1) Base Station Densification Gains: We define the BS densification
gain ρ(λ1, λ2) > 0 as the effective increase in data
rate relative to the increase in network density, which is a proxy
here for cost. Specifically, if we achieve a data rate R1 (could
be any measure thereof, e.g., edge rate or aggregate) when the
BS density is λ1 BSs/km2 and then we consider a higher BS
density λ2, with corresponding rate R2, then the densification
gain is the slope of the rate increase over that destiny range: