MODULATION TECHNIQUES AND EQUALIZATION AND DIVERSITY

EC3501 WIRELESS COMMUNICATION UNIT II - MOBILE RADIO PROPAGATION

UNIT-II
MOBILE RADIO PROPAGATION

Large Scale Path Loss: Introduction To Radio Wave Propagation - Free Space Propagation Model –
Three Basic Propagation Mechanism: Reflection – Brewster Angle- Diffraction- Scattering. Small
Scale Fading And Multipath: Small Scale Multipath Propagation, Factors Influencing Small-Scale
Fading, Doppler Shift, Coherence Bandwidth, Doppler Spread And Coherence Time. Types Of Small-
Scale Fading: Fading Effects Due To Multipath Time Delay Spread, Fading Effects Due To Doppler
Spread.


2.1 Large Scale Path Loss: Introduction to Radio Wave Propagation
2.2 Free Space Propagation Model
2.3 Three Basic Propagation Mechanism:
2.4 Reflection
2.4.1 Reflection from dielectrics
2.4.2 Brewster Angle
2.4.3 Reflection from perfect conductors
2.6 Ground reflection (Two-ray model)
2.7 Diffraction
2.7.1 Fresnel Zone Geometry
2.7.2 Knife-edge diffraction model
2.7.3 Multiple Knife-edge diffraction
2.8 Scattering
2.8.1 Radar Cross Section Model
2.9 Small Scale Fading and Multipath: Small Scale Multipath Propagation
2.9.1 Factors Influencing Small-Scale Fading
2.9.2 Doppler Shift
2.10 Parameters of mobile multipath channels
2.10.1 Time dispersion parameters
2.10.2 Coherence Bandwidth
2.10.3 Doppler Spread and Coherence Time
2.11 Types of Small-Scale Fading:
2.11.1 Fading Effects Due To Multipath Time Delay Spread
2.11.1.1 Flat fading
2.11.1.2 Frequency selective fading
2.12.2 Fading Effects Due To Doppler Spread.
2.12.2.1 Fast fading
2.12.2.2 Slow fading


Page No. 1

,EC3501 WIRELESS COMMUNICATION UNIT II - MOBILE RADIO PROPAGATION

2.1 (Large Scale Path Loss) Introduction to Radio Wave Propagation
 The mechanisms behind electromagnetic wave propagation are reflection, diffraction, and
scattering.
 Most cellular radio systems operate in urban areas where there is no direct line-of-sight path
between the transmitter and the receiver.
 The presence of high rise buildings causes severe diffraction loss.
 Due to multiple reflections from various objects, the electromagnetic waves travel along
different paths of varying lengths.
 The interaction between these waves causes multipath fading at a specific location.
 The strengths of the waves decrease as the distance between the transmitter and receiver
increases.

 Propagation models have traditionally focused on predicting the average received signal
strength.

 Propagation models that predict the mean signal strength for an arbitrary transmitter-
receiver (T-R) separation distance.
 These are useful in estimating the radio coverage area of a transmitter and are called
large-scale propagation models.
 Because, they characterize signal strength over large T-R separation distances (several
hundreds or thousands of meters).

 On the other hand, propagation models that characterize the rapid fluctuations of the
received signal strength over very short travel distances (a few wavelengths) or short
time durations (on the order of seconds) are called small-scale or fading models.


2.2 Free Space Propagation Model
1. Explain how signal propagates against free space attenuation and reflection. [8m-May 2014]
2. How the received signal strength is predicated using the free space propagation model? Explain.
[10m - Nov 2012]
3. Explain the free space path loss and derive the gain expression. [8m - May 2012] [Nov/Dec 2019]
4. Describe briefly about free space propagation model. [April/May 2018] [April/May 2019]
5. Derive the received power in dBm for a free space Propagation model. [April/May 2021][April/May
2023]

 The free space propagation model is used to predict received signal strength.
 The transmitter and receiver should have a clear line-of-sight path between them.
 The free space received power is given by the Friis free space equation,
P G G 2
Pr d   t t 2 r 2  (1)
4  d L
where, Pr d   Received power
Pt  Transmitted power
Gt  Transmitter antenna gain
Gr  Receiver antenna gain
d  T-R separation distance in meters
  Wavelength in meters
L  System loss factor


