Free Space Propagation Model 2q1k1n

Mobile Radio Propagation – Large Scale Path Loss Free Space Propagation Model

What

are reasons why wireless signals are hard to send and receive?

 The

mobile radio channel places fundamental limitations on the performance of wireless communication systems

 Wireless

transmission paths may be: * Line-of-Sight [LOS] * Non Line-of-Sight [NLOS] : Obstructed by buildings, mountains, and foliage

 Radio

channels are extremely difficult to analyze (time varying)

 Modeling

random

and

radio channels have been one of the difficult paths of mobile radio system design.

Propagation Mechanism.. The

radio, microwave, infra-red and visible light portions of the electromagnetic spectrum can all be use to transmit information. Information can be sent by modulating the Amplitude, Frequency or Phase of the wave

Properties of Radio Waves Are

easy to generate Can travel long distances Can penetrate buildings May be used for indoor and outdoor communication Are omni-directional-can travel in all directions Can be narrowly focused at high frequencies (greater than 100 MHz) using parabolic antenna (like satellite dishes)

Properties of Radio Waves… Frequency

dependence Behave more like light at higher frequencies ◦ Difficult to ing obstacles ◦ More direct path ◦ Absorbed by rain Behave

more like radio wave at lower frequencies ◦ Can obstacles ◦ Power falls off sharply with distance from sources

Subject

to interference from other radio wave sources

Problems Unique to Wireless systems Interference

from other service

providers Interference from other s (same network) ◦ CCI due to frequency reuse ◦ ACI due to Tx/Rx design limitations & large number s sharing finite BW Shadowing

◦ Obstructions to line-of-sight paths weak received signal strength

Fading ◦ When no clear line-of-sight path exists, signals are received that are reflections off obstructions and diffractions around obstructions ◦ Multipath signals can be received that interfere with each other ◦ Fixed Wireless Channel → random & unpredictable  must be characterized in a statistical fashion  field measurements often needed to characterize radio channel performance

Propagation Models Propagation

models – Focused on predicting the average received signal strength at a given distance from the transmitter. Signal strength in close spatial proximity to a particular location. Propagation models that predict the mean signal strength for an arbitrary transmitter –receiver [T-R] separation distance are useful in estimating the radio coverage are of a transmitter.

Propagation

models that characterize the rapid fluctuations of the received signal strength over very short travel distances (or) short time duration are called- Small Scale fading.

Free Space Propagation Model Free

space propagation model – transmitter and Receiver have a clear line of sight (LoS) path between them. * Satellite Communication * Microwave Link Free space propagation – Received power decays as function of the T-R separation distance.

The

free space power received by a receiver antenna which is separated from a radiating transmitter antenna by a distance d, given by the Friis free space equation Pr(d) = (PtGtGrλ2)/((4π)2d2L)

Pt

- Transmitted Power Pr - Received Power

Gt

- Transmitter Antenna Gain

Gr

– Receiver Antenna Gain

 L

d – T-R Separation distance

– System loss factor not related to propagation (L≥1)  λ – Wavelength meters

Antenna Gain Gain

of an antenna is related to its effective aperture Ae (i.e) G = 4πAe/ λ2 Ae – Physical size of antenna

λ

is related to carrier frequency λ = c/f = 2πc/wc f is the carrier frequency Wc is the carrier frequency in radians per second C – speed of light meters/sec

The

miscellaneous losses L (L≥1) are usually due to transmission line attenuation, filter losses, and antenna losses in the communication systems. Where L =1 indicates no loss in the system hardware. Friis free space equation shows that the received power falls off as the square of T-R separation distance. Received power decays with distance at a rate of 20dB/decay.

An

isotropic radiator is an ideal antenna which radiates power with unit gain uniformly in all direction. The effective isotropic radiated power (EIRP) is define as EIRP = PtGt In

practice effective radiated power (ERP) is used instead of EIRP.

Path loss The

path loss – Difference (dB) between the effective transmitted power and the received power – may or may not include the effect of the antenna gains.

PL(dB)

= 10log (Pt /Pr )= -10 log [(GtGrλ2)/ ((4π)2d2)]

When

antenna gains are excluded then PL(dB) = 10log (Pt /Pr )= -10 log [λ2/ ((4π)2d2)]

Friis

free space model is only a valid predictor for Pr for values of d which are in the far-field of the transmitting antenna. The far-field (or) Fraunhofer region of a transmitting antenna is defined as the region beyond the far-field distance df df D-

=2D2/ λ Largest physical dimension of

Far-field

region df must satisfy

df>>D

and df >> λ

Frirs

free space model equation does not hold for d=0. Large scale propagation model use a close in distance d0 – received power reference point. The received power Pr (d), at any distance d>d0

The

received power in free space at a distance greater than d0 is given by Pr (d) = Pr (d0 )(d0 /d)2 d≥d0 ≥ df Received power level in dBm or dBW  Pr (d)dBm = 10log[Pr (d0)/0.001W]

+20log(d0/d ) The reference d0 for practical system 1m in indoor environments 100m to 1Km in outdoor environments