LTE: How Network knows exactly when the UE is going to transmit the RACH?

Question

Actually i am trying to learn RACH procedures and getting lots of hurdles to understand. I am posting my queries whatever is coming to my mind so that i will get the descriptive answers. I hope someone will help me here to learn it. Thanks in advance.

Answer 1

The network knows when to expect RACH from the UE because RACH locations are configured in SIB2, and defined in 36.331, (simply search for prach-Config).

For FDD there are 2 basic configuration parameters, first is the prach-ConfigIndex, which indicates which subframes are viable for transmission of RACH preambles.

A second paramameter, prach-FreqOffset, which basically indicates the number of RB up from the 0 RB index where the 6 RB of the PRACH are allotted in the time-frequency grid defined in 36.211.

Note that the PRACH itself does not have exactly the same OFDM signal structure of the rest of the resource grid, but does fit within a 6 RB window in frequency.

In FDD, only one PRACH opportunity is available per subframe, and thus FDD only employs 10 RA-RTNIs.

For TDD multiple PRACH channels may exist per subframe, see 36.211 section 5.7

Hope that helps.

Comments

nicely explained.