This is the 2nd step in RACH procedure.
In this chapter we shall understand below 2 points:
1. How does a UE receive Msg2 ?
2. What are the contents of Msg2 ?
1. How does a UE receive Msg2 ?
1. gNB will send an DCI scrambled with RA-RNTI value that was calculated in previously. [RA-RNTI = RA-RNTI= 1 + s_id + 14 × t_id + 14 × 80 × f_id + 14 × 80 × 8 × ul_carrier_id]
2. UE will parse the PDCCH (DCI) used for downlink with the corresponding RA-RNTI within RAR-window. The window is specified by the parameter ra-ResponseWindow.
3. DCI format 1_0 with RA RNTI is the DCI format used for scheduling RAR PDSCH. This DCI needs to be found in Common Search Space, which is defined by the parameter ra-SearchSpace.
4. Resource Allocation Type for the Msg2 PDSCH would be Resource Allocation Type 1
5. Frequency Domain Resource Allocation for the PDSCH is specified by DCI format 1_0 with RA RNTI.
6. Time Domain Resource Allocation for the PDSCH is specified by DCI format 1_0 with RA RNTIand PDSCH-ConfigCommon.
7. If the UE is able to demodulate the DCI using RA-RNTI, then it will decode PDSCH payload.
8. Then the UE will check if RAPID (random access preamble identity) is the same as the RACH preamble ID when it sent Msg1. If it is the same, then UE thinks that the Msg2 demodulation is successful
9. If UE did not recieve response in “ra-ResponseWindow” or fails to decode or RAPID did not match after decoding, then UE will think Msg2 reception has failed and will trigger Msg1 retransmission.
2. What are the contents of Msg2 ?
According to TS 38 213 a MAC PDU (Random Access Response) will contain below subPDU.
Each MAC subPDU consists one of the following:
a MAC subheader with Backoff Indicator only;
a MAC subheader with RAPID only (i.e. acknowledgment for SI request);
a MAC subheader with RAPID and MAC RAR.
Below is the data structure of a MAC PDU that carries RAR.
-> BI (Backoff Indicator): It is used to generate a random backofff time. UE can retransmit Msg1 after the backoff time is completed.
-> RAPID: Used to carry random access preamble ID
-> RAR: It contains TA, UL GRANT information and TC-RNTI.
Ul Grant is used for Msg3 including PUSCH time domain, frequency domain resource allocation, frequency hopping, MCS, Msg3 power control and CSI request
-> E(Extension): Used to indicate whether this MAC subPDU is the last one in a string.
– set to “1” to indicate at least another MAC subPDU follows.
– set to “0” to indicate that this MAC subPDUis the last MAC subPDU
-> T(Type): Used to indicate whether the MAC subheader is RAPID subheader or BI subheader.
– set to “0” to indicate the presence of a BI field in the subheader (BI).
– set to “1” to indicate the presence of a RAPID field in the subheader (RAPID);
-> R: reserved bit, set to “0”;