What are the preamble formats defined for LTE FDD and TDD ?

Question

What are all major differences between TDD and FDD radio frames in the context of radio channels and their configuration ?

Answer 1

There five PRACH preamble formats which have different lengths for the cyclic prefix, useful part of the symbol, and guard period:

Preamble format       Tcp              Tseq       guard period    
0                     3168 Ts         24576 Ts          2976 Ts
1                     21024 Ts        24576 Ts         15840 Ts
2                     6240 Ts         2.24576 Ts        6048 Ts
3                     21024 Ts        2.24576 Ts       21984 Ts
4                     448 Ts          4096 Ts            288 Ts

Note that Preamble Format 4 is only applicable for TDD in special subframes (subframe 1 or 6) and with Special Subframe Configuration that results in UpPTS with 2 symbols duration i.e. the Preamble Format 4 PRACH sits in UpPTS.

Formats 2 and 3 have two repetitions of the nominal PRACH sequence which provides more total transmit energy and therefore allows for detection at lower SNRs. Also, Format 1 versus 0 and Format 3 versus 2 have a longer guard period, allowing for a larger cell size. The downside is that when the cyclic prefix time, sequence time and guard period are totaled up, some of the formats require multiple subframes for transmission:

Preamble format   Number of subframes
0                                1
1                                2
2                                2
3                                3
4                                1