Why PRACH is used in LTE and how to plan PRACH/ preamble format in a network?

Can anyone help me on to understand ?
Why PRACH is used in LTE and how to plan PRACH/ preamble format in a network ?

Hi Experts.
In LTE When UE use preamble format 2 & 3, what is the advantage of preamble format 2 & 3 over preamble format 1 & 2?
For which purpose preamble format 4 is designed by 3GPP?

I think in LTE, each cell support only one type of preamble format, normally FM0.

Each will give more or less cell radius and trades on the number of simultaneous UL request it can handle.

According to “LTE in Bullets” book, page 250 (and Table 170 - PRACH format parameters):

Format 4 can only be used by TDD when using special subframe configurations with UpPTS lengths of 4384 Ts (2 symbols with normal cyclic prefix) and 5120 Ts (2 symbols with extended cyclic prefix). This format has a short preamble sequence.