在RRC消息中，对于NB-IoT的sib2-nb中并不存在随机接入前导码组啊，不知道该如何选择前导码
RACH-ConfigCommon-NB-r13 ::=    SEQUENCE {
  preambleTransMax-CE-r13        PreambleTransMax,
  powerRampingParameters-r13      PowerRampingParameters,
  rach-InfoList-r13          RACH-InfoList-NB-r13,
  connEstFailOffset-r13        INTEGER (0..15)          OPTIONAL,  -- Need OP
  ...
}

RACH-InfoList-NB-r13 ::=  SEQUENCE (SIZE (1.. maxNPRACH-Resources-NB-r13)) OF RACH-Info-NB-r13

RACH-Info-NB-r13  ::=    SEQUENCE {
  ra-ResponseWindowSize-r13      ENUMERATED {
                      pp2, pp3, pp4, pp5, pp6, pp7, pp8, pp10},
  mac-ContentionResolutionTimer-r13  ENUMERATED {
                      pp1, pp2, pp3, pp4, pp8, pp16, pp32, pp64}
}