• The preamble format determines the length of the Cyclic Prefix and Sequence
• FDD has 4 preamble formats (for different cell sizes) and 16 RA slot configurations (for different bandwidth)
• The start of Random Access Preamble transmission assumes Timing Advance = 0 except for handover that UE can assume a time difference between current cell and target cell
• Each random access preamble occupies a bandwidth corresponding to 6 consecutive resource blocks
• The diagram considers RACH structure for FDD
PRACH
The transmission of a random access preamble, if triggered by the MAC layer, is restricted to certain time and frequency resources. These resources are enumerated in increasing order of the subframe number within the radio frame and the physical resource blocks in the frequency domain such that index 0 correspond to the lowest numbered physical resource block and subframe within the radio frame.
Note that cyclic prefix is extended to cope with large delay uncertainty (for different cell sizes). More information can be found in TS36.211 Section 5.7.
