Cell selection and re-selection in lte

  • Monday, July 11 at10:18AM
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Cell selection

Initial cell selection: In this procedure, UE scans the frequency bands without the use of prior stored data. Once the suitable cell is found, the UE camps on the selected cell.
• Stored information cell selection: In this procedure, the UE utilizes the carrier frequency information, cell parameters etc. of the previous measurements. Once the suitable cell is selected, UE camps on it.
Cell selection criteria S has to be met to select the suitable cell which states that:

π‘†π‘Ÿπ‘₯π‘ π‘Ÿπ‘£>0
While,
π‘†π‘Ÿπ‘₯π‘ π‘Ÿπ‘£=π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘Ÿπ‘Žπ‘¦−τ€΅«π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘ π‘ +π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘ π‘ π‘ π‘œπ‘œπ‘¦π‘Ÿπ‘ τ€΅―−π‘ƒπ‘ π‘ π‘šπ‘π‘Ÿπ‘ π‘¦π‘Žπ‘ π‘ π‘ π‘ 
Where, π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘Ÿπ‘Žπ‘¦=π‘€π‘’π‘Žπ‘ π‘’π‘Ÿπ‘’π‘‘ 𝑅π‘₯ 𝑙𝑒𝑣𝑒𝑙 π‘£π‘Žπ‘™π‘’π‘’ (𝑅𝑆𝑅𝑃) π‘†π‘Ÿπ‘₯π‘ π‘Ÿπ‘£=𝐢𝑒𝑙𝑙 π‘†π‘’π‘™π‘’π‘π‘‘π‘–π‘œπ‘› 𝑅π‘₯ 𝑙𝑒𝑣𝑒𝑙 π‘£π‘Žπ‘™π‘’π‘’ (𝑑𝐡) π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘ π‘ =π‘…π‘’π‘žπ‘’π‘–π‘Ÿπ‘’π‘‘ π‘šπ‘–π‘›π‘–π‘šπ‘’π‘š 𝑅π‘₯ 𝑙𝑒𝑣𝑒𝑙 π‘£π‘Žπ‘™π‘’π‘’ (𝑅𝑆𝑅𝑃) π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘ π‘ π‘ π‘œπ‘œπ‘¦π‘Ÿπ‘ =𝑂𝑓𝑓𝑠𝑒𝑑 π‘‘π‘œ π‘ π‘–π‘”π‘›π‘Žπ‘™π‘™π‘’π‘‘ π‘„π‘Ÿπ‘₯π‘ π‘Ÿπ‘£π‘šπ‘ π‘ (𝑑𝐡) π‘ƒπ‘ π‘ π‘šπ‘π‘Ÿπ‘ π‘¦π‘Žπ‘ π‘ π‘ π‘ =π‘ƒπ‘œπ‘€π‘’π‘Ÿ πΆπ‘œπ‘šπ‘π‘’π‘›π‘ π‘Žπ‘‘π‘–π‘œπ‘› (𝑑𝐡)
Once the suitable cell has been selected and the camped upon, the UE starts to measure neighbouring cells for the reselection process. UE measures the neighbouring cells in the neighbour cell list of the serving cell. To decrease the frequency of neighbouring cell measurements, a threshold signal level is defined for the serving cell so that UE does not need to perform the measurement if the serving cell measured value exceeds the threshold. The threshold has been defined for both inter-frequency and intra-frequency cell selections.

Intra-frequency measurement criteria is that if π‘†π‘¦π‘Ÿπ‘Ÿπ‘£π‘ π‘ π‘”π‘ π‘Ÿπ‘ π‘ ≤Sintrasearch, intra-frequency neighbour search should be initiated and inter-frequency measurement criteria is that if π‘†π‘¦π‘Ÿπ‘Ÿπ‘£π‘ π‘ π‘”π‘ π‘Ÿπ‘ π‘ ≤Snon-intrasearch , inter-frequencies or inter-RAT frequency neighbour search should be initiated. π‘†π‘¦π‘Ÿπ‘Ÿπ‘£π‘ π‘ π‘”π‘ π‘Ÿπ‘ π‘  is the π‘†π‘Ÿπ‘₯π‘ π‘Ÿπ‘£ value of the serving cell.

Cell re-selection

Cell re-selection is performed when the above mentioned measurement criteria is ful-filled. In the case of intra-frequency cell re-selection, ranking criteria is used. The serv-ing cell and the neighbouring cells are ranked based on measured data. The best ranked cell is re-selected.
The serving cell is ranked as
𝑅𝑆=π‘„π‘šπ‘Ÿπ‘Žπ‘¦,𝑦+π‘„β„Žπ‘¦π‘¦

While the neighbouring cell is ranked as
𝑅𝑁=π‘„π‘šπ‘Ÿπ‘Žπ‘¦,𝑠−π‘„π‘ π‘œπ‘œπ‘¦π‘Ÿπ‘ 

π‘„π‘šπ‘Ÿπ‘Žπ‘¦,𝑦 is the measured RSRP value of the serving cell while π‘„π‘šπ‘Ÿπ‘Žπ‘¦,𝑠is the measured RSRP value of the neighbouring cell. The hysteresis value π‘„β„Žπ‘¦π‘¦ is added so that the frequent cell-reselection is prevented. Once the neighbouring cell is better ranked, the cell transition occurs after π‘‡π‘Ÿπ‘Ÿ−π‘¦π‘Ÿπ‘ π‘Ÿπ‘ π‘ π‘ π‘ π‘  time. This also helps to reduce the frequency of cell re-selection.

 



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