1. Mobile Communications
Part IV- Propagation Characteristics
Professor Z Ghassemlooy
Faculty of Engineering and
Environment
University of Northumbria
U.K.
Z. Ghassemlooy

2. Contents Radiation from Antenna Propagation Model (Channel Models)
Free Space Loss
Plan Earth Propagation Model
Practical Models
Summary
Z. Ghassemlooy

3. Wireless Communication System
Channel code word
Source
Encoder
Channel
Mod-
ulator
Message Signal
Modulated
Transmitted Signal
Wireless Channel
User
Source
Decoder
Channel
Demod-
ulator
Estimate of
Message signal
channel code word
Received
Signal
Z. Ghassemlooy

4. Basic Questions Tx: What will happen if the transmitter Channel:
changes transmit power ?
changes frequency ?
operates at higher speed ?
Channel:
What will happen if we conduct this experiment in different types of environments?
Desert
Metro
Street
Indoor
Rx:
What will happen if the receiver moves?
Z. Ghassemlooy

5. Antenna - Ideal
Isotropics antenna: In free space radiates power equally in all direction. Not realizable physically d d E H Pt
EM fields around a transmitting antenna
, a polar coordinate
d- distance directly away from the antenna.
 is the azimuth, or angle in the horizontal plane.
 is the zenith, or angle above the horizon.
Distance d
Area
Z. Ghassemlooy

6. Antenna - Real
Not isotropic radiators, but always have directive effects (vertically and/or horizontally)
A well defined radiation pattern measured around an antenna
Patterns are visualised by drawing the set of constant-intensity surfaces
Z. Ghassemlooy

7. Antenna – Real - Simple Dipoles
Not isotropic radiators, e.g., dipoles with lengths /4 on car roofs or /2 as Hertzian dipole
Example: Radiation pattern of a simple Hertzian dipole
shape of antenna is proportional to the wavelength
Largest dimension of the antenna La = /2
/4
/2
side view (xy-plane) x y
side view (yz-plane) z
top view (xz-plane)
simple
dipole
Z. Ghassemlooy

8. Antenna – Real - Sdirected and Sectorized
Used for microwave or base stations for mobile phones (e.g., radio coverage of a valley)
side view (xy-plane) x y
side view (yz-plane) z
top view (xz-plane)
Directed
top view, 3 sector x z
top view, 6 sector
Sectorized
Z. Ghassemlooy

9. Antenna - Ideal - contd.
The power density of an ideal loss-less antenna at a distance d away from the transmitting antenna is:
Gt is the transmitting antenna gain
The product PtGt : Equivalent Isotropic Radiation Power
(EIRP)
which is the power fed to a perfect isotropic antenna to
get the same output power of the practical antenna in hand.
Note: the area is for a
sphere.
W/m2
Z. Ghassemlooy

10. Antenna - Ideal - contd.
The strength of the signal is often defined in terms of its Electric Field Intensity E, because it is easier to measure.
Pa = E2/Rm where Rm is the impedance of the medium. For
free space Rm = 377 Ohms..
V/m
Z. Ghassemlooy

11. Antenna - Ideal - contd.
The receiving antenna is characterized by its effective aperture Ae, which describes how well an antenna can pick up power from an incoming electromagnetic wave
The effective aperture Ae is related to the gain Gr as follows:
Ae = Pr / Pa => Ae = Gr2/4
which is the equivalent power absorbing area of the antenna.
Gr is the receiving antenna gain and  = c/f
Z. Ghassemlooy

12. Signal Propagation (Channel Models)
Z. Ghassemlooy

13. Basic Digital Communication System Model
Input
data
Pulse
generator TX
filter
channel
x(t)
Receiver
A/D +
Channel noise
n(t)
Output data
HT(f)
HR(f)
y(t)
Hc(f)
Tb: Symbol period
Z. Ghassemlooy

14. Fourier Representation of Periodic Signals1 1 t t
ideal periodic signal
real composition
(based on harmonics)

15. Signals II Different representations of signals
amplitude (amplitude domain)
frequency spectrum (frequency domain)
phase state diagram (amplitude M and phase  in polar coordinates)
Composite signals mapped into frequency domain using Fourier transformation
Digital signals need
infinite frequencies for perfect representation
modulation with a carrier frequency for transmission (->analog signal!)
f [Hz]
A [V] 
I= M cos 
Q = M sin 
t[s]

