First of all you have to find the signal strengths [of individual BTS] using some propagation prediction tool. A propagation prediction tool simply models the radio channel between a BTS and a receiver (mobile station, user equipment etc.) and then evaluates the signal strength of the individual BTS at each of the receivers using a propagation model (or path loss model).

Now there are three types of propagation models:

1. Empirical models: based on equations derived from massive measurements campaign. Pros: Simple to use, easy to compute, requires less knowledge of the environment. Cons: Less accurate.

e.g. Power law model, Okumura-hata models, Cost 231-Hata model (extension of Okumura hata model). Mostly used now days by radio network planners in Urban and sub-urban environments (outdoor only)

2. Basically same as the empirical models but to some extent also takes the layout of the propagation environment into consideration. It models some aspects of the propagation mechanism e.g. 'diffraction' analytically. Pros: More accurate than empirical models. Cons: a little more detail about the propagation model is required. In some instances the semi-empirical model is shown to have underestimated the signal strengths as compared to empirical model.

e.g. Cost231-Walfish-Ikegami model, Allsebrook Parson model, Flat-edge model etc.

Can be used to model propagation in a dense urban environment.

Note: The above two models have some major limitations, for instance, the models only consider the 2 dimensional environment, and therefore cannot be used to model a radio channel in a 3D propagation environment e.g. signal strengths on different indoor floor levels etc.

To tackle this problem we use deterministic models.

3. Deterministic model (based on ray tracing) - It provides analytical model for the radio wave. The model adapts to the propagation environment by using advanced Ray optic methods. These types of models are well suited for modelling, Outdoor, Outdoor-Indoor, Indoor-Outdoor, and Indoor propagation channels.

Pros: Very accurate

Cons: You need to have a detailed digital map of the propagation environment. The downside is that its computationally intensive and very time consuming.

The accuracy of the deterministic model depends on:

1. the accuracy a digital map (how much fine details about the environment it has).

2. the maximum number of allowed propagation mechanisms (reflections, diffraction and penetrations) and single ray can pass through.

These models are seldom used by radio network planners as they are time consuming and computationally intensive.

So the choice of propagation model depends upon your requirements.

I use Wireless Insite by Remcom, (its a proprietary software and not free). There used to be an academic version of a software called 'RPS simulator' by Actix which was free to download, but I guess they have stopped distributing that.

Ok now back to your question;

So once you select an appropriate model you can find the signal strengths of individual BTS.

Then for the coverage plot; first of all you need to find the best serving BTS at a certain location/point on a map and then by comparing the signal strengths of the individual BTS (if the criteria for best server is based on highest signal strength) you select the BTS with highest signal strength as the best server.

Now the question remains; how to plot the coverage of the BTS. In wireless insite what I do is, I uniformly spread the receiver points (mobile stations, User equipment etc.) across the map, equally spaced apart e.g. 1 m for each other. After running the propagation prediction, I get the signal strengths of individual BTS on each of these points along with the coordinates (x,y) of these receiver points. Then I import those raw data values along with the coordinates to Matlab. Process the raw data for finding the best server and then use scatter function to plot the coverage map of the individual base stations.

Depending on some tools, you might have this facility built-in to the tool. Actix RPS has this option of plotting the best serving map once you compute the signal strengths of the BTS.

Hope this helps.