mac-ContentionResolutionTimer，这个参数，文档说明是“MAC冲突解决定时器:取值范围0~7，enumerate(8,16,24,32,40,48,56,64) sf RA过程中UE等待接收Msg4的有效时长。当UE初传或重传Msg3时启动。在超时前UE收到Msg4或Msg3的NACK反馈，则定时器停止。定时器超时，则随机接入失败，UE重新进行RA。”
- if mac-ContentionResolutionTimer expires:
- discard the Temporary C-RNTI;
- consider the Contention Resolution not successful.
- if the Contention Resolution is considered not successful the UE shall:
- flush the HARQ buffer used for transmission of the MAC PDU in the Msg3 buffer;
- increment PREAMBLE_TRANSMISSION_COUNTER by 1;
- If PREAMBLE_TRANSMISSION_COUNTER = preambleTransMax + 1:
- indicate a Random Access problem to upper layers.
- based on the backoff parameter in the UE, select a random backoff time according to a uniform distribution between 0 and the Backoff Parameter Value;
- delay the subsequent Random Access transmission by the backoff time;
- proceed to the selection of a Random Access Resource (see subclause 5.1.2).
直接随机接入，并且还要考虑前导码次数是否达到最大次数，否则还有个backoff time时间惩罚