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iBwave Propagation Models
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iBwave Propagation Models Fast Ray Tracing (FRT) is an extremely powerful propagation model. If used in free space, (amongst many other things) it can be set-up to model antenna contour with different colors for different signal levels. The following document describes the comparison and the required steps to set-up iBwave Design so that path loss exponent (PLE) and fast ray tracing (FRT) show the same results.
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Introduction Path Loss Exponent: Path Loss Exponent is a popular way of calculating the path loss of an RF signal, defining at the same time what is commonly known as “antenna contour”. The formula for the calculation is as follows:
Where n is called the exponent factor and (sigma) the standard deviation. If n is equal to 2 and sigma equal to 0, it is equal to free space path loss calculations. Often we find values for n and sigma in literature for different building types and frequencies, so for instance n=2.4; =9.6 for an office with soft partitions at 900 MHz. Sigma can be set as a margin. Fast Ray Tracing: Fast Ray Tracing is a sophisticated way of calculating path loss (Propagation Module req uired) in environments other than free space. It uses a 3D model of t he environment where the materials were specified to have certain RF-losses (and other details) for different frequencies. Further below, we have highlighted the required steps to set it up such that it can emulate the above path loss exponent formula, so that eventually an antenna contour can be d isplayed, ideally with different colors for different RF values. For FRT the calculated loss for all areas that have line of sight (and obstacles) between the transmitter and the receiver the calculation is done as a function of:
Where the path loss exponent Y1 (Gamma 1) is applied to the direct line of sight path only. Increasing this value increases the path loss. In fact, it has the s ame impact on the FRT calculation as n for PLE. For FRT, we can also set a safety margin and include this in the downlink propagation calculations when using FRT, the selection can be made when creating the output map, see following screen shot:
Depending on the technology, these parameters can be set under “Project Properties”, “Calculations”, “Prediction”, “Network Optimisation”, as shown in the following screenshot for an WCDMA example
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The total value for Body Loss, Fast Fading Margin and antenna gain reflects margins that correspond to sigma in PLE. So, in short, if we set Gamma1 in FRT to the same value as n in PLE and Fast Fading Margin in FRT to the same value as –sigma in PLE, you will get the same results. Last but not least, we have to mention that FRT takes the real radiation pattern of the antenna into consideration, which can cause discrepancies when using antennas with a radiation pattern other than an ideal omni and also in the vertical pattern, when looking at circles for omni antennas (antenna contour). Example:
The following shows an example for a comparison for PLE and FRT to achieve the same results, looking at a 2100 UMTS WCDMA signal with an antenna contour of -85 dBm in purple for PLE, using purple to green colour transition for this value for FRT. Also, using an omni antenna with the following radiation pattern:
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Prediction height for this example is set equal to component height to minimize the impact of the vertical pattern. PLE settings:
FRT settings:
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1. Wall loss factor 1 (p1) : Multi wall loss correction factor. Both p1 and p2 (see next parameter) are used to reduce the multi-wall penetration loss in the direct path only. Thus, increasing p1 will decrease the multi-wall penetration loss. The principle of applying p1 and p2 is very similar to the Reduction factor parameter in the COST 231 model, where the reduction factor reduces the attenuation effect every time a new wall is being penetrated by a signal path. Thus, the first wall encountered by a signal path has a higher impact on the attenuation of the signal. 2. Wall loss factor 2 (p2) : Multi wall loss correction factor. Both p1 (see previous parameter) and p2 are used to reduce the multi-wall penetration loss in the direct path only. Thus, increasing p2 will decrease the multi-wall penetration loss. 3. DLOS (Y1): The path loss exponent Y1 (Gamma 1) is applied to the direct line of sight path only. Increasing this value increases the path loss. 4. RP (Y2): The path loss exponent Y2 (Gamma 2) is applied to the reflected paths only. Increasing this value increases the path loss. 5. DNLOS (Y3): The path loss exponent Y3 (Gamma 3) is applied to the direct non-line of sight path. Increasing this value increases the path loss.
Result:
A slight offset can be seen due to resolution, but it is obvious from the above, that the purple circle (antenna contour using PLE) is equal.
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