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PRACH Root Sequence Planning

1. . Zadoff-Chu Sequence length=839 in LTE.

2. Every Cell needs 64 preambles.
3. To generate the needed 64 preamble for each cell, we use cyclic shift(CS):
4. We need a larger CS for a larger cell radius, since the larger the cell radius, the longer the
transmission delay
5. From 4, we need more root sequences to generate the needed 64 preambles for a
larger cell radius.
6. But too many root sequences used in a cell is not good because sequences obtained
from cyclic shifts of different ZC sequences are not orthogonal.
7. Each cell, then must have a different RootSequenceIndex to avoid the reception of false
preambles in adjacent eNodeBs.

PrachCS and rootSeqIndex
o PrachCS defines the number of cyclic shifts
(in terms of number of samples) used to
generate multiple preamble sequences
from a single root sequence
o Example based on PrachCS=12 -> number
of cyclic shifts: 119
Root sequence length is 839 so a cyclic shift of
119 samples allows ROUNDDOWN
(839/119)= 7 cyclic shifts before making a
complete rotation (signatures per root
o 64 preambles are transmitted in the
PRACH frame. If one root is not enough to
generate all 64 preambles then more root
sequences are necessary
To ensure having 64 preamble sequences
within the cell it is necessary to have
ROUNDUP (64/7)= 10 root sequences per cell

Parameters Related To RSI

Parameter Name
Allowed Range -
Engineering unit recomned values Parameter Description
NCs Configuration(Zero
correlationZone config) 0 to 15 12
parameter used to calculate cyclic shifts
of the root Zadoff-chu sequence in the

High speed flag
TRUE -high
Speed , Fasle
other wise FALSE
Defines the use of restricted set of cyclic
shifts of the Zadoof chu root sequence
used for high speed moving UE
ALU Algorithm RootSequenceIndex

LA6.0.1, rootSequenceIndex (or RSI) is only derived from
rootSequenceIndex when bit 24 of LteCell::spare4 is set to 0.

When bit 24 of LteCell::spare4 is set to 1, the RSI is derived
automatically as follows

RSI = C1 + mod(floor(PCI/3)N, C2),

Also note that if auto RSI is used (i.e. bit 24 of LteCell::spare4
is set to 1),
parameter rootSequenceIndex will not be synched up with
the actual RSI value
ALU Parameter list
3GPP Allocation of sequence