Random access: random access preamble (MSG1)V15.11.0

Physical random access procedure is triggered upon request of a PRACH transmission by higher layers or by a PDCCH order. A configuration by higher layers for a PRACH transmission includes the following:

  • A configuration for PRACH transmission
  • A preamble index, a preamble SCS, PPRACH,target, a corresponding RA-RNTI, and a PRACH resource

A PRACH is transmitted using the selected PRACH format with transmission power PPRACH,b,f,c(i), on the indicated PRACH resource.

A UE is provided a number N of SS/PBCH blocks associated with one PRACH occasion and a number R of contention based preambles per SS/PBCH block per valid PRACH occasion by ssb-perRACH-OccasionAndCB-PreamblesPerSSB in RACH-ConfigCommon.

  • N < 1: one SS/PBCH block is mapped to 1/N consecutive valid PRACH occasions, and
    R contention based preambles with consecutive indexes associated with the SS/PBCH block per valid PRACH occasion start from preamble index 0
  • N ≥ 1: N SS/PBCH blocks are associated with one PRACH occasion, and
    each SSB is associated with R preambles, with index starts from , where is provided by totalNumberOfRA-Preambles in RACH-ConfigCommon and is an integer multiple of N, 0 ≤ n ≤ N - 1.
ssb-perRACH-Occasion (N)CB-PreamblesPerSSB (R)totalNumberOfRA-Preambles ()
20
PRACH preamble index applicable for an SSB
SSBPreamble idx
00,...,9
110,...,19
ssb-occasion association

For link recovery, a UE is provided N SS/PBCH blocks associated with one PRACH occasion by ssb-perRACHOccasion in BeamFailureRecoveryConfig. For a dedicated RACH configuration provided by RACH-ConfigDedicated, if cfra is provided, a UE is provided N SS/PBCH blocks associated with one PRACH occasion by ssb-perRACHOccasion in occasions.

  • N < 1: one SS/PBCH block is mapped to 1/N consecutive valid PRACH occasions.
  • N ≥ 1: all consecutive N SS/PBCH blocks are associated with one PRACH occasion.

How to map SSBs to valid PRACH occasions

SS/PBCH block indexes provided by ssb-PositionsInBurst in SIB1 or in ServingCellConfigCommon are mapped to valid PRACH occasions in the following order:

  1. in increasing order of preamble indexes within a single PRACH occasion. Defined by N, which is ssb-perRACHOccasion. If N < 1, 1 / N SSBs will be mapped to one PRACH occasion.
  2. in increasing order of frequency resource indexes for frequency multiplexed PRACH occasions. If msg1-FDM (possible values 1, 2, 4, 8) in RACH-ConfigGeneric > 1, there'll be multiple PRACH occasions in one time instance, multiplexed in frequency domain.
  3. in increasing order of time resource indexes for time multiplexed PRACH occasions within a PRACH slot. the time domain PRACH configuration is indicated by prach-ConfigurationIndex in RACH-ConfigGeneric, which is an index to the configuration tables defined in TS 38.211 clause 6.3.3.2.
  4. in increasing order of indexes for PRACH slots

Definition of terminologies:

