This is accomplished through the use of dipole/point sources. There are two ways of accomplishing this.
First you could run multiple (many) simulations with randomized positioned and polarized sources with different phase offsets. However, the number of simulations required to do this is excessive because the sample size must be sufficiently large that averaging the results produces a reasonable result.
The other option is to run a number of simulations each with only one point source with polarization and position. Then collect the results of these coherent simulations by adding them incoherently [1/n*(|E1|^2+|E2|^2+|E3|^2+…+|En|^2)].
You will not be able to use the built in normalized power spectrum as this is not available with point sources. You would need to setup a measurement configuration to measure what came from the source and manual normalize the data. One option would be to run simulations with just the source and then simulations with source and structure and you could normalize them this way.
Scott