1. Lecture 2 - EE 359: Wireless Communications - Fall 2009
Signal Propagation. Path Loss Models.
Lecture Outline
ā¢ Overview of Signal Propagation
ā¢ Free Space Path Loss Model
ā¢ Ray Tracing Path Loss Models
ā¢ Simpliļ¬ed Path Loss Model
ā¢ Empirical Path Loss Models
1. Signal Propagation Characteristics:
ā¢ Path loss: power falloļ¬ relative to distance
ā¢ Shadowing: random ļ¬uctuations due to obstructions
ā¢ Flat and frequency selective fading: caused by multipath
2. Transmitted and received signals:
ā¢ Transmitted Signal:, with power Pt, is s(t) = {u(t)ej(2Ļfct)} = sI(t) cos(2Ļfct + Ļ0) ā
sQ(t) sin(2Ļfct + Ļ0), where u(t) = sI(t) + jsQ(t) is the complex baseband signal with band-
width B, fc is the carrier frequency, and Ļ0 is the initial phase. For simplicity we assume u(t)
real for propagation model analysis.
ā¢ Received signal:r(t) = {v(t)ej(2Ļfct)}, where v(t) = u(t)āc(t) for c(t) the baseband channel
model.
ā¢ Doppler frequency shift fD = (v/Ī») cos(Īø) may also be introduced in the received signal
3. Free space path loss model:
ā¢ Typically used for unobstructed LOS signal path.
ā¢ Received signal is
r(t) = {
u(t)
ā
GlĪ»ej2Ļd/Ī»
4Ļd
ej(2Ļfct+Ļ0)
}
with received power
Pr = Pt
ā
GlĪ»
4Ļd
2
.
ā¢ Power falls oļ¬ proportional to the ratio of wavelength over distance squared. The inverse
frequency dependence is due to the eļ¬ective aperture of the receiver.
ā¢ Power falls oļ¬ proportional to net antenna gain Gl.
ā¢ Model not accurate for general environments.
2. 4. Ray tracing approximation:
ā¢ Represent wavefronts as simple particles.
ā¢ Simplest models only determine reļ¬ections.
ā¢ Scattering and defraction can also be included.
5. Two path model:
ā¢ One LOS path, one reļ¬ected path.
ā¢ At small distances, power falls oļ¬ proportional to d2 (free space loss on both paths).
ā¢ Above some critical distance dc, received power given by
Pr ā Pt
ā
Glhthr
d2
2
.
ā¢ Above dc, power falls oļ¬ proportional to d4 and is independent of signal wavelength (frequency)
ā¢ Model not generally accurate for cities or indoors.
6. General Ray Tracing:
ā¢ Incorporates all signal components: reļ¬ections, scattering, and diļ¬raction.
ā¢ Reļ¬ected rays have power falloļ¬ proportional to d2 by free space path loss model. Scattered
and refracted rays have power falloļ¬ that depends on exact distance of scattering or refractive
object from transmitter and receiver.
ā¢ If objects are more than a few wavelengths from receiver, typically neglect scattering and
refraction.
ā¢ Most computer packages for channel simulation in indoor/outdoor environments use general
ray tracing for path loss.
ā¢ Model requires detailed site information.
7. Simpliļ¬ed path loss model:
ā¢ Capture main characteristics of ray tracing using simpliļ¬ed model Pr = PtK d0
d
Ī³
, where K
is a constant factor (Pr(d0)/Pt), d0 is a reference distance, and Ī³ is the path loss exponent.
ā¢ Path loss exponent is function of carrier frequency, environment, obstructions, etc. Typically
ranges from 2 to 8 (at around 1 GHz).
ā¢ Model captures main characteristics of ray tracing: good for high-level analysis.
8. Empirical Models:
ā¢ Irregular terrain, like in cities, doesnāt lend itself to simple analytical path loss models.
ā¢ Empirical path loss models based on extensive measurements.
ā¢ Local mean attenuation: empirical measurements of local received power.
ā¢ Okumura Model: Empirical model for irregular terrain.
3. ā¢ Hata Model: Analytical approximation to Okumura model.
ā¢ Cost 231 Extension to Hata Model: Extends Hata model to 2 GHZ and to lower mobile
antenna heights. Widely used in 2G simulations.
ā¢ Piecewise linear models capture multiple slopes associated with path loss.
ā¢ Models not accurate and donāt generalize well to other environments.
Main Points
ā¢ Path loss models simplify Maxwellās equations. The models vary in complexity and accuracy.
ā¢ Power fallof with distance is proportional to d2 in free space model, d4 in two path model.
ā¢ General ray tracing requires detailed site speciļ¬c information. Typically generated with computer
packages.
ā¢ Main characteristics of ray tracing models captured in simpliļ¬ed path loss model.
ā¢ Empirical models widely used to study cellular system performance via simulation. Typically
awkward for analysis.