Random Access preamble

Gaurav · 4G Newbie Joined: 09 Oct 2009 Posts: 1

What happens when two UEs transmit same Random Access (RA) preamble to eNodeB. How the contention or conflict get resolved?[/b]

jbkchoudary · 4G Newbie Joined: 04 Nov 2009 Posts: 1

Hi Gaurav,

When two UE's send same preamble to eNodeB , eNodeB responds to the preamble and sends message-2 in downlink. Now both the UE's receive the same message-2 in which parameters for msg-3 transmission in Uplink will be same. So both the UE's tries to send the msg-3 on same UL resources given in msg-2.

Now eNodeB cannot decode the msg-3 due to overlap of msg-3's from both UE's. eNodeB will not send any msg-4. So both the UE's back off after contension resolution timer expires and resend the preamble again.

This time there is less probability to have both UE's to select same preamble again

Note: This my vary with eNodeB implementations from vendors

- JBK

ykhaled · 4G Newbie Joined: 13 Jul 2011 Posts: 7

You are referring to contention resolution message.
Assume two or more UEs send the same random preamble out of 64 on the same time and frequency on the PRACH. So these UEs have the same frequency/time signature.
These preambles will collide at eNodeB and it will send Random Access Response (RAR) specifying the identity that all the UEs with the same signature will be able to receive and decode and all the colliding UEs will receive the same Temporary C-RNTI and they will collide when transmitting their L2/L3 message on the scheduled uplink transmission. This might cause interference off course. If none of these UE's transmission was decoded properly; they have to re-transmit their Preamble. If one was decoded properly by eNodeB, then it will response with L2/L3 message including UE identity.
Now all the colliding UEs will receive this message as well; but it will only be accepted by the UE that have a matching identity.