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 /...