5G • RAN
Root Sequence Index
PRACH Root Sequence Index
Logical index broadcast in SIB1 determining the root sequence used to generate 64 PRACH preambles.
Technical Explanation
In SIB1, prach-RootSequenceIndex defines the starting logical root sequence. The UE generates all 64 preambles by taking consecutive cyclic shifts of the root. If 64 preambles cannot be generated from one root (due to large cyclic shift Ncs for large cells), the UE steps to the next logical root sequence in the 3GPP mapping table.
Key Functions
- Configures the initial mathematical root for 64 cell preambles
- Mapped logically in 3GPP tables to minimize adjacent-cell interference
- Determines how many root sequences are needed based on Ncs
Specifications
3GPP TS 38.211, TS 38.331
Interfaces
Uu
Related 5G Concepts
Want to memorize 5G concepts like this one?
Study it with SuperMemo SM-2 spaced repetition flashcards.