Radio Signal Path Loss
- summary, tutorial or overview about the essentials of radio signal path loss, its causes and prediction, and its use in radio coverage and wireless survey tools.
Radio signal path loss is a particularly important element in the design of any radio communications system or wireless system. The radio signal path loss will determine many elements of the radio communications system in particular the transmitter power, and the antennas, especially their gain, height and general location. The radio path loss will also affect other elements such as the required receiver sensitivity, the form of transmission used and several other factors.
As a result, it is necessary to understand the reasons for radio path loss, and to be able to determine the levels of the signal loss for a give radio path.
The signal path loss can often be determined mathematically and these calculations are often undertaken when preparing coverage or system design activities. These depend on a knowledge of the signal propagation properties.
Accordingly, path loss calculations are used in many radio and wireless survey tools for determining signal strength at various locations. These wireless survey tools are being increasingly used to help determine what radio signal strengths will be, before installing the equipment. For cellular operators radio coverage surveys are important because the investment in a macrocell base station is high. Also, wireless survey tools provide a very valuable service for applications such as installing wireless LAN systems in large offices and other centres because they enable problems to be solved before installation, enabling costs to be considerably reduced. Accordingly there is an increasing importance being placed onto wireless survey tools and software.
Signal path loss basics
The signal path loss is essentially the reduction in power density of an electromagnetic wave or signal as it propagates through the environment in which it is travelling.
There are many reasons for the radio path loss that may occur:
- Free space loss: The free space loss occurs as the signal travels through space without any other effects attenuating the signal it will still diminish as it spreads out. This can be thought of as the radio communications signal spreading out as an ever increasing sphere. As the signal has to cover a wider area, conservation of energy tells us that the energy in any given area will reduce as the area covered becomes larger.
- Absorption losses: Absorption losses occur if the radio signal passes into a medium which is not totally transparent to radio signals. This can be likened to a light signal passing through transparent glass.
- Diffraction: Diffraction losses occur when an object appears in the path. The signal can diffract around the object, but losses occur. The loss is higher the more rounded the object. Radio signals tend to diffract better around sharp edges.
- Multipath: In a real terrestrial environment, signals will be reflected and they will reach the receiver via a number of different paths. These signals may add or subtract from each other depending upon the relative phases of the signals. If the receiver is moved the scenario will change and the overall received signal will be found vary with position. Mobile receivers (e.g. cellular telecommunications phones) will be subject to this effect which is known as Rayleigh fading.
- Terrain: The terrain over which signals travel will have a significant effect on the signal. Obviously hills which obstruct the path will considerably attenuate the signal, often making reception impossible. Additionally at low frequencies the composition of the earth will have a marked effect. For example on the Long Wave band, it is found that signals travel best over more conductive terrain, e.g. sea paths or over areas that are marshy or damp. Dry sandy terrain gives higher levels of attenuation.
- Buildings and vegetation: For mobile applications, buildings and other obstructions including vegetation have a marked effect. Not only will buildings reflect radio signals, they will also absorb them. Cellular communications are often significantly impaired within buildings. Trees and foliage can attenuate radio signals, particularly when wet.
- Atmosphere: The atmosphere can affect radio signal paths. At lower frequencies, especially below 30 - 50MHz, the ionosphere has a significant effect, reflecting (or more correctly refracting) them back to Earth. At frequencies above 50 MHz and more the troposphere has a major effect, refracting the signals back to earth as a result of changing refractive index. For UHF broadcast this can extend coverage to approximately a third beyond the horizon.
These reasons represent some of the major elements causing signal path loss for any radio system.
Predicting path loss
One of the key reasons for understanding the various elements affecting radio signal path loss is to be able to predict the loss for a given path, or to predict the coverage that may be achieved for a particular base station, broadcast station, etc.
Although prediction or assessment can be fairly accurate for the free space scenarios, for real life terrestrial applications it is not easy as there are many factors to take into consideration, and it is not always possible to gain accurate assessments of the effects they will have.
Despite this there are wireless survey tools and radio coverage prediction software programmes that are available to predict radio path loss and estimate coverage. A variety of methods are used to undertake this.
Free space path loss varies in strength as an inverse square law, i.e. 1/(range)^2, or 20 dB per decade increase in range. This calculation is very simple to implement, but real life terrestrial calculations of signal path loss are far more involved.
Most path loss predictions are made using techniques outlined below:
- Statistical methods: Statistical methods of predicting signal path loss rely on measured and averaged losses for typical types of radio links. These figures are entered into the prediction model which is able to calculate the figures based around the data. A variety of models can be used dependent upon the application. This type of approach is normally used for planning cellular networks, estimating the coverage of PMR (Private Mobile Radio) links and for broadcast coverage planning.
- Deterministic approach: This approach to radio signal path loss and coverage prediction utilises the basic physical laws as the basis for the calculations. These methods need to take into consideration all the elements within a given area and although they tend to give more accurate results, they require much additional data and computational power. In view of their complexity, they tend to be used for short range links where the amount of required data falls within acceptable limits.
These wireless survey tools and radio coverage software packages are growing in their capabilities. However it is still necessary to have a good understanding of radio propagation so that the correct figures can be entered and the results interpreted satisfactorily.
Radio signal path loss can be caused by many factors. Only in the free space scenario is the calculation straightforward. In a terrestrial environment there are many factors that affect the actual RF path loss. When planning any radio or wireless system, it is necessary to have a broad understanding the elements that give rise to the path loss, and in this way design the system accordingly. ........
By Ian Poole
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