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RF penetration demonstration

During infantry training, soldiers are shown firsthand the impact of different weapons upon different materials to help them make better decisions about good cover versus bad cover. Spoiler: The railway sleeper doesn’t make it 🙁

As tactical radios have moved several hundred megahertz up the spectrum from their cold-war VHF roots, material attenuation is a serious issue which needs demonstrating to enable better route selection and siting. Unlike shooting at building materials it’s hard to visualise invisible radio signals, and therefore teach good siting, but equally important as ground based above-VHF signals are easily blocked in urban environments.

This blog provides a visual demonstration of the physical relationship between different wavelengths and attenuating obstacles only. It does not compare modulation schemas, multi-path, radios or technologies.

Bricks and wavelengths

Clutter data refers to obstacles above the ground such as trees and buildings. Cloud-RF has 9 classes of clutter data within the service which you can use and build with. Each class (Bricks +) has a different attenuation rate measured in decibels per metre (dB/m). This rate is a nominal value based upon the material density and derived from the ITU-R P.833-7 standard and empirical testing with broadcast signals in European homes.

A signal can only endure a limited amount of attenuation before it is lost into the noise floor. In free space attenuation is minimal but with obstacles it can be substantial. This is why a Wi-Fi router in a window can be hard to use within another room in the house but the same router is detectable from a hill a mile away.

The attenuation rate is an average based upon a hollow building with solid walls.

Common building materials attenuate signals to different amounts based on their density and the signals wavelength.

A higher wavelength signal such as L band (1-2GHz) will be attenuated more than VHF (30-300MHz) for example.

A long wavelength signal like HF will suffer minimal attenuation making it better suited to communicating through multiple brick walls.

The layer cake house

A brick house is not just brick. It’s bricks, concrete blocks, glass, insulation, stud walls, furniture and surfaces of varying absorption and reflection characteristics. Modelling every building material and multi-path precisely, is possible, given enough data and time due to the exponential complexity of multi-path but wholly impractical.

A trade-off for accurate urban modelling is to assign a local attenuation value. It’s local since building regulations vary by country and era so a 1930s brick house in the UK has different characteristics to a 1960s timber house in Germany. Taking the brick house we can identify the nominal value by adding up the materials and dividing it by the size.

For example, 2 x solid 10dB brick walls plus a 5 dB margin for interior walls and furniture would be 25dB. Divide this by a 10m size and you have 2.5dB/m. Using some local empirical testing you can quickly refine this for useful value for an entire city (assuming consistent architecture) but in reality the *precise* value will vary by each property, even on a street of the same design, due to interior layouts and furniture.

Range setup

We created nine 4 metre tall targets using each of the 9 clutter classes in attenuation order from left-to-right, measuring 10x10m and fired radio-bulletsTM at them from a distance of 300m using the same RF power of 1W.

The following bands were compared: HF 20MHz, VHF 70MHz, UHF 700MHz, UHF 1200MHz, UHF 2.4GHz. SHF 5.8GHz.

The ITU-R P.525 model was used to provide a consistent reference.

Only the stronger direct-ray is modelled. Multipath effects mean that reflections will reach into some of the displayed null zones, with an inherent reflection loss for each bounce, but these are nearly impossible to model accurately and in a practical time.

Here are the results.

HF 20MHz

VHF 70MHz

UHF 700MHz

UHF 1200MHz

UHF 2.4GHz

SHF 5.8GHz

Findings

  • Dense materials, especially concrete, attenuates higher frequency signals more than natural materials like trees
  • Lower UHF signals perform much better than SHF with the same power
  • Higher frequencies with low power can be blocked by a single house, even after only 300m
  • HF eats bricks for breakfast!

Summary

Modern tactical UHF radios, and their software eco-systems, are unrecognisable from their cold-war VHF ‘voice only’ ancestors in terms of capabilities but have an Achilles heel in the form of material penetration. To get the best coverage the network density must be flexed to match the neighbourhood.

This is obvious when comparing rolling terrain with a urban environment but the building materials and street sizes in the urban environment will make a significant difference too. Ground units which communicated effectively in a city in one country may find the same tactics and working ranges ineffective in another city with the same radios and settings. Understanding the impact of material penetration will help planning and communication.

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DIY clutter

DIY clutter
In this video, a Port in west Africa poorly served by high resolution data is enhanced with DIY clutter. The result shows substantial attenuation from the shipping containers which due to their dynamic nature would not be current in commercial data.

Summary

High quality clutter data is necessary for accurate radio planning but it’s not always available when and where you need it. Using the new ‘My clutter’ feature at CloudRF you can define your own and use it in seconds. The data can be layered on top of existing data, regardless of resolution, to enhance accuracy with material attenuation conforming to ITU standards for forests.

Clutter data

Clutter data in modelling refers to objects on the earth’s surface. In radio this is typically buildings and trees which attenuate signals. These must be factored in to deliver accurate predictions. It’s normally very expensive and the market for this data is worth billions due to demand by global telecommunications firms. This puts it out of reach of most small businesses and organisations.

Material attenuation

Different materials attenuate RF in different ways. The impact depends upon the wavelength (eg. WiFi can’t go through thick walls) and the material (concrete absorbs more RF than wood). For more on this subject see the land cover blog here.

How

Use the form in the ‘Model’ menu to either define your own polygons and lines or upload your own bulk clutter as a KML file containing polygons.

Why

Here’s a few reasons why DIY clutter is necessary:
  • Based on market pricing it would cost over a billion dollars to purchase ‘commercial’ clutter data for the earth.
  • Based on experience, the lead time for clutter in Africa can be 6 weeks.
  • The expensive clutter data is out of date by the time you buy it. Shipping containers, construction, transport will change and they affect RF coverage.
  • Commercial clutter data doesn’t let you model future construction projects eg. a new building
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Modelling trees and buildings

3D buildings Land Cover or ‘clutter’ data describes obstacles on the earth’s surface a radio wave will have to negotiate like trees or buildings. The Land cover data is layered on top of the terrain data which can be either a smooth(er) Digital Terrain Model (DTM) or a rougher ground-with-clutter ‘Digital Surface Model’ (DSM) . For DTM and DSM this will allow you to simulate attenuation from a forest or city where it might not otherwise be represented in the data resulting in much more accurate results. It also means you can enhance basic coarse terrain data with fresh high resolution land cover to reflect recent construction developments.

Obstacles and attenuation

Radio waves are attenuated differently by different materials like vegetation and man-made buildings. The impact varies by frequency with very short wavelength signals like WiGig at 60GHz struggling to penetrate a paper wall whilst a long wavelength VHF signal can breeze through multiple brick walls. For accurate modelling its essential that land cover is considered otherwise you run a risk of an unrealistic prediction which will bear little resemblance to real world results.

Trees

Trees attenuate differently with dense coniferous pine forests attenuating the most. An ITU standard, ITU-R P.833-7 “Attenuation in Vegetation” exists to describe the impact of a forest of different signals. There have been many academic studies into this subject but the summary of this standard for a mixed deciduous/coniferous forest is as follows:
Frequency MHzAttenuation dB/m
1060.04
4660.12
9490.17
18520.3
21180.34
No two forests are the same but if you err on the side of caution you can budget for their impact with a rule of thumb that 10m of mixed forest is equivalent to 2dB of attenuation at 1GHz, 4dB at 2GHz and 8dB at 4GHz. Trees are defined in the Land cover used by the system with attenuation values aligned to ITU-R P.833-7 which scale with wavelength so the same forest block will attenuate a WiFi signal more than a PMR446 signal. The resolution varies by region with 30m for CONUS, 100m for Europe and 500m for the rest of the world.

Buildings

Man-made buildings are even less predictable due to the variety in size, density and materials used. Many studies have been conducted into building attenuation but they are region specific due to construction materials and designs. A good reference is a UK paper by OFCOM which merges multiple research papers and has a useful table of attenuation by material and frequency on page 39.
MaterialAttenuation dB/m
at 1GHz
Attenuation dB/m
at 10GHz
Concrete24>50
Brick3232
Plasterboard11>50
Wood5>50
Glass344
Ceiling board110
Chipboard22>50
Floorboard4>50
The system currently has four classes of building attenuation for high to low intensity developments. The attenuation rate is 1% of the solid material attenuation rate (eg. Brick is 32dB/m so a brick house in CloudRF is 0.32dB/m) since most buildings are largely hollow.

Land cover data

To enhance DTM and DSM models with 3D clutter, Land Cover data can be layered on top of the terrain to apply representative attenuation. This Land Cover data has been sourced from a variety of sources with up to 30m resolution. The total possible resolution possible is determined by the highest resolution data so if you are in New York City for example where there is 2m LIDAR / DSM data available, your effective resolution will be 2m.

30m Digital surface model

30m Digital surface model plus 30m land cover

2m Digital surface model (LIDAR)

2m Digital surface model (LIDAR) plus 30m land cover

Propagation models

Propagation models vary in complexity from the simple ‘one liners’ like the free-space-path-loss model to the incredibly complex Irregular Terrain / Longley Rice model. Most models are simple and must be used within their parameter limits (especially with empirical ‘measured’ models) otherwise you could get wildly inaccurate results. A good example is the well known Hata model which was designed for elevated cellular base stations serving mobile subscribers which were lower than it. If you use this model at the bottom of a hill you can get some incredibly unlikely results as the simple model has no concept of terrain only A to B. By using Land cover, the output from these simple models can be enhanced greatly to provide a result which is sensitive to changes in the terrain along a given path, similar to how the ITM model works.
A UHF repeater at the foot of some hills with 20m DSM only. By default the Sleipnir engine will restrict coverage to line-of-sight for simple models like Hata.
With knife-edge-diffraction enabled, the Hata coverage is free to roam beyond line-of-sight. The coverage becomes very optimistic to the west up in the hills as Hata has no concept of terrain and expects a clear shot from the base station to the mobile station.
With knife-edge and 30m Land cover enabled, the optimistic Hata coverage is still free to roam beyond line-of-sight but is now severely constrained by the dense forests and urban developments without modifying the model itself.

Forest example

Modelling little forest blocks far away from your tower is easy with accurate DSM data but modelling a huge forest where your tower is within it is a harder problem. Heights are all defined as relative to the ground so if you have a 10m tall forest which is represented as raised earth in a DSM model and your tower is 12m tall you will end up with a tower which is in fact 22m above the ground – not ideal! Instead, when working with the 30m DSM you should define your height as the height above the canopy which is 2m. Here’s a comparison using 30m DSM and 30m Land cover in west Virginia.

Free space path loss prediction for an outdoor WiFi router, 30m DSM.

30m DSM plus 30m Land cover.

Urban example

To demonstrate the attenuation of buildings, this example has an emitter (LTE eNodeB on 800MHz) equidistant between a city and some countryside. The attenuation of the urban land cover becomes obvious once applied which contrasts with the open fields and water.

Free space path loss prediction for an LTE eNodeB, 30m DSM

Free space path loss prediction for an LTE eNodeB, 30m DSM with 30m Land cover

Summary

Land cover is essential for accurate planning and is now supported at 30m resolution. Coupled with high resolution data like 2m LIDAR, you can now accurately model attenuation of different materials in a cluttered environment. More land cover data is planned for the near future along with an upgraded ‘my clutter’ interface to allow you to define your own forests or housing developments for areas where data may not be available.