Most common problems in LTE networks

3dB consult has listed the most common problems seen in LTE networks and now they provide some suggestions how to tackle them.

Load Balancing

For LTE load balancing operators can use Inter-Frequency Load Balancing feature. This feature manages traffic load between different carrier frequencies (relocates connected UE’s to carriers that are less loaded).

To take LB decision traffic load knowledge is needed (how many E-RAB are in the cell and how many it is expected to accommodate). Based on the load information of other cells (LB needs to be configured between related cells), source cell determines the amount of traffic load to perform a handover (HO is performed using normal IF HO procedure, A5). It is needed to set a minimum load difference with other cells (consider that a low threshold would be constantly balancing) and a maximum load to trigger the HO.

Interference

LTE cells use same RF channels and create interference between each other. Although OFDM modulation uses orthogonal sub-carrier signals, in practice transmitters and receivers are not so accurate and they add a frequency deviation that causes the sub-carriers no longer to be orthogonal. As a result, inter-carrier interference (ICI) is produced.


Since the same frequency is used over and over again, ICI can limit system performance in terms of throughput and spectral efficiency, especially for users located at cell edge. So careful management of inter-cell interference is very important in LTE to improve system performance.

Some LTE features are intended to mitigate, cancel or avoid interference between sites. Strategies such as coordinated multi-point transmission/reception (CoMP) and inter-cell interference coordination (ICIC) are specifically designed to manage interference on the cell boundary.

CSFB Trigger events

CS fallback to UMTS can be performed with different solutions: RRC Connection Release with Redirection without Sys Info, RRC Connection Release with Redirection with Sys Info, PS handover with DRB.

CS fallback to GERAN can be performed with different solutions: RRC Connection Release with Redirection without Sys Info, RRC Connection Release with Redirection with Sys Info, PS handover, Cell Change order without NACC, Cell change order with NACC.


RwR (Release with Redirect): Does not use events for CSFB. RRC Connection Release with redirection info – in this case RRC Connection release message contains the target UTRAN frequency information. Then the UE moves to the target RAT and searches for an acceptable cell by utilizing the frequency information within the RRC Connection Release.

PS HO: This feature minimizes the interruption on PS data session, when the eNodeB transfers the UE to UTRAN. It supports blind handover and the handover to a WCDMA frequency based on measurements. The measurements use B1 and B2 events (B1 Inter RAT neighbour becomes better than threshold, B2 PCell becomes worse than threshold1 and inter RAT neighbour becomes better than threshold2).

  • Event B1: Neighbour becomes better than absolute threshold;
  • Inequality B1-1 (Entering condition)
  • Inequality B1-2 (Leaving condition)
  • Event B2: PCell becomes worse than absolute threshold1 AND Neighbour becomes better than another absolute threshold2.

  • Inequality B2-1 (Entering condition 1)
  • Inequality B2-2 (Entering condition 2)
  • Inequality B2-3 (Leaving condition 1)
  • Inequality B2-4 (Leaving condition 2)


Please refer to 3GPP TS 36.331 for more details or contact 3dB consult

Call Muting in LTE

There is a great number of reasons that could cause a mute call in LTE. We recommend operators to check the following issues:

  • Sometimes the problem could come from the UE. Check if mute calls come from a specific model. VoLTE is a quite new technology and not all the terminals can understand the dialog to the network properly. If this is the case a UE firmware update is needed.

  • Poor 4G RF conditions may cause packet loss. Try to avoid ping-pong HO
  • Check PDCP PDU sequence number length for specified QCI. The same value must be set for all nodes in the network to assure PDCP data decoding. Review also RLC PDU sequence number length for the same reason. Isolated node with different value will cause mute and drop calls in its area
  • Check if UE has Noise Cancellation activated.

Of course this is just the tip of the iceberg. For more information about this or other 2G/3G/4G topics, get in contact with 3dB. We will go much deeper than this, right down to the bottom of the issue.

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