In case homogeneous network all base stations have similar transmit power levels, antenna patterns, receiver noise,So all of the UEs get the same kind of dat flow and power levels so its very hard to decide when placing a base station.
But in case of heterogeneous network pico/femto/relay base stations present in a network and they provides at different power levels.For homogeneous network the signal coming from other station act as noise but in case of heterogeneous network they take in to consideration.
These low power level nodes like pico/femto cells can be deployed at coverage holes created due to macro cells deployment.
Source
http://www.qualcomm.com/media/documents/files/lte-heterogeneous-networks.pdf