what is meant by PRACH preamble offset index in LTE?

Question

Compressed Mode Frame structures and Types of compressed mode with detail Explain?

Answer 1

You might either be referring to Preamble configuration index or PRACH frequency offset index

Preamble configuration index:
These are the timing and frequency resources that are enumerated in increasing order of the subframe number within the radio frame. They are just indexes that represent the PRB's such that index 0 correspond to the lowest numbered PRB. Table 5.7.1-2 explains the indexing
Please see 36.211 chapter 5.7 for more details

PRACH frequency Offset (n_RA_PRBoffset):
represents the first physical resource block n_RA_PRB allocated to the PRACH opportunity. this is given as n_RA_PRB = n_RA_PRBoffset, where the parameter prach-FrequencyOffset, n_RA_PRBoffset is expressed as a physical resource
block number configured normally in SIB2 and it always fulfills the following criteria
0<=n_RA_PRBoffset<=N_UL_RB-6. Please see 36.211 chapter 5.7 for more details

From 36.311 sib2 in PRACH-ConfigInfo we find prach-FreqOffset

PRACH-ConfigInfo ::= SEQUENCE { 
prach-ConfigIndex INTEGER (0..63), 
highSpeedFlag BOOLEAN, 
zeroCorrelationZoneConfig INTEGER (0..15), 
prach-FreqOffset INTEGER (0..94) 
}