Random Access Procedure in LTE

Background

When you switch on smartphone for the very first time, it will start searching for the network. There is a possibility that there are many networks or to put in other words , there are many frequencies from different operators available in the air to which UE (user equipment) can connect. Therefore, UE  needs to synchronize to each frequency and check whether this is frequency from the right operator to which it wants to connect to. UE does this by going through very initial synchronisation process. Once synchronized UE reads the master information block and System information blocks to check whether this is the right PLMN. Lets assume that it finds that PLMN value to be correct and so UE will proceed with reading System information block 1 and System information block 2. The next step is known as Random Access Procedure in which the network for the first time knows that some UE is trying to get access.

At this stage, UE does not have any resource or channel available to inform network about its desire to connect to it so it will send its request over the shared medium. Now there are two possibilities at this stage, either there are many other UEs in the same area (same cell) sending same request in which there is also a possibility of collision among the requests coming from various other UEs. Such random access procedure is called contention based Random access procedure. In second scenario, network can inform UE to use some unique identity to prevent its request from colliding with requests coming from other UEs. The second scenario is called contention free or non contention based random access procedure.

RACH preambles

The concept of RACH preamble though a little confusing is important in understanding the random access procedure.

When UE sends the very first message of random access procedure to some network, it basically sends specific pattern or signature which is called RACH preambles. The preamble value differentiate requests coming from different UEs. But if two UEs uses same RACH preambles at same time then there can be collision. There are totally 64 such patterns or signature available to the UE for the very first message of random access procedure and UE will decide any one of them randomly for contention-based random access procedure but for non-contention based procedure, actually network will inform UE about which one to use

In case, when UE goes from idle state to RRC connected state, there is no way for network to inform UE about which preamble out of 64 values should be used. Therefor UE has no choice but to use one of the preambles randomly which also result in possibility of collision if the same preamble is being used by another UE, provided the requests comes at same time (same frame)

In another scenario  if UE has to take handover to another eNB, in this case actually the UE can be informed about which preamble it can use, since UE is already in connected state

Steps of Random access procedure

Random access procedure consist of four steps explained below (Only contention based procedure is shown below)

Step 1: Msg1

• UE selects one of the 64 available RACH preambles
• Now UE also needs to give its own identity to the network so that network can address it in next step. The identity which UE will use is called RA-RNTI (Random access radio network temporary identity). Basically its not some value sent by UE but interestingly RA RNTI is determined from the time slot number in which the preamble is sent
• If UE does not receive any response from the network, it increases its power in fixed step and sends RACH preamble again

Step 2: Msg2

• eNodeB sends "Random Access Response" to UE on DL-SCH (Downlink shared channel) addressed to RA-RNTI calculated from the timeslot in which preamable was sent, as explained in step 1 (about RA-RNTI calculation)
• The message carries following information 
• Temporary C-RNTI: Now eNB gives another identity to UE which is called temporary C-RNTI (cell radio network temporary identity) for further communication
• Timing Advance Value: eNodeB also informs UE to change its timing so it can compensate for the round trip delay caused by UE distance from the eNodeB
• Uplink Grant Resource: Network (eNodeB) will assign initial resource to UE so that it can use UL-SCH (Uplink shared channel)

Step 3: Msg3 

• Using UL-SCH, UE sends "RRC connection request message" to eNodeB
• UE is identified by temporary C-RNTI (assigned in the previous step by eNodeB)
• The message contains following
• UE identity (TMSI or Random Value )
• TMSI is used if UE has previously connected to the same network. With TMSI value, UE is identified in the core network 
• Random value is used if UE is connecting for the very first time to network. Why we need random value or TMSI? Because there is possibility that Temp-CRNTI has been assigned to more than one UEs in previous step, due to multiple requests coming at same time (Collision scenario explained later)
• Connection establishment cause: The shows the reason why UE needs to connect to network

Step 4: Msg4

• eNodeB responds with contention resolution message to UE whose message was successfully received in step 3. This message is address towards TMSI value or Random number (from previous steps) but contains the new C RNTI which will be used for the further communication

Collision Scenario

The above example didn't consider any collision. Collision can occur because of following example scenario

• Lets assume two UEs send same RACH preamble at same time in step 1
• Same Temp C-RNTI and up-link grant will be received by two UEs in step 2
• In step 3 eNodeB may be able to receive Msg3 from only one UE or none of them due to interference. 
• In step 4 the UE which does not receive Msg4 from eNodeB will back-off after expiration of RACH specific timers. Possibility is also that none of them receive Msg4 
• UE which receive msg4 will move to next step and decode RRC connection setup message

For more LTE call flows, please check out this tool