PRACH Planning in LTE
PRACH Planning Principle
•In LTE it is necessary the radio planner to selects the preamble format for each cell based on maximum
estimated cell range. Typical preamble format will be ‘Preamble Format 0’, allowing for cell sizes up to 15km.
Other preamble formats allow for larger cell ranges.
•PRACH parameters should be planned. PRACH transmission can be separated by:
– Time (prachConfIndex) specifies in which subframes the RA can occur
PRACH Configuration Index cannot be the same for different cells at the eNB
– Frequency (prachFreqOff) avoid overlapping with PUCCH and PUSCH or splitting the PUSCH
area.PRACH area is next to PUCCH area either at upper or lower border of frequency band,
For simplicity use same configuration for all cells
– Sequence (PRACH CS and RootSeqIndex) the cyclic shift ensures sufficient separation between the
preambles
Use different sequences for all neighbour cells
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PRACH Configuration Index (1)
The PRACH Configuration
• Chosen after selecting the
Preamble format
• The PRACH capacity is
determined by the SFN and
subframe figures
• In the case of FDD, the PRACH
cannot be multiplexed in the
frequency domain, i.e. only 1
PRACH resource per
subframe
Recommendation:
Configure different
PRACHconfiguration
Indexes at cells belonging
to the same site. E.g.:
3/4/5 if RACH density=1 or
6/7/8 if RACH density=2
(Preamble Format 0)
Frequency Offset (1)
– Defines the position of the PRACH preamble
within the channel bandwidth
– PRACH should be positioned adjacent to the
PUCCH
PUCCH
PUCCH
2 ms
Resource Block
signalled in SIB2
PRACH
PRACH Frequency Offset (2)
•
Indicates the first PRB available for PRACH in the UL frequency band
– PRACH area (6 PRBs) should be next to PUCCH area either at upper or lower border of
frequency band in order to maximize the PUSCH area but not overlap with PUCCH area
– Parameter is configured based on the PUCCH region i.e. its value depends on how many
PUCCH resources are available.
– If PRACH area is placed at the lower border of UL frequency band then:
PRACH-Frequency Offset= roundup [PUCCH resources/2]
•
If PRACH area is placed at the upper border of the UL frequency band then:
PRACH-Frequency Offset= NRB -6- roundup [PUCCH resources/2]
NRB: Number of Resource Blocks
Root Sequence Index
– The allocated ‘root sequence’ index broadcast in SIB2 is a logical index. The actual physical index is
obtained using a look-up table defined within 3GPP TS 36.211 (part of the table is shown below)
– Each logical rootSeqIndex is associated with a single physical root sequence number.
– The reuse distance of ‘root sequences’ should be maximised
– The eNB could be configured with all cells using the same root sequences, provided the cells use
different preamble configuration indexes.
PRACH Cyclic Shift for FDD preambles
– Cyclic shift is used to assure sufficient separation between the preambles
– The propagation delay and the cyclic separation are directly related to the cell range
Recommendation: assume all cells have same size=> same PrachCS
The table highlights how the intra-cell interference is optimized with respect to cell size: the smaller the cell size, the
larger the number of orthogonal signatures and the better the detection performance.
Zero Correlation Zone (1)
– Selecting
• Zero Correlation Zone
• High Speed Flag
are prerequisites to planning the ‘Root Sequence Index’
High Speed Flag
= FALSE
High Speed Flag
= TRUE
– The Zero Correlation Zone determines the size of the cyclic shift used to generate the PRACH
sequence from the ‘root sequence’
– Large cyclic shift (large Zero Correlation
Zone) required to support larger cell
ranges
– Large cyclic shift means that fewer
PRACH sequences can be generated
from each ‘root sequence’
– PRACH sequences generated from
different ‘root sequences’ are not
orthogonal
Zero Correlation Zone (2)
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There are 838 ‘root sequences’ from which to generate the PRACH sequences(64)
Each ‘root sequence’ has a length of 839
Each cell requires 64 PRACH sequences
The number of PRACH sequences which can be generated from each ‘root sequence’ is given by:
PRACH Sequences per Root Sequence = ROUNDDOWN(839 / Cyclic Shift)
– The number of root sequences
required per cell is then given by:
ROUNDUP(64 / PRACH
Sequences per Root Sequence)
– This determines the size of
the reuse pattern when
planning ‘root sequences’
PRACH Planning
Wrap Up
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Steps:
- Define the prachConfIndex
Depends on preamble format (cell range)
It should be different for each cell of a site
- Define the prachFreqOff
Depends on the PUCCH region
It can be assumed to be the same for all cells of a network (simplification)
- Define the PrachCS
Depends on the cell range
If for simplicity same cell range is assumed for all network then prachCS is the same for all
cells
- Define the rootSeqIndex
It points to the first root sequence
It needs to be different for neighbour cells
rootSeqIndex separation between cells depends on how many are necessary per cell
(depends on PrachCS)
PRACH Planning example
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Assumptions:
- prachConfIndex=3 for all cells
preamble format =0
One PRACH opportunity per 10ms
- prachFreqOff=6 for all cells
PRACH starts at sixth PRB in frequency domain
- Define the prachCS=8 for all cells
Max cell range = 5.5km
Each cell consumes 4 root sequences
Exercise
– Plan the PRACH Parameters
for the sites attached in the
excel
– Assumptions:
• PUCCH resources =6
• Cell range = 12km (all
cells have same range)
• BW:10MHz
Exercise Solution