where E0 is the transmitted signal amplitude at a reference distance d0, and d is the propagation
distance for the LOS component
Certain Assumptions
1. ht & hr >> λ
2. ht & hr << d
3. Earth may be assumed to be flat
 Two ray model considers both the direct
path and a ground reflected propagated
path between transmitter and receiver.

 According to the law of reflection in a dielectric,
and
 The E-field due to the LOS component at the receiver
can be expressed as
 The E-fleld for the ground reflected wave, which has
a propagation distance of d", can be expressed as
 We know from the laws of reflection in dielectric,
the value of reflection coefficient Г is (-1)
 Total resultant E-field given by
where Γ is the reflection coefficient for ground.
 The electric field received by receiver
is can be expressed as
 Two propagating waves arrive at the receiver, one LOS wave which travels a distance of d’ and another
ground reflected wave, that travels d”.
By law of reflection in dielectric , = -1

Fig shows the method of images is used to find the path difference LOS & the ground reflected paths.
 Both rays travelled from different paths & have different distances so it is must to find path
difference

Using the method of images,
 we find the path difference between LOS & the
ground reflected path dg which is denoted as Δ
 Assuming now that ht, hr << d, the expression
for Δd can be approximated as
and
 Put the value of dLOS & dg in above equation we
get
 So, the Path differencd Δ is

2. Phase difference between two E -Field
components denoted as
3. Time delay between the arrival of the two
components easily find using following
relations denoted as
 Using Path difference Δ we can find out the value of Phase difference & Time delay
1. Path Difference Δ

By referring phasor diagram the total E field
received by receiver can be expressed as
Fig. shows Phasor diagram of electrical field
components of Line of sight, ground reflected
& total E field received
By using trigonometric identities
By increasing distance d the E tot(d) decays an oscillatory fashion

sin When
= .
=
> ≈
=
=
 We can represent the power in terms of E Field as
Power = (
 Received power is given by
= (
=
=
 As total power radiated is =
=
=
Total received Power
is given by

1. The received power falls off at a rate of 40dB/decade
2.The received power & path loss becomes independent of frequency
 The path loss for Two Ray model is expressed as
= 40logd – (10log )
=
Point be noted
1. At small T-R separation distances,
total E-Field is calculated using
equation
2. Where the ground appears in the first
Fresnel zone between Tx & Rx
for = π
d=

OUTDOOR PROPAGATION MODEL
 Several models have been developed to accurately model the received signal strength in
practical wireless scenario
 In mobile communication system radio transmission often takes place over irregular terrain
 So for estimation the Path Loss we consider terrain profile, which may vary from simple
curved earth profile to a highly mountainous profile also consider the presence of trees,
buildings & other obstacles.
 A number of propagation models are available to predict path loss over irregular terrain.
 Some of the models which are commonly used for outdoor propagation model are
1. Longly-Rice Model
2. Durkin’s Model-A case study
3. Okumura Model
4. Hata Model
5. PCS Extenssion to Hata Model

HATA MODEL
 The HATA Model [HAT90] is imperical formulation of graphical Path Loss data provided by
Okumura model
 Is valid from 150MHz to 1500 MHz
 Hata represents the urban area propagation loss
 Where,
fc - is frequency in MHz from 150MHz to 1500 MHz
- is effective transmitting (base station) antenna height( in m) from 30 m to 200m
- is effective receiving (mobile) antenna height( in m) from 1 m to 10 m
d - is the T-R separation distance (in km)
a( ) - is the correction factor for effective mobile antenna height which is the function of the size
of the coverage area
- 13.82 log - a( ) + (44.9 - 6.55 log ) logd

HATA MODEL
 For small and medium size city , the mobile antenna correction factor is given by
 To obtain the path loss in a suburban area , The standard Hata formula is modified as
a( ) - 0.7) - (1.56 - 0.8) dB
 and for large city , it is given by
a( ) 1.54 – 1.1 dB for ≤ 300 MHz
a( ) 11. - 4.97 dB for ≥ 300 MHz
- 5.4
open rural + 18.33
 Formulae for open rural area is modified as,
------ a
------ b
------ c

HATA MODEL
In Figure the simulated path loss in three types of environments are plotted

PCS EXTENSION TO HATA MODEL
 The proposed model for path loss is
 Where,
fc - 1500 MHz to 2000 MHz
- 30 m to 200m
- 1 m to 10 m
d - 1 km to 10 km
a( ) - is defined in equation a, b & c
- 13.82 log - a( ) + (44.9 - 6.55 log ) logd +
 The European cooperative for scientific & technology research (EURO-COST) formed the COST-
231 working committee to develop an extended version of Hata model.
 COST-231 proposed the following formulae to extend Hata’s model to 2 GHz.
0 dB for medium size city suburban areas
3 dB for metropolitan centres
=

