Radio Propagation
4.1 Propagation Mechanisms
4.1.2 Propagation Models for Path Loss (Global Mean) Prediction
22 Introduction to Mobile Network Engineering
20 dB/decade
32 dB/decade
41 dB/decade Signal level,
dB
10 dB 20 dB Distance, km
20 dB
Open space Suburbian Urban
Figure 4.2 Path loss in a mixed environment.
Multipath propagation gives rise to a fine structure of signal fluctuations, which results in corresponding variations (fast fading) in the input signal to the mobile unit. Averaging with respect to the fast fading structure gives the local mean of the propagation path loss or received signal strength.
In a typical propagation situation there are considerable shadow effects. The propa- gation loss with respect to the local mean is generally much greater than when there is free-space propagation, owing to the strong influence of the terrain. This influence manifests itself partly as an increase in the average (‘global’) propagation loss and partly as a varying loss due to the irregularity of the terrain. The large-scale effects can be esti- mated from data on the terrain and built up areas. The result of the calculation is the global mean. However, the detailed structure causes random variations in the local mean of the signal received by the mobile unit (called shadow fading, slow fading orlognormal fading).
Following this discussion, it is useful to divide propagation loss into three components:
theglobal meanof the propagation loss, that is the deterministic part, and two random elements resulting from shadow fading (slow fading) and multipath propagation (fast fading), see Figure 4.3. The global mean of the received signal power is typically inversely proportional to the fourth power of the propagation distanceP∼R−n(n=4), wheren is the propagation exponent.
global mean
average level tens of
wavelength Slow log-normal
fading
average level Fast Rayleigh fading
λ2 Pathloss
Distance Figure 4.3 Three components in path loss.
EIRP
Path loss Signal level
Distance Tx Antenna/feeder
gain & loss, Lf Pt
EIRP
Rx Antenna/feeder
gain & loss, Lf Pr (Pmin)
Figure 4.4 Basic components of link budget.
Introducing losses in the antenna-feeder system for transmitter and receiver,Lftand Lfr, respectively, the output power at antenna connectorPt, one may obtain a more gen- eral equation for EIRP that includes feeder losses:
EIRP=Pt+Gt−Lft, dBm (4.14)
in dB notation.
The received powerPrat the receiver input takes propagation path lossLinto account and is given by
Pr=Pt+Gt−Lft−L+Gr−Lfr, dBm (4.15)
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Similar to transmitter EIRP, one can define a ‘design level’ of minimal received power at the receiving antenna input,Pmin. Then the relation between acceptable path loss and design level is as follows:
L=MAPL=EIRP−Pmin (4.16)
The main goal of propagation modelling is to predict path lossL as accurately as possible, allowing the range of radio system to be determined before installation. The maximum rage of the system occurs when the received power drops belowPmin, a level that provides acceptable communication quality. The value ofLfor which this power level is received is the maximum acceptable path loss,MAPL.
A general model for propagation loss is very often semi-empirical and intended to provide a general estimate of radio propagation based on nominal characteristics rather than specific path data. During the radio planning process, any generic model is cali- brated based on specific land-usage data and measurements.
The tuning of propagation model involves determination different coefficient values in the propagation equation so that the residual RMS value of the error reaches the global minimum; that is, the lowest possible value. General semi-empirical propagation equation is as follows:
Pr=Pt+Gt+Gr−L=Pt+Gt+Gr+CCT+Cdlog(R) +Cdhlog(R)log(hB) +Chlog(hB) +CdkKdk+CdrKdr+CClKCl (4.17) where:
Pr = received power, dBm Pt = transmit power, dBm
CCT = the fixed correction term that accounts for effects of frequency and other non-specific factors of the model
R = the distance from transmitter, km;
hB = the base-station antenna height, m;
Kdk = a knife-edge diffraction loss;
Kdr = a rounded-hill diffraction loss and KCl = the clutter loss.
Table 4.1 Pre-set values for correction coefficients in semi-empirical equation for propagation loss.
Parameter Initial Value
Cd −44.9
Cdh 6.55
Ch 0
Cdk −0.5
CCl 1
The parametersCCTandKClcombine to give a power level at the distance of 1 m. The initial value is normally zero and is then tuned to ensure it provides an overall RMS error of about 6–7 dB for the entire data set for each base station and the whole clutter.
The parameters in (4.17) are defined in Table 4.1.
Apparently, the optimal set of parameters and coefficients in equation (4.17) produce a global minimum of prediction errors only for specified clutter or land-usage type. The method based on the tuning of the equation (4.17) model involves dividing the pre- diction area into a series of clutter and terrain categories; namely, open, suburban and urban. These are summarized as follows:
• Open area: Open space, no tall trees or buildings in the path, a plot of land cleared for 300–400 m ahead; for example, farmland, open fields.
• Suburban area: Village or highway scattered with trees and houses, some obstacles near the mobile but not very congested.
• Urban area: Built up city or large town with large buildings and houses with two or more storeys, or larger villages with close houses and tall, thick tree cover.
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