What is Beam-based Mobility Robustness Optimization in Open RAN?

  • My friend: Hi Ibrahim, I have a question for you.

    • Me: Please, go ahead.
  • My friend: I was wondering what is meant by Beam-based Mobility Robustness Optimization in Open RAN?

    • Me: Actually, MRO (Mobility Robustness Optimization) is a well-know concept that appeared in SON (Self Organizing Networks) since LTE Release 9 and its main target is to configure some of the parameters [such as CIO (Cell individual Offset), TTT (Time to trigger), etc.] at the eNB to delay or advance the HO between two neighboring cells to improve the handover performance. In 5G, each cell has multiple beams radiating in different directions and so instead of having one border between two cells during handover, we have now multiple borders as UE served by one beam at the serving cell can make handover to different beams at the target cell and hence the concept of MRO changed to be bMRO (Beam-based Mobility Robustness Optimization).
  • My friend: So, how bMRO is working?

    • Me: Simply we replace the usage of the parameters on cell-to-cell basis, to be used on beam-to- beam basis. So, instead of having a single CIO+TTT value pair for the cell1→cell2 border, we are going to employ a CIO+TTT value pair for the Source Cell [Beam1] →Target Cell [Beam2] and so we have optimized values based on beams not cells. This will enhance the mobility performance specially at the cell borders.
  • My friend: But how can this be deployed in Open RAN?

    • Me: The near-RT RIC may host an xApp to optimize inter-cell beam mobility such as bMRO. In this case the near-RT RIC might for example configure beam individual offsets and beam individual TTT for inter-cell mobility decisions and these decisions will be based on the operator objective such as reducing the number of TE (Too Early) and TL (Too Late) handovers or ping-pong handovers, also to improve the HO success rate at specific situations when performance is degraded. And of course, all this can be optimized via AL/ML based on the performance.
  • My friend: Thank you so much. You made it very clear.

    • Me: You are most welcome.

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