What is a Propagation Model

What is a propagation model?

• A mathematical model used by computer planning tools to predict coverage from a radio transmitter.– Typical inputs• 3D terrain data• Land use data (Clutter)• Building outlines• Building heights

Propagation Modeling for UMTS

Model Types (1)Empirical (Statistical)

• Well understood. 2G standard.• Lower computational overhead.• Wide variety available.• Less accurate in small urban cells. • Subjective clutter classification.• Only accurate in environment where

measurements were taken.

Deterministic (Ray tracing)• Few proven models available• Higher computational overhead• More map data required• More suited to small urban sites• Higher accuracy• Higher resolution


Street Canyon• More map data required• More suited to microcells

Model Types (2)• In practice, few planning tools use a single mode of model and most are a

hybrid of these methods.• Macrocell models are normally based on statistical techniques.• A large variety of Microcell models have been proposed.


Statistical Models• Carrier wave measurements are

made from test transmitters.• The measurements are plotted vs.

log(distance).• A straight line is fitted through the

data.• A basic y=mx +c formula can be

used to estimate path loss.• The formula can be modified to

account for other factors eg. Tx height, Rx height & terrain effects. Plot of measurements vs. log(distance)


Deterministic Models• The planning tool traces rays from

each site through an accurate 3D representation of an urban area.

• Typically a major component of the calculated path loss is free space loss.

• The effect of reflections diffraction and absorption due to buildings can be incorporated into the model.

Reflected ray Diffracted ray


Popular Macrocell Models• Okumura - Hata

– Industry standard.– Not well suited for ranges under 1km.– Upper frequency limit 2GHz.

• Wavecall– Not so well accepted.– Fully deterministic no calibration required.– Suitable for urban macro and microcells.– Available for many common planning systems.


Popular Microcell ModelsPropagation Modeling for UMTS

Prediction Model Method Used Parameters 2D 3D Over Roof-top Required Data ResultsUniversity of Lund Dual Slope

Empirical 2D Building

LayoutPath Loss

University of Karlsruhe Ray Launching Reflected +Penetrated Rays

2D BuildingLayout

Path Loss + CIR

CNET Ray Tracing9 Reflections +Ground + Street

Diffraction

2D BuildingLayout Path Loss

Swiss Telecom Ray Tracing Reflection +Diffraction

2D BuildingLayout

Path Loss + CIR

Ericsson Mathematical WalfischIkegami Method

2D BuildingLayout

Path Loss

Telecom Street Canyon Walfisch BertoniMethod

2D BuildingLayout

Path Loss

Walfisch Ikegami Semi -Empirical WalfischIkegami Method

2D BuildingClasses

Path Loss

University of Valencia Ray TracingScattering +Diffraction

Walfisch BertoniMethod

2D BuildingLayout +

Building Height

Path Loss

CNET Ray Launching Reflection +Diffraction

3D BuildingLayout

Path Loss + CIR

ASCOM ETH Ray TracingReflection fromWall and Street

2D BuildingLayout +

Building HeightPath Loss + CIR

University of Stuttgard Ray Launching6 Reflections + 2

Diffractions 2D Building

Layout +Building Height

Path Loss + CIR

University of Karlsruhe Ray TracingThe Most

Dominant Raysare Considered

2D Building

Layout +Building Height

Path Loss + CIR

Map data requirements (1)• The minimum requirements for

map data are:– Height– Clutter

• Some propagation models also require building information:– Building vectors– Building rasters


Map data requirements (2)

• The resolution of map data will depend on several things.– Budget

• You can pay as much as you like for map data.• Better data requires deeper pockets.

– Area type• Urban areas warrant higher resolution data (1 - 50 m).• Rural areas warrant lower resolution data (50 - 200 m).

– Model type• Microcell models require better data than Macrocell models.• Macrocell models typically use 20 - 200 m.• Microcell models typically require 1 - 20 m.


Carrier Wave (CW) Measurements• CW measurements are accurate

radio measurements used to calibrate propagation models.

• A number of temporary test sites are used for the test transmissions.

• Signal strength measurements and GPS fixes are made along predefined routes.

• These measurements must be averaged before they can be used for model calibration.

GPS


CW Test Sites• A typical network would require 3 different propagation models eg.