RECEIVER NOISE COMPUTATION
 Noise at the receiver arises due to thermal effects is known as thermal noise.
 The noise Power Spectral Density (PSD) η denotes the noise power per
hertz of bandwidth. Hence, the total noise power is given as
 It is very important to accurately characterize noise power to compute the signal-to-noise power
ratio at the receiver and the resulting bit-error-rate performance.
Noise power = η × B
 Further, the noise power spectral density η can be derived as η = kTF
Where,
k = 1.38 × is the Boltzmann constant,
T - is the temperature in Kelvin,
F - is the noise figure.
 Thus Noise Power (Thermal noise power multiplied by noise figure) Noise power = kTF × B
 Noise temperature is the noise introduced by the receiver , is given as = (F-1)
(T is the room temperature)

SMALL SCALE FADING
 Defination:- The rapid fluctuations of the amplitude, phases in received signal over very short travel
distances (a few wave length) for short time duration (on the order of second) called small scale
fading or simply Fading.
- rapid changes in signal over short time interval
- random frequency variation due to Doppler shifts
- Time dispersion (echoes) caused
 Effects:- 3 most important effects are

SMALL SCALE FADING
 Factors influencing small scale fading
1. Multipath propagation
- As the environment changes continuously
because of reflection objects & scatters present in
channel which affects the signal in terms of
Phase, Frequency & time
- Creates signal smearing due to ISI
2. Speed of mobile
- random frequency modulation due to
different Doppler shift at each multipath
component
- It may be +ve or -ve
3. Speed of surrounding objects
- If surrounding objects move at greater
rate than mobile which cause small scale
fading
- otherwise motion of surrounding
objects may be ignored
4. Transmission BW of signal
- Transmitted signal BW > BW of Channel
signal will be distorted
- Transmitted signal BW < BW of Channel
amplitude change rapidly but
signal will not be distorted in time

DOPPLER SHIFTS
Definition:- The Doppler shift or Doppler effect is defined as the change in wavwlength or
frequency of the waves with respect to the observer who is in motion relative to the wave source.
Fig shows: A mobile moving
constant velocity (v), along a path
segment having length d between
point X & Y, while it receive signal
from remote source S .
Δl = d cosθ Δl = v Δt cosθ
Where, Δt time required to travel distanve from X to Y
θ is same at X & Y
Phase change Δϕ Δϕ =
Δl
Δϕ =
v Δt
cosθ
Hence Doppler change fd, is given by
Phase change Δϕ
=
Δϕ
Δt
= cosθ

EXAMPLE
Consider a transmitter which radiates a sinusoidal carrier
frequency of 1850 MHz. For a vehicle moving 60 mph, compute
the received carrier frequency if the mobile is moving
a) directly towards the transmitter
b) directly away from the transmitter
c) in a direction which is perpendicular to the direction of
arrival of the transmitted signal.

CHANNEL ESTIMATION IN WIRELESS SYSTEMS
 to detect the transmitted symbol x (k) at the receiver,
one needs to know the channel coefficient h.
 A popular scheme is through the transmission of pilot or training symbols
 Consider again the wireless channel model given as
 The estimate x (k) of the symbol x (k) can then
be recovered from y (k) simply as
 Consider transmission of L pilot symbols then received
symbols for this given by
y(k) = hx(k) + n(k)
where h is the flat-fading channel coefficient
x (k) = 1/h y(k) zero-forcing receiver
 The process of computing this channel coefficient h at the wireless receiver is termed channel
estimation and is an important procedure in every wireless communication system
( ) (k) = h ( ) (k) + n (k)
 Due to presence of noise in a system y(k) ≠ x(k) for any value of k, so h can be estimated as a
minimum value of cost function

 Due to presence of noise in a system y(k) ≠ x(k) for any value of k, so h can be estimated as a
minimum value of cost function
 To minimize the error function ξ (h) above is to differentiate it and set it equal to zero.
 This procedure yields
 This estimate of h gives minimum value of error function so called as Least square estimate
channel estimate
for complex numbers

DIVERSITY
1. Principal of Diversity
2. Need of Diversity
3. Types of Diversity
1. Principal of Diversity
Antennas are strategically
separated, while one antenna may sees a signal
Null, one of the other antennas may see a signal
Peak , & the receiver is able to select the antenna
with the best signal at any time.
2. Need of Diversity :- To compensate the fading
effect, Improve Link Quality
3. Types of Diversity :- Macroscopic Diversity &
Microscopic Diversity
• Space/Spatial Diversity
-TX Diversity & RX Diversity
• Time Diversity
• Frequency Diversity
• Angular Diversity
• Polarization Diversity
Types of Microscopic Diversity

REFERENCES
1. Rappaport, T. S., “Wireless Communications-Principles
and Practice”, Pearson, 2nd Edition.
2. Jagannatham, A. K., “Principles of Modern Wireless
Communication Systems”, McGraw-Hill Education.
V P Shinde

Unit I Introduction of Wirelecc Channel.pdf

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