16. Channel Models High degree of variability (in time, space etc.)
Large signal attenuation
Non-stationary, unpredictable and random
Unlike wired channels it is highly dependent on the environment, time space etc.
Modelling is done in a statistical fashion
The location of the base station antenna has a significant effect on channel modelling
Models are only an approximation of the actual signal propagation in the medium.
Are used for:
performance analysis
simulations of mobile systems
measurements in a controlled environment, to guarantee repeatability and to avoid the expensive measurements in the field.
Z. Ghassemlooy

17. Channel Models - Classifications
System Model - Deterministic
Propagation Model- Deterministic
Predicts the received signal strength at a distance from the transmitter
Derived using a combination of theoretical and empirical method.
Stochastic Model - Rayleigh channel
Semi-empirical (Practical +Theoretical) Models
Z. Ghassemlooy

18. Channel Models - Linear
Finite delay
Constant
Linear channel
h(t) H(f)
y(t) = k x(t-td)
x(t)
No Noise
Therefore for a linear channel is:
Amplitude
distortion
Phase
distortion
The phase delay
Describes the phase delayed experienced by each frequency component
Z. Ghassemlooy

19. This can be modelled as an
ideal low pass filter
non-ideal low pass filter
This smears the transmitted signal x(t) in time causing the effect of a symbol to spread to adjacent symbols when a sequence of symbols are transmitted.
The resulting interference, intersymbol interference (ISI), degrades the error performance of the communication system.
Channel
Z. Ghassemlooy

20. Power Delay Profile RMS Delay Spread () = 46.4 ns
Received Signal Level (dBm)
-105
-100
-95
-90 50
100
150
200
250
300
350
400
450
Excess delay (ns)
RMS Delay Spread () = 46.4 ns
Mean Excess delay () = 45 ns
Maximum Excess delay < 10 dB = 110 ns
Noise threshold
Z. Ghassemlooy

21. Power Delay Spread - Example
0 dB
-10 dB
-20 dB
-30 dB  1 2 5
(µs)
Z. Ghassemlooy

22. Channel Models – Multipath Link
The mathematical model of the multipath can be presented using the method of the impulse response used for studying linear systems.
y(t) = k x(t-td)
y(n)= k x(n-N)
x(t) = (t)
x(n)= (n)
Linear channel
h(t) H(f)
h(n)
Channel is usually modeled as Tap-Delay-Line
Z. Ghassemlooy

23. Channel Models – Multipath Link
Time variable multi-path channel impulse response
Where a(t-) = attenuated signal
Time invariant multi-path channel impulse response
Each impulse response is the same or has the same statistics, then
Where aie(.) = complex amplitude (i.e., magnitude and phase) of the
generic received pulse.
 = propagation delay generic ith impulse
N = number signal arriving from N path
(.) = impulse signal (delta function)
Z. Ghassemlooy

24. Channel Models – Multipath Link
Channel transfer function
Multipath Time
Mostly used to denote the severity of multipath conditions.
Defined as the time delay between the 1st and the last received impulses.
Coherence bandwidth - on average the distance between two notches
Bc ~ 1/TMP
Z. Ghassemlooy

25.  RMS delay spread Bc Freq. domain view Time domain view
High correlation of amplitude
between two different freq.
components
Range of freq over
which response is flat Bc
 RMS delay spread
Freq. domain view
For 0.9 correlation
For 0.5 correlation
Z. Ghassemlooy

26. Power Delay Spread - Example
0 dB
-10 dB
-20 dB
-30 dB  1 2 5
(µs)
Signal bandwidth for Analog Cellular = 30 KHz
Signal bandwidth for GSM = 200 KHz
Z. Ghassemlooy

27. Multipath Delay
With a 52 meter separation, in a LOS environment, a large ns excess delay
NLOS excess delay over 423 meters extended to ns.
Office building: RMS delay spread = ns
Cell size (km)
Max Delay Spread
Pico cell
0.1
300 nn
Micro cell 5
15 us
Macro cell 20
40 us
Z. Ghassemlooy

28. Inter-symbol interference (ISI)
Channel is band limited innature
Limited frequency response  unlimited time response
Channel has multipath
Tx filter –
There are two major ways to mitigate the detrimental effect of ISI.
The first method is to design bandlimited transmission pulses (i.e., Nyquist pulses) which minimize the effect of ISI.
Equalization BS 1 2 0
Building MU
Z. Ghassemlooy

29. ISI
Symbol time
Pr()
4.38 µs
0 dB
-10 dB
-20 dB 
-30 dB  1 2 5
(µs) 1 2 5
(µs)
4.38
Symbol time > 10*  No equalization required
Symbol time < 10*  Equalization will be required to deal with ISI
In the above example, symbol time should be more than 14µs to avoid ISI.
This means that link speed must be less than 70Kbps (approx)
Z. Ghassemlooy

