Simple estimate of signal to interference ratio with randomly located antennas

We consider a propagation model under path loss, shadowing and multipath effects, where each cell are a Voronoi tessellation and the generating points of this cells are a uniform Poisson point process in R2. In addition, the antennas are located in each point generated by the Poisson process. With this model, we calculate the mean interference to signal ratio ISR, which actually gives us a physical information, but once we are interest in apply this mean in the Shannon's formula, we simulated the real mean value of signal to interference ratio SIR and compared the result with the calculated ISR-1.