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The UE Cannot Access the Cells Due to the Optical Module

Publication Date:  2013-10-26 Views:  60 Downloads:  1
Issue Description

In the early stage of network deployment in country P, 10 sites are deployed. The customer chooses the first three deployed sites for the test. The UE can access only one cell and fails to access other eight cells. According to the traced result on the OMT, the UE can receive the eNodeB signals. Therefore, the customer considers that this problem occurs on the entire network.

None
Handling Process

Possible causes:

1. Faults on the wireless network: BBUs and RRUs are faulty. Or the parameters (UE parameters, eNodeB parameters, and EPC parameters) are incorrectly set.

2. Transmission faults: Route configurations are incorrect and transmission faults occur.

3. EPC faults: Parameters used in connecting the wireless network are incorrectly set, and HSS registration parameters are incorrectly configured and NE parameters on the EPC are incorrect.
Problem analysis:

1. In a field commercial site, the UE can receive the MIB and SIB information sent by the eNodeB. Meanwhile, the UE can camp on the cells. Check the UE parameters, eNodeB parameters, and S1 status.
The result shows that they are normal. Therefore, the problem is not caused by improper radio parameter settings or transmission faults.

2. According to the messages on the OMT, the UE sends the MM_ATTACH_REQ&RRC_CONN_REQ message but does not receive the RRC connections setup message sent by the eNodeB. Therefore, the RRC connection is not set up. The problem occurs on the radio network side.
Check the modules of BBUs and RRUs.
The modules are normal. Therefore, the hardware may be faulty.

3. The UE cannot access the other eight cells. The following figure lists the result of observing the OMT key event message.

 

In the random access process, the UE sends the random preamble sequences. However, in the RAR preamble ID match process, the received IDs are not those used in the uplink. Therefore, the UE resends the random preamble sequences. Before the RAB setup, the UE experiences a random access process in the physical layer. The UE is allowed to send the layer-2/layer-3 RAB messages. Physical-layer synchronization is required. If the random access fails, the uplink physical-layer synchronization cannot be ensured. As a result, the RRC connection cannot be set up.

Therefore, it is suspected that this problem is caused by delay in RRUs or BBUs. RRU engineers check the negotiation rates of the optical modules in the eight cells. The negotiated rate is 1.25 Gbit/s. The optical modules used in the eight cells are 1.25 Gbit/s optical modules.

Different optical modules have different rates. The clock frequencies of the RRU are also different. The reported delay is also different. The baseband board performs delay compensation. Otherwise, the physical-layer synchronization cannot be ensured and the UE cannot access the cells.

In the original network design, the 1.25 Gbit/s optical modules are used to support establishment of 10 MHz cells or cells with higher bandwidth. The baseband board can support delay compensation for the 1.25 Gbit/s optical modules. However, in the actual R&D process, the baseband board does not support delay compensation. Therefore, the UE cannot access the cells. Therefore, the problem is solved after changing the optical modules.
Root Cause
None
Solution
The UE cannot access the eight cells. The UE parameters, eNodeB parameters, and transmission status are normal. According to the key events observed on the OMT, the event is delay. The baseband board does not support delay compensation for the 1.25 Gbit/s optical modules. The problem is solved after the modules are replaced.
Suggestions

1. Different optical modules have different rates. The RRU reports different delays. Currently, the baseband board does not support delay compensation for 1.25 Gbit/s optical modules. Therefore, the 1.25 Gbit/s optical modules should be replaced.

2. When a UE cannot access cells, analyze the signaling to identify the causes.  

END