30. Propagation Path Loss The propagation path loss is LPE = LaLlf Lsf
where
La is average path loss (attenuation): (1-10
km),
Llf - long term fading (shadowing): 100 m
ignoring variations over few wavelengths,
Lsf - short term fading (multipath): over fraction
of wavelength to few wavelength.
Metrics (dBm, mW)
[P(dBm) = 10 * log[ P(mW) ]
Z. Ghassemlooy

31. Propagation Path Loss – Free Space
Power received at the receiving antenna
at far field only
Thus the free space propagation path loss is defined
as:
Isotropic antenna has unity gain (G = 1) for both
transmitter and receiver.
Z. Ghassemlooy

32. Propagation - Free Space–contd.
The difference between two received signal powers in
free space is:
If d2 = 2d1, the P = -6 dB i.e 6 dB/octave or 20 dB/decade
Z. Ghassemlooy

33. Propagation - Non-Line-of-Sight
Generally the received power can be expressed as:
Pr  d-v
For line of sight v = 2, and the received power
Pr  d-2
Average v over all NLOS could be
For non-line of sight with no shadowing, received
power at any distance d can be expressed as:
100 m< dref < 1000 m
Z. Ghassemlooy

34. Propagation - Non-Line-of-Sight
Log-normal Shadowing
Where Xσ: N(0,σ) Gaussian distributed random variable
Z. Ghassemlooy

35. Received Power for Different Value of Loss Parameter v
Distance (km) 10 20 30 40 50
-70
-80
-90
-100
-110
-120
-130
-135
Received power (dBm)
v = 2, Free space
v = 3 Rural areas
v = 4,
City and urban
areas
Z. Ghassemlooy

36. Propagation Model- Free Space
In terms of frequency f and the free space velocity of
electromagnetic wave c = 3 x 108 m/s it is:
Expressing frequency in MHz and distance d in km:
Z. Ghassemlooy

37. Propagation Model- Free Space (non-ideal, path loss)BS MU
Non-isotropic antenna gain  unity, and there are
additional losses Lad , thus the power received is:
d > 0 and L 0
Thus for Non-isotropic antenna the path loss is:
Note: Interference margin can also be added
Z. Ghassemlooy

38. Propagation Model- Free Space (non-ideal, path loss)
Considering loss S due to shadowing
Z. Ghassemlooy

39. Propagation Model - Mechanisms
Reflection
Diffraction
Scattering
Z. Ghassemlooy
Source: P M Shankar

40. Channel Model- Plan Earth Path Loss - 2 Ray Reflection
In mobile radio systems the height of both antennas (Tx. and Rx.) << d (distance of separation) hb dd dr hm
Direct path (line of sight)
Ground reflected
path d
From the geometry dd = [d2 + (hb - hm )2]
Z. Ghassemlooy

41. Channel Model- Plan Earth Path Loss - contd.
Using the binomial expansion
Note d >> hb or hm.
Similarly
The path difference d = dr - dd = 2(hbhm )/d
The phase difference
Z. Ghassemlooy

42. Channel Model- Plan Earth Path Loss– contd.
Total received power
Where  is the reflection coefficient.
For  = -1 (low angle of incident) and .
Z. Ghassemlooy

43. Channel Model- Plan Earth Path Loss– contd.
Therefore:
Assuming that d >> hm or hb, then
sin x = x for small x
Thus
which is 4th power law
Z. Ghassemlooy

44. Channel Model- Plan Earth Path Loss– contd.
Propagation path
loss (mean loss)
Compared with the free space = Pr = 1/ d2
In a more general form (no fading due to multipath),
path attenuation is
LPE increases by 40 dB each time d increases by 10
Z. Ghassemlooy

45. Channel Model- Plan Earth Path Loss– contd.
Including impedance mismatch, misalignment of antennas, pointing and polarization, and absorption The power ration is:
where
Gt(θt,φt) = gain of the transmit antenna in the direction (θt,φt) of receive antenna.
Gr(θr,φr) = gain of the receive antenna in the direction (θr,φr) of transmit antenna.
Γt and Γr = reflection coefficients of the transmit and receive antennas
at and ar = polarization vectors of the transmit and receive antennas
α is the absorption coefficient of the intervening medium.
Z. Ghassemlooy

46. LOS Channel Model - Problems
Simple theoretical models do not take into account many practical factors:
Rough terrain
Buildings
Refection
Moving vehicle
Shadowing
Thus resulting in bad accuracy
Solution: Semi- empirical Model
Z. Ghassemlooy

