How to get the Preamble index

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There are two cases: Contention-based and Contention-free randomaccess.

For Contention-based one, UE selects randomly the value ofpreamble index. (First, UE decides that preamble is chosen from group A orgroup B. If group B exists and the size of Msg3 is greater thanmessageSizeGroupA and pathloss is less than PCMAX,c – preambleInitialReceivedTargetPower

- deltaPreambleMsg3 – messagePowerOffsetGroupB，then select group B；otherwiseselect group A. If UE failed in random access with sending Msg3, then it shouldchoose the same group as last time. Then UE should choose a preamble randomlyfrom the chosen group.

For Contention-free one, eNodeB assigns a dedicated preamble indexfor UE to avoid collision. eNodeB do this by two ways: through ra-PreambleIndexof RACH ConfigDedicated (in handover procedure), or through the Preamble Indexfield of DCI format 1A in the random access triggered by PDCCH order.

Simply put, eNodeB sends PRACH configindex and PRACH frequencyoffsetthrough SIB2 to determine the set of time-frequency resources that the cell canuse to transmit preamble. When UE initiates RA, it selects a resource from it(which may also be specified by eNodeB) to send preamble. Because eNodeB doesnot know which time-frequency resource the UE will send the preamble, it willdetect and receive the preamble on all the indicated preamble time-frequencyresources.

To be more specific, For FDD, UE looks up table 5.7.1-2 of 36.211with PRACH configindex to get the preamble format and the system frame andsubframe number that can be used to transmit preamble, so as to determine theoptional time domain resources. The starting RB in the frequency domain isobtained by PRACH frequencyoffset, so as to determine the frequency domainresources (FDD has only one frequency domain resource in a subframe, so it isfixed).

For TDD, look up table 5.7.1-3 and table 5.7.1-4of 36.211 with PRACH configindex to get the preamble format and the quads（fRA, t(0)RA, t(1)RA, t(2)RA ) that can be used to transmit the preamble. Thet(0)RA, t(1)RA , t(2)RAdetermines the system frame and subframe number that can be used to transmitpreamble in the time domain, so as to determine the optional time domainresources. PRACH frequencyoffset and fRA determines the optionalfrequency domain resources (TDD may have multiple frequency domain resources ona subframe, so it is optional).