The analysis requires data on building footprints.
Here, we'll use building footprints from OpenStreetMap. We can download vector data from OpenStreetMap with the QuickOSM plugin.
1. Download and install the QuickOSM plugin, if you don't have it yet.
2. Click the 
icon in the main toolbar to open the QuickOSM dialog.
3. If you use it for the first time, you need to click that you understand the copyrights.
4. In the QuickOSM dialog type building as key and leave value open.
5. Click the 
button to define the extent. You can use the layer extent of your point cloud data.
6. Expand Advanced section and uncheck the boxes before Points, Lines and Multilinestrings. In this way, we'll only download polygons.
Your dialog should now look like this:
7. Click 
to run the query and close the dialogue when the building have been downloaded.
The buildings are in a temporary format (note the 
icon). We need to save the layer and reproject it to the same projection as the point cloud layer.
8. Right-click on the building layer and choose Export | Save features as... from the context menu.
9. Save the file to an ESRI Shapefile and call it buildings.shp. Change the CRS to the one of your point cloud layer.
10. Click OK.
11. Click again OK if the popup to Select Datum Transformations appears. We'll just use the defaults here.
12. Right-click on the temporary building layer and choose Remove Layer... from the context menu. Click OK in the popup to confirm.
Now all input data is ready. Don't forget to regularly save your project.