Page No. 2

,EC3501 WIRELESS COMMUNICATION UNIT II - MOBILE RADIO PROPAGATION

 The losses are usually due to
a. Transmission line attenuation
b. Filter losses
c. Antenna losses in the communication system.
 The gain of an antenna is
4A
G  2e  (2)

where, G  Gain of an antenna
Ae  Effective aperture
Effective aperture, Ae is physical size of the antenna.
c 2c
 Wavelength is     (3)
f c
where f  carrier frequency in Hertz
c  Carrier frequency in radians per second
c  Speed of light given in meters/s.
 An isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all
directions. It is used to reference antenna gains in wireless systems.
 The Effective Isotropic Radiated Power (EIRP) represents the maximum radiated power
available from a transmitter in the direction of maximum antenna gain.
It is defined as EIRP  Pt Gt  (4)
 The Effective Radiated Power (ERP) denotes the maximum radiated power as compared to a
half-wave dipole antenna (Instead of an Isotropic antenna).
 Antenna gains are given in units of
dBi (dB gain with respect to an isotropic antenna) or
dBd (dB gain with respect to a hail-wave dipole antenna)
 The path loss is defined as the difference between the effective transmitted power and the
received power.
 The path loss represents signal attenuation as a positive quantity measured in dB.
 The path loss for the free space model when antenna gains are included is given by
P  G G 2 
PL dB   10 log t   10 log  t 2r 2   (5)
Pr  4  d 
 The path loss for the free space model when antenna gains are excluded is given by
P  2 
PL dB   10 log t   10 log  2 2 
 (6)
Pr  4  d 
 Far-field or Fraunhofer region:
The far-field or Fraunhofer region of a transmitting antenna is defined as the region beyond
the far-field distance df, which is related to the largest linear dimension of the transmitter
antenna aperture and the carrier wavelength.
 The Fraunhofer distance is given by
2D 2
df   (7)

where, D  Largest physical linear dimension of the antenna.
df  Far-field distance
d f must satisfy
d f  D  (8)
d f    (9)


Page No. 3

, EC3501 WIRELESS COMMUNICATION UNIT II - MOBILE RADIO PROPAGATION


 The received power, Pr d  at any distance d  d 0 , may be related to Pr at d 0 .
 The received power in free space at a distance greater than d0 is given by
2
d 
Pr d   Pr d 0  0  d  d0  d f  (10)
 d ,
 dBm or dBW units are used to express received power levels.
 The received power in dBm, is given by
 P d   d 
Pr  d  dBm  10 log  r 0   20 log 0  d  d0  d f  (11)
 0.001W   d ,
Pr  d 0  is in units of Watts.
******
Problem 1: A communication system has the following parameters: Pt =5W, Gt (db)=13dB,
G r (dB)=17dB, d =80km, f=3GHz. Determine the value of the received power. [6m - May
2013][April/May 2019]
Given
Transmitted power, Pt =5W
Transmitter antenna gain, Gt =13dB
Receiver antenna gain, G r =17dB
T-R separation distance in meters, d =80Km
Frequency=3GHz
W.K.T., System loss factor, =1
To Find
Received power, Pr d 
Solution
c 3  10 8 m / s
   0.1m
f 3  10 9 Hz
Pt Gt Gr 2 5  13  17  0.12
Pr d   =  10.9446  10 12W
4 2 2
d L 4  2
80  10 
3 2
1
*****


2.3 THREE BASIC PROPAGATION MECHANISM:
1. In free space propagation describe how the signals are affected by reflection, diffraction
and scattering. [16m - May 2016]
2. Explain in brief about the three propagation mechanisms which have impact on
propagation in mobile environment. [8m - May 2015, 8m -Nov 2013, 8m-Nov 2012]
3. Explain the different types of multipath propagation in wireless communication.
[10m - Nov 2014].


 Reflection, diffraction, and scattering are the three basic propagation mechanisms which influence
propagation in a mobile communication system.
 Received power (or its reciprocal, path loss) is the most important parameter predicted by large
scale propagation models (based on the physics of reflection, scattering, and diffraction.)

Page No. 4