Short preamble and msg1 frequency start

Specs & Protocol Testing
1

Hi Experts,
Question for Nokia NR n78 band: why do they use short preambles and always a different msg1 frequency start?
And rootsequenceindex always either 0 or 64?
Is there any limitation?

2

For different in msg1 freq start that is their strategy to avoid collision on freq domain.
For rach format not sure why they are not extending to long, but could be some sw limitation.
Here also still using short format.

3

Yes you are correct. It’s because phy limitation.
I mean sw phy limitation.

4

Short preamble as specified by 3gpp used for large subcarrier spacing (15, 30, 60, 120 kHz) used for less cell range.
So for n78 short preamble used with 30 kHz.
Long preamble used for small subcarrier spacing i.e. 15 kHz used to support large cell range.
It’s used for fdd only .
Msg1 Freq start specifies the frequency domain position of the prach.

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5

Root sequence index 0 or 64 means.
How it will generate 64 Preambles using only one RSI?

6

in case zeroCorrelationZoneConfig =15 mean NCS =69

amount of preamble per root sequence: round-down (139/69) = 2
required amount of root sequences per cell: round-up (64/2) = 32

32 Roots needed to generate 64 preambles (2*32)

in case one RSI mean NCS =2.

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7

Long preamble with SCS 30 KHz requires 2 consecutive UL slots. The TDD pattern may be limiting factor if vendor supports

8

But basically ZCS(ZeroCorrelationZoneConfig) will be sent to UE via RRC will single root index then how can UE will use other root index to complete 64 ? Is it really valid ?

Because to my understanding UE will get the ZCS, PRACH config idx and etc So ZCS value will be single root index value based on this root sequence will be used to generate the preamables but How UE will use other root index to complete 64 preambles ? Is it valid ?

Could you please help to understand this confusion ?

Considering your example :

//*
in case zeroCorrelationZoneConfig =15 mean NCS =69

amount of preamble per root sequence: round-down (139/69) = 2
required amount of root sequences per cell: round-up (64/2) = 32

32 Roots needed to generate 64 preambles (2*32)

\\*

9

1)Root Sequence Index: It indicates first Zadoff-Chu root sequence from which preambles start to be generated.
2)To generate the 64 preamble sequences, multiple root Zadoff–Chu sequences must be used in the cell.
3)If one root is not enough to generate 64 preambles, next roots are taken
4)Neighbour cells cannot have RS pools overlapping: this can create PRACH collisions.

Above example:
NCS config equal to 69 means 32 root sequence indices (out of 138) must be used for 1 CELL , (138 roots capacity is enough just for 4 cells)

Cell1 :RSI from 0 to 31
Cell2 :RSI from 32 to 63
Cell3 :RSI from 64 to 95
Cell4 :RSI from 96 to 127
5th cell utilizes remaining free RSIs from the end of the pool and takes further RSIs from the beginning

in cell configuration :it will like 0/32/64 for cell1/2/3

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10

Ok got it, Thank you for detailed information.

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11

Hi Experts, is the Msg1 Freq start a parameter controlled in Ericsson system ? how do we check it in node ?