  • Association period: starting from frame 0, for mapping SS/PBCH blocks to PRACH occasions, is the smallest value in the set determined by the PRACH configuration period according Table 8.1-1 such that SS/PBCH blocks are mapped at least once to the PRACH occasions within the association period.
  • : obtained from the value of ssb-PositionsInBurst in SIB1 or in ServingCellConfigCommon.
    If after an integer number of SS/PBCH blocks to PRACH occasions mapping cycles within the association period there is a set of PRACH occasions or PRACH preambles that are not mapped to SS/PBCH blocks, no SS/PBCH blocks are mapped to the set of PRACH occasions or PRACH preambles.
  • Association pattern period: includes one or more association periods and is determined so that a pattern between PRACH occasions and SS/PBCH blocks repeats at most every 160 msec. PRACH occasions not associated with SS/PBCH blocks after an integer number of association periods, if any, are not used for PRACH transmissions.
Toolbox: mapping SSB to PRACH occasion
ssb-perRACH-
Occasion (N)
PRACH cfg
period [ms]
SSB period
[ms]
# occasions per
PRACH cfg period
# invalid occasions per
SSB period due SSB
# invalid occasions per
10ms due to DL
Time [ms]020406080100120140
PRACH cfg period idx01234567
Association period idx01234567
# valid occasions4242424242424242
PRACH occasion index
SSB 00, 12, 240, 12, 240, 12, 240, 12, 240, 12, 240, 12, 240, 12, 240, 12, 24
SSB 10, 12, 240, 12, 240, 12, 240, 12, 240, 12, 240, 12, 240, 12, 240, 12, 24
SSB 21, 13, 251, 13, 251, 13, 251, 13, 251, 13, 251, 13, 251, 13, 251, 13, 25
SSB 31, 13, 251, 13, 251, 13, 251, 13, 251, 13, 251, 13, 251, 13, 251, 13, 25
SSB 42, 14, 262, 14, 262, 14, 262, 14, 262, 14, 262, 14, 262, 14, 262, 14, 26
SSB 52, 14, 262, 14, 262, 14, 262, 14, 262, 14, 262, 14, 262, 14, 262, 14, 26
SSB 63, 15, 273, 15, 273, 15, 273, 15, 273, 15, 273, 15, 273, 15, 273, 15, 27
SSB 73, 15, 273, 15, 273, 15, 273, 15, 273, 15, 273, 15, 273, 15, 273, 15, 27
SSB 84, 16, 284, 16, 284, 16, 284, 16, 284, 16, 284, 16, 284, 16, 284, 16, 28
SSB 94, 16, 284, 16, 284, 16, 284, 16, 284, 16, 284, 16, 284, 16, 284, 16, 28
SSB 105, 17, 295, 17, 295, 17, 295, 17, 295, 17, 295, 17, 295, 17, 295, 17, 29
SSB 115, 17, 295, 17, 295, 17, 295, 17, 295, 17, 295, 17, 295, 17, 295, 17, 29
SSB 126, 18, 306, 18, 306, 18, 306, 18, 306, 18, 306, 18, 306, 18, 306, 18, 30
SSB 136, 18, 306, 18, 306, 18, 306, 18, 306, 18, 306, 18, 306, 18, 306, 18, 30
SSB 147, 19, 317, 19, 317, 19, 317, 19, 317, 19, 317, 19, 317, 19, 317, 19, 31
SSB 157, 19, 317, 19, 317, 19, 317, 19, 317, 19, 317, 19, 317, 19, 317, 19, 31
SSB 168, 20, 328, 20, 328, 20, 328, 20, 328, 20, 328, 20, 328, 20, 328, 20, 32
SSB 178, 20, 328, 20, 328, 20, 328, 20, 328, 20, 328, 20, 328, 20, 328, 20, 32
SSB 189, 21, 339, 21, 339, 21, 339, 21, 339, 21, 339, 21, 339, 21, 339, 21, 33
SSB 199, 21, 339, 21, 339, 21, 339, 21, 339, 21, 339, 21, 339, 21, 339, 21, 33
SSB 2010, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 34
SSB 2110, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 3410, 22, 34
SSB 2211, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 35
SSB 2311, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 3511, 23, 35

For a PRACH transmission triggered by a PDCCH order, the PRACH mask index field, if the value of the random access preamble index field is not zero, indicates the PRACH occasion for the PRACH transmission where the PRACH occasions are associated with the SS/PBCH block index indicated by the SS/PBCH block index field of the PDCCH order.

For a PRACH transmission triggered by higher layers, if ssb-ResourceList is provided, the PRACH mask index is indicated by ra-ssb-OccasionMaskIndex which indicates the PRACH occasions for the PRACH transmission where the PRACH occasions are associated with the selected SS/PBCH block index.

