LTE architecture

I understand that the LTE uses a different set of transmission antennas compared to those used for the 3G service, is that correct ? Do the LTE towers transmit and receive 3G as well, or is this an entirely different system ? If the LTE system is not available to my phone, it will use 3G: is this all done via the LTE system or is my phone interacting with two different services ?

Also: I noticed that the LTE antennas are positioned geographically at sites other than the towers using the regular GPRS. Why is this ?