LTE (lena) model Handover problem
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Taehyoung Shim
Jan 11, 2014, 7:29:35 AM1/11/14
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Hi, I'm ns-3 LTE model user and I have a question.
In lena-x2-handover-measures.cc, there is a problem if we simulated more than ten UEs.
The message in the problem is, msg ="should trigger HO Preparation Failure, but it is not implemented".
How could we fix this problem?
PLZ, help.
Taehyoung Shim
Jan 11, 2014, 7:32:50 AM1/11/14
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At simultaneously more than 15 UEs requesting handover, always the problem occurred.
Budiarto Herman
Jan 24, 2014, 4:41:22 PM1/24/14
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Hi,
I'm afraid I'm not competent enough to completely answer your question. Here I'm sharing some investigation that I've done on this issue, but please someone do correct me if I'm wrong.
The root cause of the problem is in the Random Access (RA) procedure. During the handover preparation phase, the target cell must dedicate a unique RA preamble ID for each UE undergoing the handover (i.e., the incoming UE). Each cell has 64 IDs available, but only 14 of them are available for this kind of RA procedure (i.e., non-contention-based RA). When an ID is allocated for an incoming UE, it will stay reserved for 400 ms. After 400 ms has expired, the ID becomes available again for another incoming UE.
In your simulation, there were 15 UEs undergoing handover to the same cell at the same time. When the target cell attempted to allocate a preamble ID for the 15th UE, it failed because it ran out of preamble ID, hence throwing the error that you got.
I suppose the ideal solution would be to implement handover preparation failure as a response to this kind of situation, as highlighted by the error message.
Alternatively, you could try to increase the default 14 slots of preamble IDs by decreasing the NumberOfRaPreambles attribute of LteEnbMac (the default is 50). As a side effect, I presume that there will be a higher chance of RA preamble collisions during connection establishment. Collisions may delay connection establishments by several milliseconds, but at least it doesn't fatally disrupt the simulation.
Regards,
-budi-
I'm afraid I'm not competent enough to completely answer your question. Here I'm sharing some investigation that I've done on this issue, but please someone do correct me if I'm wrong.
The root cause of the problem is in the Random Access (RA) procedure. During the handover preparation phase, the target cell must dedicate a unique RA preamble ID for each UE undergoing the handover (i.e., the incoming UE). Each cell has 64 IDs available, but only 14 of them are available for this kind of RA procedure (i.e., non-contention-based RA). When an ID is allocated for an incoming UE, it will stay reserved for 400 ms. After 400 ms has expired, the ID becomes available again for another incoming UE.
In your simulation, there were 15 UEs undergoing handover to the same cell at the same time. When the target cell attempted to allocate a preamble ID for the 15th UE, it failed because it ran out of preamble ID, hence throwing the error that you got.
I suppose the ideal solution would be to implement handover preparation failure as a response to this kind of situation, as highlighted by the error message.
Alternatively, you could try to increase the default 14 slots of preamble IDs by decreasing the NumberOfRaPreambles attribute of LteEnbMac (the default is 50). As a side effect, I presume that there will be a higher chance of RA preamble collisions during connection establishment. Collisions may delay connection establishments by several milliseconds, but at least it doesn't fatally disrupt the simulation.
Regards,
-budi-
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