The PRACH occasions are mapped consecutively per corresponding SS/PBCH block index. The indexing of the PRACH occasion indicated by the mask index value is reset per mapping cycle of consecutive PRACH occasions per SS/PBCH block index. The UE selects for a PRACH transmission the PRACH occasion indicated by PRACH mask index value for the indicated SS/PBCH block index in the first available mapping cycle.

For the indicated preamble index, the ordering of the PRACH occasions is

  1. in increasing order of frequency resource indexes for frequency multiplexed PRACH occasions
  2. in increasing order of time resource indexes for time multiplexed PRACH occasions within a PRACH slot
  3. in increasing order of indexes for PRACH slots

For a PRACH transmission triggered upon request by higher layers, a value of ra-OccasionList, if csirs-ResourceList is provided, indicates a list of PRACH occasions for the PRACH transmission where the PRACH occasions are associated with the selected CSI-RS index indicated by csi-RS. The indexing of the PRACH occasions indicated by ra-OccasionList is reset per association pattern period.

Table 8.1-1: Mapping between PRACH configuration period and SS/PBCH block to PRACH occasion association period
PRACH configuration period (msec)Association period (number of PRACH configuration periods)
10{1, 2, 4, 8, 16}
20{1, 2, 4, 8}
40{1, 2, 4}
80{1, 2}
160{1}

How to decide if a PRACH occasion is valid

For paired spectrum or supplementary uplink band all PRACH occasions are valid.

For unpaired spectrum:

  • if a UE is not provided tdd-UL-DL-ConfigurationCommon, a PRACH occasion in a PRACH slot is valid if it does not precede a SS/PBCH block in the PRACH slot and starts at least Ngap symbols after a last SS/PBCH block symbol, where Ngap is provided in Table 8.1-2.
  • If a UE is provided tdd-UL-DL-ConfigurationCommon, a PRACH occasion in a PRACH slot is valid if
    • it is within UL symbols, or
    • it does not precede a SS/PBCH block in the PRACH slot and starts at least Ngap symbols after a last downlink symbol and at least Ngap symbols after a last SS/PBCH block symbol, where Ngap is provided in Table 8.1-2.

For preamble format B4: Ngap = 0.

Table 8.1-2: Ngap values for different preamble SCS μ
Preamble SCS [kHz]Ngap
1.250
5
152
30
60
120
valid prachValid/invalid PRACH occasions for TDD

When to transmit PRACH after PDCCH order

If a random access procedure is initiated by a PDCCH order, the UE, if requested by higher layers, transmits a PRACH in the selected PRACH occasion for which a time between the last symbol of the PDCCH order reception and the first symbol of the PRACH transmission is

NT,2 + ΔBWPSwitching + ΔDelay msec, where
  • NT,2 is a time duration of N2 symbols corresponding to a PUSCH preparation time for UE processing capability 1 with SCS configuration μ. I.e., N2 = {10, 12, 23, 36} for μ = {0, 1, 2, 3}
  • μ = min{μPDCCH order, μPRACH transmission }. For a PRACH transmission using 1.25 kHz or 5 kHz SCS, the UE determines N2 assuming SCS configuration μ = 0.
  • ΔBWPSwitching = 0 if the active UL BWP does not change. Otherwise, ΔBWPSwitching is defined in TS 38.133
  • ΔDelay = 0.5 msec for FR1 and ΔDelay = 0.25 msec for FR2.

What happens if PRACH collides with PUSCH/PUCCH/SRS

For single cell operation or for operation with carrier aggregation in a same frequency band, a UE does not transmit PRACH and PUSCH/PUCCH/SRS

  • in a same slot, or,
  • when a gap between the first or last symbol of a PRACH transmission in a first slot is separated by less than N symbols from the last or first symbol, respectively, of a PUSCH/PUCCH/SRS transmission in a second slot where
    • N = 2 for μ = 0 or μ = 1, N = 4 for μ = 2 or μ = 3.
    • μ is the SCS configuration for the active UL BWP.