LTE random access procedure after cell search

After acquired common RRC configuration information. UE knows random access configuration and is ready to do random access to get RRC dedicated configuration from cell. Following RRC IEs provided RACH and PRACH configuration.

1. Interpretation of 3GPP 36.331 6.3.2 Radio Resource Config Common: prach-Config

IE prach-ConfigIndex, by mapping to 3GPP 36.211 TABLE 5.7.1-2 (FDD), -3 (TDD), UE knows it can send PRACH sequence at subframe 1 of any radio frame. 

IE prach-FreqOffset, UE knows the first PRB to transmit PRACH. 

IE rootSequenceIndex, IE zeroCorrelationZoneConfig and IE highSpeedFlag, by mapping to 3GPP 36.211 TABLE 5.7.2-2, UE know NCS value, this value is used for preamble generation. 

2. Interpretation of 3GPP 36.331 6.3.2 Radio Resource Config Common: rach-ConfigCommon

IE numberOfRA-Preambles, value represents number of non-dedicated random access preamble

IE powerRampingStep, value represents power ramping factor, used to calculate preamble received target power (PREAMBLE_RECEIVED_TARGET_POWER)

IE preambleInitialReceivedTargetPower, used to calculate preamble received target power. 

PREAMBLE_RECEIVED_TARGET_POWER = preambleInitialReceivedTargetPower + DELTA_PREAMBLE + (PREAMBLE_TRANSMISSION_COUNTER – 1) * powerRampingStep 

IE preambleTransMax, value represents maximum number of preamble transmission. PREAMBLE_TRANSMISSION_COUNTER has to be smaller than preambleTransMax. 

IE ra-ResponseWindowSize, value is in unit of number of subframes, represents duration of RA response window. Following is quoted from 3GPP 36.321 5.1.4

"Once the Random Access Preamble is transmitted and regardless of the possible occurrence of a measurement gap, the UE shall monitor the PDCCH of the PCell for Random Access Response(s) identified by the RA-RNTI defined below, in the RA Response window which starts at the subframe that contains the end of the preamble transmission [36.211] plus three subframes and has length ra-ResponseWindowSize subframes."

IE mac-ContentionResolutionTimer, value is in unit of subframes. Following is quoted from 3GPP 36.321 3.1 

"Specifies the number of consecutive subframe(s) during which the UE shall monitor the PDCCH after Msg3 is transmitted."

IE maxHARQ-Msg3Tx, value represents Maximum number of Msg3 HARQ transmissions, used for contention based random access.

3. RACH process flow chart quoted from http://www.sharetechnote.com/