– Urban– Suburban– Rural

• Each model would require measurements from 10 - 15 test sites for calibration.

• The test sites used for propagation modeling should:– Be representative of typical cellular sites.– Should be free of obstructions.


CW Test Routes• CW test routes must be planned carefully.• Routes must avoid:

– Elevated sections of road.– Cuttings.– Tunnels.– Bridges.

• Sufficient measurements must be made in each clutter type for the model to be valid.

• Typically the distances driven for each site would be in the order of– 80km per urban test site.– 160km per rural test site.


CW Measurement Equipment

• Measurements should be distance based.– Measurements should be taken no closer than 0.38l.

• Equipment can be:– Distance triggered.– Time triggered.

• GPS outputs position every 1 second.– Position interpolation is required for each

measurement.• Test mobile measurements are NOT suitable.


Model Calibration Exercise

• In this exercise you will calibrate an empirical propagation model at 900 MHz.

• The model calibration process will not change at 2.2GHz.– Start Enterprise.– Log into the “UMTS #16” database.• UserId = “demouser”• Password = “demouser”

– Start the “Model Cal” Project.


Create a UMTS Propagation Model• Add a new propagation model.

– Type - Standard macrocell– Name 900MHz

• Set up a propagation model with the default parameters.


Parameter SettingModel Type Standard MacrocellFrequency 900

Mobile Rx Height 1.5Effective Earth Radius 8491.2

K1 135K2 38K3 -2.55K4 0K5 -13.82K6 -6.55K7 0.7

Eff. Ant. Height RelativeDiffraction Epstein Peterson

Merge knife edges 0

Load a CW measurement file• Open the CW measurement analysis window.• Use the Add button to load the CW file.• Use the options button to check the CW measurement options.

– Model•20m resolution.•900 MHz prediction model.

– Filter•Deselect all clutter types.•Radius 0 » 100000m.•Signal -110dBm » -40dBm.•Visibility: Check both boxes.

– Display•Check the “Overall Summary” and “Clutter Summary” boxes.


Perform a preliminary analysis (1)• Press the Analyse button.• Identify which clutter types are

invalid due to insufficient measurements.– In this case assume clutter types

with fewer than 100 samples are invalid.

– In practice the actual threshold would be much higher than this.


Perform a preliminary analysis (2)• Open the “CW measurement

options” window. Highlight all the invalid clutter types on the “Filter” tab.– These clutter types will be

excluded from further analysis.– The final propagation model will

not be valid for these clutter types.

– This is one reason why careful selection of the drive route is important.

• Perform the analysis a second time to check that only valid clutter types remain.


Perform a preliminary analysis (3)• Use the “Graph” button to plot:

– Received Level vs log(distance)– Error vs log(distance)

• These can be used to spot potential problems with CW measurement files.

• The example here shows a potential problem possibly caused caused by collecting data in underpasses and road cuttings. Such measurements should be removed prior to calibration.


Model Calibration (1)• The propagation model can now be calibrated.• Mostly a trial and error process.

– Change one model parameter at a time.– Re-analyse the modified propagation model and see if the model improves or

deteriorates.– Repeat the previous two steps until the error has been minimised for that

parameter.– Moving on to another parameter.– Changing one parameter might affect the optimum value of another

parameter so when all parameters have been calibrated the process might need repeating until convergence has been reached.


Model Calibration (2)• There are different approaches to model

calibration.• For this exercise follow the flowchart.• K3 and K4 are not altered. This is

because they relate to mobile height which in a typical cellular system is constant making these coefficients redundant.

• This process can be extremely laborious. A couple of shortcuts are possible though.– K1 can be derived form the overall

mean error– Clutter offsets can be derived from the

mean error of each clutter type.


Repeat calibration of K factors

K2

K1

Effective Antenna Height Algorithm

K6

K7

K5

Diffraction Algorithm

Clutter Offsets

Start

Finish

Model Calibration (3)

• The model calibration process can be extremely laborious.

• A couple of shortcuts are possible though.– K1 can be derived from the overall mean error.– A clutter offset can be derived from that clutter

types mean error.• A good propagation model will have a

standard deviation error of 6-8 dB.


Documents

What is a Propagation Model