Cellular Network Plaaning

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  • Topic: Fading, Rayleigh fading, Link budget
  • Pages : 40 (3032 words )
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  • Published : December 12, 2012
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Cellular Network Planning
and Optimization
Part II: Fading
Jyri Hämäläinen,
Communications and Networking Department,
TKK, 17.1.2007

Outline
Modeling approaches
Path loss models
Shadow fading
Fast fading

2

Modeling approaches

3

Fading seen by moving terminal
Fast fading
Power

Modeling approach:
+20 dB

1. Distance between
TX and RX =>
path loss
2. Shadowing by
large obstacles =>
shadow fading
3. Multi-path effects
=> fast fading - 20 dB

Path loss

Lognormal
fading
Path loss

Time
4

Path Loss
Path loss is distance dependent mean
attenuation of the signal.
Once the allowed path loss of a certain system
is known we can solve the maximum distance
between transmitter and receiver and compute
the relative coverage area.
Suitable path loss model depends on the
environments (macro-cell, micro-cell, indoor)
Outdoor to outdoor models
Outdoor to indoor models
Indoor models
5

Shadow Fading
Shadow fading is used to model variations in
path loss due to large obstacles like buildings,
terrain conditions, trees.
Shadow fading is also called as log-normal
fading since it is modeled using log-normal
distribution
In cell dimensioning/link budget shadow fading
is taken into account through a certain margin
(=shadow fading margin)

6

Path loss + shadow fading

Signal strength in dB’s

Log-normal distribution

Path loss

Standard deviation e.g. +/-8 dB

Distance between TX and RX in logarithmic scale
7

Fast Fading
Fast fading is also called as multi-path fading since it is mainly caused by multi-path reflections of a transmitted waves by local scatterers such as human build structures or natural obstacles Fast fading occurs since MS and/or scatterers nearby MS are moving Signal strength in the receiver may change even tens of decibels within a very short time frame

Signal coherence distance = separation between locations where fast fading correlation is negligible. Signal coherence distance is half of the carrier wavelength
f = 2GHz => coherence distance = c/(2*f) =7.5 cm
Coherence time = time in which MS travels coherence distance Coherence time depends on MS speed.

In cell dimensioning/link budget fast fading is taken into account through a certain margin (=fast fading margin)

8

Fast Fading
Scatterers

a1 (t )e jφ1 ( t )

Especially the changes in
component signal phases
create rapid variations in
sum signal

a1 (t + t0 )e jφ1 ( t +t0 )

Sum signal at time t
Sum signal at time t+t0

S (t ) = a1 (t )e jφ1 (t ) + ... + a5 (t )e jφ5 ( t )

S (t + t0 ) = a1 (t + t0 )e jφ1 ( t +t0 ) + ... + a5 (t + t0 )e jφ5 ( t +t0 ) 9

Path loss models

10

Content
We recall first two important path loss models for macro- and micro-cell environments
I Model: Classical Okumura-Hata
Okumura-Hata is based on only few parameters but it works well and is widely used to predict path loss in macro-cell environments

II Model: COST 231 or Walfisch – Ikegami
This model is suitable for both macro- and micro-cell environments and it is mode general than Okumura-Hata. Walfisch – Ikegami models propagation phenomena more accurately but in cost of increased complexity.

Then we consider path loss in urban environment when both transmitter and receiver are below the rooftop (Berg model)
Outdoor to outdoor model
Path loss of RS – MS signal in street canyon II Model: BRT – BRT, NLOS (Berg model)

Finally, we discuss shortly on outdoor-to-indoor modeling
Terminology
ART= Above Roof Top
BRT = Below Roof Top
LOS = Line-of-Sight
NLOS = Non Line-of-Sight

11

General path loss model/outdoor
Outdoor path loss models are usually given in the form

L= A+ 10 ⋅ n ⋅ log 10 ( R )

(*)

(in decibels)

Here
R is the distance between TX and RX
A and n are constants. Values of these constant are
depending on the various parameters such as carrier
frequency, antenna heights etc

An other form for formula (*)

~n
~
L /...
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