36.213 随机接入过程中的疑问

pinky66

在36.213 随机接入流程中
6.1.1       Timing

For the L1 random access procedure, UE’s uplink transmission timing after a random access preamble transmission is as follows.
If a PDCCH with associated RA-RNTI is detected in subframe n, and the corresponding DL-SCH transport block contains a response to the transmitted preamble sequence, the UE shall, according to the information in the response, transmit an UL-SCH transport block in the first subframe , , if the UL delay field in section 6.2 is set to zero. The UE shall postpone the PUSCH transmission to the next available UL subframe if the field is set to 1.

问题是：  在什么场景下MSG3在UE恻需要延迟发送（UL delay == 1）？下一个有效发送的时刻怎么计算？