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What happens when two UEs transmit same Random Access

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

posted Jul 18, 2013 by Nora Jones

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2 Answers

+1 vote

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

answer Jul 18, 2013 by Salil Agrawal
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If eNodeB receives same RAs at the same time , then eNB drops both. After Backoff timer again both UE's start sending preamble. 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

answer Jul 22, 2013 by anonymous
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