47. Sem-iempirical Model
Practical models are based on combination of measurement and theory. Correction factors are introduced to account for:
Terrain profile
Antenna heights
Building profiles
Road shape/orientation
Lakes, etc.
Okumura model
Hata model
Saleh model
SIRCIM model
Outdoor
Indoor
Y. Okumura, et al, Rev. Elec. Commun. Lab., 16( 9), 1968.
M. Hata, IEEE Trans. Veh. Technol., 29, pp , 1980.
Z. Ghassemlooy

48. Okumura Model
Widely used empirical model (no analytical basis!) in macrocellular environment
Predicts average (median) path loss
“Accurate” within dB in urban and suburban areas
Frequency range: MHz
Distance: > 1 km
BS antenna height: > 30 m.
MU antenna height: up to 3m.
Correction factors are then added.
Z. Ghassemlooy

49. Hata Model
Consolidate Okumura’s model in standard formulas for macrocells in urban, suburban and open rural areas.
Empirically derived correction factors are incorporated into the standard formula to account for:
Terrain profile
Antenna heights
Building profiles
Street shape/orientation
Lakes
Etc.
Z. Ghassemlooy

50. Hata Model – contd. The loss is given in terms of effective heights.
The starting point is an urban area. The BS antennae is mounted on tall buildings. The effective height is then estimated at km from the base of the antennae.
P M Shankar
Z. Ghassemlooy

51. Hata Model - Limits Frequency range: 150 - 1500 MHz
Distance: 1 – 20 km
BS antenna height: m
MU antenna height: 1 – 10 m
Z. Ghassemlooy

52. Hata Model – Standard Formula for Average Path Loss for Urban Areas
Correction Factors are:
Large cities
Average and small cities
Z. Ghassemlooy

53. Hata Model – Average Path Loss for Urban Areas contd.
Carrier frequency
900 MHz,
BS antenna height
150 m,
MU antenna height
1.5m.
Z. Ghassemlooy
P M Shankar

54. Hata Model – Average Path Loss for Suburban and Open Areas
Suburban Areas
Open Areas
Z. Ghassemlooy

55. Hata Model - Average Path Loss
S. Loyka, 2003,
Introduction to Mobile Communications
Z. Ghassemlooy

56. Improved Model
Hata-Okumura model are not suitable for lower BS antenna heights (2 m), and hilly or moderate-to-heavy wooded terrain.
To correct for these limitations the following model is used [1]:
For a given close-in distance dref. the average path loss is:
Lpl = A + 10 v log10 (d / dref) + s for d > dref, (dB)
where
A = 20 log10(4 π dref / λ)
v is the path-loss exponent = (a – b hb + c / hb)
hb is the height of the BS: between 10 m and 80 m
dref = 100m and
a, b, c are constants dependent on the terrain category
s is representing the shadowing effect
[1] V. Erceg et. al, IEEE JSAC, 17 (7), July 1999, pp
Z. Ghassemlooy

57. Improved Model Terrains Model Type A Type B Type C parameter
The typical value of the standard deviation for s is between 8.2
And 10.6 dB, depending on the terrain/tree density type
Terrain A: The maximum path loss category is hilly terrain with moderate-to-heavy tree densities .
Terrain B: Intermediate path loss condition
Terrain B: The minimum path loss category which is mostly flat terrain with light tree densities
Z. Ghassemlooy

58. Losses
Summary of penetration losses through various common building materials at 28 GHz. Both of the horn antennas have 24.5 dBi gains with 10
half power beamwidth [28].
Penetration losses for multiple indoor obstructions in an office environment at 28 GHz. Weak signals are denoted by locations where the SNR
was high enough to distinguish signal from noise but not enough for the signal to be acquired, i.e. penetration losses were between 64 dB to 74 dB
relative to a 5 m free space test. No signal detected denotes an outage, where penetration loss is greater than 74 dB relative to a 5 m free space test [28].
Z. Ghassemlooy

59. Reflection Coefficients
Comparison of reflection coefficients for various common building materials at 28 GHz. Both of the horn antennas have 24.5 dBi gains with 10 half power beam-width.
Z. Ghassemlooy

60. Summary
Attenuation is a result of reflection, scattering, diffraction and
reflection of the signal by natural and human-made structures
The received power is inversely proportional to (distance)v,
where v is the loss parameter.
Studied channel models and their limitations
Z. Ghassemlooy

61. Questions and Answers Tell me what you think about this lecture
Next lecture: Multi-path Propagation- Fading
Z. Ghassemlooy