8.5.18 Selection of RACH TTI

25.3313GPPProtocol specificationRadio Resource Control (RRC)Release 17TS

Tools: ARFCN - Frequency Conversion for 5G NR/LTE/UMTS/GSM

8.5.18.1 FDD

In FDD mode, a RACH may employ either 10 or 20 ms TTI. The supported TTI is indicated as a semi-static parameter of the RACH Transport Format in system information. The UE shall select an appropriate TTI length from the RACHs included in the list of candidate PRACH(s) according to the following rule.

The UE shall:

1> if only RACHs with one particular TTI length are included in the list of candidate PRACH(s):

2> select this TTI length and proceed as specified in subclause 8.5.17.

1> if both PRACHs with 10ms and 20ms TTI lengths are included in the list of candidate PRACH(s):

2> perform TTI selection as follows:

3> when the UE calculates the initial preamble transmit power ("Preamble_Initial_Power") as specified in subclause 8.5.7:

4> select a TF to be employed for calculation of a transmit power margin as follows:

5> from the TFs supported by all candidate PRACHs keep those which correspond to a single transport block of all configured RLC sizes (i.e., in idle mode, the RLC size applicable for RB0, in connected mode, the RLC sizes configured with explicit "RB mapping info"). If more than a single TF remain applicable, the UE may select any of these. Preferably the UE should select the TF which is intended to be used at the next transmission or, if such information is not available, the TF corresponding to the largest configured RLC size.

4> calculate a transmit power margin,

Margin = {min(Maximum allowed UL tx power, P_MAX) – max(Preamble_Initial_Power,
Preamble_Initial_Power + DPp-m + 10*log10(1 + (bd /bc)2))

where "Maximum allowed UL tx power" is the maximum allowed uplink transmit power indicated in system information (in dBm), and P_MAX is the maximum RF output power of the UE (dBm). The margin shall be calculated for the gain factors bd and bc of the TF selected in the step above, using 10ms TTI length.

NOTE 1: the expression Preamble_Initial_Power + DPp-m + 10*log10(1 + (bd /bc)2) represents the total RACH message power if the message would be sent after the initial preamble.

NOTE 2: For all PRACHs with the same TTI the UTRAN should set each of the IEs "Primary CPICH TX power", "Constant value" to the same value, and within these PRACH configurations, the UTRAN should, for all TFs including a single TB, set each of the IEs "Gain Factor _c ", "Gain Factor _d" and "Power offset P p-m" to the same value. If these guidelines are not followed, the UE behaviour is unspecified.

3> if the resulting "Margin" value is less than 6 dB:

4> select RACH with 20 ms TTI, and proceed as specified in subclause 8.5.17.

3> otherwise, if the last L1 message transmission on PRACH failed (see [15]):

4> the UE may select RACH with 20ms TTI length and proceed as specified in subclause 8.5.17.

3> otherwise:

4> select RACH with 10ms TTI length and proceed as specified in subclause 8.5.17.

8.5.18.2 1.28 Mcps TDD

In 1.28 Mcps TDD, a RACH may be assigned a 5, 10 or 20 ms TTI. If, in one cell, more than one RACH is defined a UE shall select the RACH that is to be used for each transmission according to the following rule:

1> if only RACHs with one particular TTI length are assigned a transport format that is suitable for the transmission of the transport block set:

2> select this RACH’s TTI length.

1> if more than one RACHs are assigned a transport format that is suitable for the transmission of the transport block set:

2> select the longest of the TTI lengths of these RACHs.